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Chapter 10.
Clinically Oriented Extrinsic and Intrinsic Factors in Etiology of Fetal Growth Restriction
Intrauterine or fetal growth restriction (FGR) is associated with a heterogenous group of conditions ranging from fetal chromosomal aberrations to maternal malnutrition. FGR is the second most important cause of perinatal morbidity and mortality (1). Adverse effects, however, are not limited to the perinatal period because FGR is associated with sequelae, such as a permanent neurological damage (2). Furthermore, a pathophysiological concept has emerged linking FGR to increased susceptibility in adulthood to diseases, such as hypertension, type-2 diabetes, and atherosclerosis (3), as well as with increased risk of neonatal morbidity and mortality and alterations in physical and mental development during early childhood (4).
10.1) Geographic peculiarities of immunoglobulins in FGR
The blood IgG level is directly related to the neonate's birth weight being higher in infants with normal birth weight than in those with low birth weight (5). The lower level of IgG in low-birth-weight (LBW) infants can probably be attributed to blockage of the IgG-specific Fc-receptor sites in the placenta due to acute atherosis and reduced uteroplacental perfusion. Cord blood levels of IgG, IgA and IgM in newborn FGR infants were found to be significantly lower than those in infants with growth that was adequate for their gestational age (6). Lower levels of cord IgG in FGR may be due to a defect in the active transport of IgG across the placenta. Lower levels of cord IgM and IgA suggest an impairment in Ig synthesis in FGR infants. No differences were observed in maternal Ig concentrations among the study groups.
In the adult human thymus, cytoplasmic IgG is detected most frequently with lesser amounts of IgA and IgM (7,8). In the fetal thymus, the significant number of Ig-containing cells present a distribution similar to that seen in the adult, with IgG predominating. IgM is demonstrated more frequently than IgA. There is a tendency for the number of Ig-containing cells to rise after birth and to continue into early adult life, sometimes continuing past the age of 20. Although Ig-containing cells represent only a minor population, their presence must be taken into account when considering the function of the thymus and the mechanism of involution.
The humoral and cellular immune status of preterm and small-for-dates babies was evaluated in Indian newborns (9). Neonates with severe FGR and preterm babies had significantly lower levels of IgG, but not of IgM or IgA. The preterm babies had a significantly higher percentage of B lymphocytes, although the absolute count was not significantly different from normal newborns. The babies with severe FGR had a significantly lower absolute count of B cells, as well as lower absolute and percentage counts of E-rosette-forming cells relative to normal newborns. These findings suggested that LBW babies with severe FGR are at greater risk of developing bacterial infection due to deficiencies both humoral and cellular immune host defenses. In contrast, preterm babies are immunologically competent, even though passively transferred maternal IgG levels are low.
Sizeable concentrations of IgG were present at birth in Nigerian LBW babies (2,500 g or less) (10). The mean IgG level in such babies (1360 mg/100 ml) was significantly lower than in babies with average and above average weight. The mean maternal and cord IgG levels were approximately equal in all except in LBW babies. Mothers with higher IgG levels than in their corresponding babies were most prominent in the LBW group. There was no significant difference between the maternal or cord serum's respective IgA and IgM levels. The mean cord serum IgA and IgM levels were about 4% and 10% respectively, of thouse in the mothers' serum. Compared with Caucasians, relatively higher quantities of IgD were found in the Nigerian mothers.
Serum levels of IgG, IgM and IgA were different between appropriate-for-date (AFD) and small-for-date (SFD) infants in
In a similar study performed in Tanzania, the mean serum levels of total protein, albumin, and IgG in mothers who delivered AFD infants were 6.8 g/100 ml, 2.9 g/100 ml, and 1,840 mg/100 ml, respectively, whereas those from their infants were 6.9 g/100 ml, 4.1 g/100 ml, and 1,471 mg/100 ml (12). The synthesis of IgM and IgA during fetal life appeared to be activated at an earlier gestational age than in infants in Western countries. This fact subsequently resulted in a higher detectable amount of IgM and IgA in the cord blood of the AFD infants. IgG in Tanzanian mothers was generally higher than in the corresponding cord blood sera, which is contrary to the finding in Europe.
10.2) Changes in the placenta as a reason for FGR
10.2.1) Morphological aspects
The placenta plays a key role in fetal nutrition. It mediates the active transport of nutrients and metabolic wastes across the barrier separating maternal and fetal compartments, and modifies the composition of some nutrients through its own metabolic activity (13). The function of the placenta is essential to the growth of a healthy fetus; it is becoming apparent that the activities of the placenta are in turn modulated by signals originating from the fetus. Communication between the placenta and fetus is especially critical in intrauterine growth retardation.
Normal fetal growth depends on the genetically predetermined growth potential and is modulated by fetal, placental, maternal, and external factors (14). Fetuses with growth restriction (FGR) are at high risk for poor short- and long-term outcomes. Although there are many underlying etiologies, FGR resulting from placental insufficiency is most relevant clinically because the outcome can be altered by appropriate diagnosis and timely delivery. A diagnostic approach that aims to separate FGR resulting from placental disease in the constitutionally small fetuses from that with other underlying causes (e.g., aneuploidy, viral infection, non-aneuploid syndromes) needs to integrate multiple imaging modalities. In placental-based FGR, cardiovascular and behavioral responses are interrelated with disease severity.
From the beginning of the pregnancy, the placenta exerts its effects on fetal growth via metabolic and endocrine mechanisms (15). To achieve this, the placenta exchanges a wide array of nutrients, endocrine signals, cytokines and growth factors between the mother and the fetus. These exchanges modulate or programmed fetal growth and development. The mother's nutritional and hormonal state from as early as the first few days after fertilization, can influence the growth rate of the placenta and the fetus as well as the length of gestation. Thus influences on placental development and their consequences will clearly have an impact on the placental control of fetal growth. Variations in the maternal environment and consequent perturbation of the metabolic and endocrine environment of the placenta and fetus are responsible for the associations between prenatal growth of the placenta and its fetus and the subsequent risk of adult disease.
Changes in the placenta are reflected in fetal weight (16). With large fetuses, there is a relatively small volume of the fibrinoid substance, moderate synthesis by the syncytial epithelium of glucosaminoproteoglycans (GAPG), slight lymphoplasma cell infiltrates with a marked suppressor influence, a lack of human lymphocyte antigens (HLA) on the trophoblast and the formation of adequate interrelations between the immune- dependent placental structure and the thymus. It may be that all of these features allow the large fetuses to be retained as fetuses for up to 38-40 weeks or even more. In cases of FGR, there is a sharp increase in the fibrinoid substance, focal enhancement of GAPG production by the syncytial cells, a decrease in the suppressor activity and the appearance of HLA on the trophoblast. This is probably a manifestation of a premature exhaustion of compensatory-adaptive reactions in the placenta.
A placental lesion, characterized by fibrinoid and trophoblastic necrosis with massive infiltration of the intervillous space by mononuclear cells (massive chronic intervillositis, MCI), was observed in cases of FGR and of sudden intrauterine fetal death (17). The lesions are characterized as chronic villitis of unknown etiology or as anchoring villitis. A high number of acute atherosis-like lesions in the spiral arteries of parietal and/or basal decidua were observed. Patients with MCI showed a higher incidence of FGR with lower than control values of infant weight, length, and pondered index. Massive chronic intervillositis may represent an extreme variant of villitis of unknown etiology.
A prominent feature of the placentation process is an extensive transformation of the 120 spiral arteries that supply the intervillous space of the placenta with maternal blood (18). The transformation implies a destruction of the muscular wall of the spiral arteries resulting in wide vessels with low resistance and high flow rates (19). In pregnancies with placental insufficiency, this adaptive remodeling of the spiral arteries is incomplete (20). In addition, a prominent feature of the insufficient placenta is thrombotic infarction (21).
Fetal growth is largely determined by the availability of nutrients to the fetus and regulated by the hormones of the fetal somatotrophic axis, and in particular insulin-like growth factor (IGF)-1 (22). Placental function in turn, is heavily influenced by the maternal and fetal growth hormones (GH). The placenta itself is also an active endocrine organ and it produces a large number of hormones including GH and IGF-1 as well their corresponding receptors. Thus the placenta can no longer be considered merely a passive conduit for fetal nutrition. Rather, it is actively involved in the integration of nutritional and endocrine signals from the maternal and fetal somatotrophic axes. Inadequate growth in utero is associated not only with adverse fetal, perinatal, and neonatal outcomes, but also with an altered propensity for disease later in life (23). Conversely, fetal overgrowth is also associated with increased medical risks for both the mother and fetus. The interaction between the fetal genome and the intrauterine environment determines in large part how fetal growth will progress. The placental, maternal, and fetal somatotrophic axes (in particular, GH and IGF-1) play key roles in modulating this interaction.
FGR is a significant cause of infant mortality and morbidity. It is now clear that FGR infants exhibit higher rates of coronary heart disease, type-2 diabetes, hypertension and stroke as adults. Therefore, fetal growth not only impacts the outcome of the perinatal period, but also impacts adult well-being (24). The etiologies of FGR are numerous, but are often associated with abnormalities in placental structure and function. The processes of implantation and placentation require the production of a plethora of growth factors, cell-adhesion molecules, extracellular matrix proteins, hormones and transcription factors. Many of these exhibit altered expression within the placenta in FGR pregnancies, particularly, with respect to placental vascular structure and function.
A morphological electron microscopy study showed that in cases of FGR, villi of human placentas appear longer, thinner, and less vascularized, compared to the normal condition (25). Fibrinoid, an extracellular material of hematic origin, frequently fills the villous stroma. The density of apical microvilli appears considerably reduced and occasional microvillus-free areas are observed. Moreover, the underlying basal membrane appears significantly thicker than that of the normal syncytiotrophoblast. A study of apoptosis as a possible cell-deletion mechanism in growth restriction showed that most of the typical apoptotic features appear indifferently in both FGR and normal pregnancy. It has been suggested that growth retardation might be correlated with a complex of structural changes, suggestive of materno-fetal traffic downregulation mechanisms.
The placenta is an important functional unit for gas transfer between mother and fetus. The placental membrane, consisting of the trophoblast layer interposed between the maternal and fetal blood, is known to play an active role in respiration intensity. It is accepted that FGR and increased rate of fetal mortality can usually be seen at high altitude. It has been shown that trophoblast cells may play an important role in the gas transfer mechanism in a hypoxic state at high altitude, such as in
Defects in all the trophoblast-differentiation pathways -- endovascular, interstitial and chorionic villous -- play a role in the pathogenesis of early-onset FGR (27). There are two types of extravillous trophoblast: endovascular trophoblast, which forms the definitive placenta by occlusion of the spiral arteriole at the implantation site, and interstitial extravillous trophoblast, responsible for the anatomical erosion of the distal spiral arteriole and the secretion of angiogenic and vasodilator signals to improve uterine blood flow. Defective endovascular erosion may render the basal plate inadequate to meet the demands of the fetus. Failed interstitial invasion of the spiral arterioles may lead to failure of local angiogenic and systemic cardiovascular adaptation signals which in turn could be the underlying basis for early-onset FGR and pre-eclampsia.
10.2.2) Biochemical aspects
FGR is considered an abnormal pregnancy. It is not a specific disease entity per se, but rather a complex of several fetal and/or maternal disorders. Analysis of the modalities of the uteroplacental circulation and maternal blood entrance has demonstrates physiological hypoxia ending with the first trimester of pregnancy. Moreover, in vitro culture of the first-trimester villous explants has shown the role of oxygen in extravillous cytotrophoblast proliferation, decidual invasion and spiral artery remodeling (28). Oxygen appears to be a key factor controlling the mechanism of placentation by regulating the transcription of several genes, such as the vascular endothelial growth factor and leptin, among others. These genes are turned on or off as a function of the oxygen partial pressure via an oxygen sensor. Oxygen is also implicated in the development of several pathologies of pregnancy. It is involved, for example, at different steps in the cascade of events leading to preeclampsia. Decreased oxygen partial pressure is considered a possible reason for abnormal development of the trophoblast villous tree in FGR, in maternal anemia or in pregnancies in regions at high altitude.
Under hypoxic conditions, litter size in rats is reduced and insulin-like growth factor binding protein (IGFBP-1), a secreted protein that binds IGFs in extracellular environments, is up-regulated in the maternal serum and in the fetal liver and heart (29). Tissue-specific induction of calcium homeostasis-related genes and suppression of growth-related genes are observed, suggesting mechanisms underlying hypoxia-related FGR. Furthermore, induction of inflammation-related genes in placentas exposed to long-term hypoxia (11 days) suggests a mechanism for placental dysfunction and impaired pregnancy outcome that accompany intrauterine hypoxia.
Hypoxia treatment not only resulted in FGR, it also caused a significant delay in the developmental rate and timing of morphogenesis in vital organs (30). Elevated IGFBP-1 mediates hypoxia-induced FGR and developmental delay by binding to and inhibiting the activities of IGFs. Knockdown of IGFBP-1 significantly alleviated the hypoxia-induced growth retardation and developmental delay. Over expression of IGFBP-1 caused growth and developmental retardation under normoxia. Furthermore, reintroduction of IGFBP-1 to the IGFBP-1 knocked-down embryos restored the hypoxic effects on fetal growth and development. The induction of IGFBP-1 expression may be a conserved physiological mechanism to restrict IGF-stimulated growth and developmental processes under hypoxic stress.
IGF-I is known to play a role in placental and fetal growth. Immunoreactive IGF-I was detected in the cyto- and syncytiotrophoblast, amnion, endothelial cells of the fetal capillaries and in the decidua in both normal and FGR placental tissue (31). Stronger immunostaining and an increased number of positively stained cells were found in the decidua of the FGR placenta. Intense immunostaining was also found in endothelial cells, smooth muscle cells and fibroblasts of the umbilical vein. IGF-I immunoreactivity was also present in the stroma (Hofbauer cells and/or fibroblasts) of FGR chorionic villi. Expression of IGF-I is normally high at specific sites in the placenta and umbilical cords, indicating a paracrine and/or endocrine function. The increased expression of IGF-I in the placenta of FGR fetuses indicates its involvement in restoring normal growth by means of a positive feedback mechanism.
Gestational and fetal size effects were found on the receptor density of endothelins (ET), potent vasoconstrictor peptides, within the trophoblastic stem villi (32). Smooth muscle cells expressed ET receptors predominantly in the proximal regions of the chorionic villous tree and in the deciduas, and at a lower density on paravascular stromal cells in the stem villi. The localization of ET-1 immunoreactivity differs in human placental tissues from third-trimester normal vs. FGR pregnancies (33). The localization of ET-1 immunoreactivity was significantly higher in the capillary endothelial cells of the trophoblastic villi as well as in the endothelial, decidual, and trophoblastic cells of the basal plate in placentas from normal pregnancies than in FGR pregnancies. It has been suggested that the lower expression of ET-1 in placental tissues from FGR pregnancies might be secondary to an adaptive mechanism designed to reduce the vasoconstrictor effect of ET-1.
Human angiogenin, a potent inducer of neovascularization, is involved in the morphological and angiogenic changes in the placenta necessary for a successful fetal outcome during pregnancy. It has been shown that placental explants from patients with FGR secrete in vitro angiogenin at 1.3- to 1.6-fold higher than normal term levels at 24 and 72 hr of culture, respectively. (34). Expression profiles of angiogenin from term and FGR cases are in agreement with its mRNA levels and immunoblot resultss. The significantly elevated levels of angiogenin in the FGR placenta may provide a molecular mechanism for the abnormal placental development.
Some hormones play an important role in regulating fetal growth. The role in this process of 11γ-HSD2, a known factor in determining fetal morbidity and in the subsequent development of cardiovascular disease in adulthood, has been studied as an example (35). In the placenta, 11γ-HSD2 activity is thought to protect the fetus from the deleterious effects of maternal glucocorticoids. Patients with apparent mineralocorticoid excess owing to mutations in the 11γ-HSD2 gene invariably exhibited reduced babies' birth weight, accompanied by reduced placental 11γ-HSD2 activity in pregnancies complicated by FGR. This is reflected by evidence of hypercortisolemia in the fetal circulation in small babies. Placental 11γ-HSD2 mRNA levels were significantly lower in FGR pregnancies than in gestationally matched, appropriately growing placentas.
10.2.3) Transport across the trophoblast in FGR
The causes of FGR are multifactorial and largely unknown; however, altered placental transport has been implicated (36). The placental transporting epithelium -- the syncytiotrophoblast -- is polarized with the apical microvillous membrane and basal plasma membrane. These two plasma membranes constitute the major barrier between the mother and her fetus, and most nutrients and metabolites are actively or passively transferred across them to and/or from the fetus. Some of the evidence for a link between disturbed placental transport across these membranes and FGR is considered below.
Development of the fetal immune system begins at the early stages of gestation (37). It is competent to respond to intrauterine infections from as early as 12 weeks and has full functional potential at birth. Maternally acquired IgG is available for up to 9 months of life until the infant's own immune system has been adequately primed and activated following first exposure to specific antigens. The normal fetomaternal immune relationship represents a remarkably harmonious association between two genetically disparate individuals. Maternal IgG antibodies are transmitted across the trophoblast by receptor-dependent mechanisms to provide immediate protection for the neonate against environmental pathogens. Leakage of fetal erythrocytes, leukocytes and platelets into the maternal circulation can elicit IgG isoantibodies that take advantage of the same mechanisms to gain access to the fetus, with pathological consequences (38). Autoantibodies in women with various disease states may also pass into the fetus, but these normally produce only mild and transient effects. The fetal trophoblast is able to act as a protective barrier by virtue of its unique properties, including a lack of conventional class I and class II human lymphocyte antigen (HLA) molecules, which render it nonsusceptible to immune attack (38). Maternal immune cell traffic across the placenta occurs only at a very low level, if at all, in normal pregnancy. This may take place to a greater degree in some of the rare instances of fetal graft-versus-host disease, but this is complicated by the associated fetal immunodeficiency.
FGR can be characterized as a condition in which the fetus has failed to achieve its genotypical growth potential. FGR is associated with increased perinatal morbidity, higher incidence of neurodevelopmental impairment, and increased risk for a number of diseases in adulthood, such as cardiovascular disease and diabetes (39,40). The altered growth pattern seen in many cases of FGR is believed to be caused by impaired placental transport functions and blood flow (36). FGR is associated with changes in fetal and/or maternal levels or placental secretion of a number of hormones and growth factors, providing the underlying rationale for most of the effectors tested. Fetal levels of IGF-I (41), leptin (42), and epidermal growth factor (43) as well as maternal levels of growth hormones (44) are reduced in FGR. FGR placentas show higher than normal placental amounts of IGF-II transcripts (45). There are contradictory results concerning maternal and fetal cytokine levels in FGR: some studies show elevated levels of cytokines, others show reduced levels, and still others show no change in cytokine production in association with FGR (46,47).
Vascular placental insufficiency is considered a common pathogenic factor in human FGR. In experiments with ischemic conditions in rabbits, it has been shown that the change in activity of the brain-type isozyme of creatine kinase (CK), involved in energy generation and regulation, can be used as a marker for responses to ischemia in rabbit tissues, including placenta (48). A significant transient increase in CK-specific activity was found in the kidney and the cerebellum but not in the other organs tested, at 24 and 48 h after ligation. This increase was not seen with adenylate kinase, another enzyme involved in energy generation and regulation. It has been proposed that an increase in CK-specific activity could serve as a metabolic marker of vascular insufficiency in rapidly developing tissues, representing part of a compensatory mechanism to overcome an energetic gap induced by ischemia and, as a result, manifested in retarded fetuses.
Amino acids have multiple functions in fetoplacental development. The supply of amino acids to the fetus involves active transport across the trophoblast and metabolism within it. Transport occurs through various amino acid-transport systems located on both the maternal- and fetal-facing membranes. The capacity of the placenta to supply amino acids to the fetus develops during pregnancy through alterations in such factors as the surface area and expression of the specific time-dependent transport system. In FGR, both placental surface area and amino acid uptake decrease in human and experimental animal models (49). In an ovine model of FGR produced by hyperthermia-induced placental insufficiency, umbilical oxygen and the rate of uptake of essential amino acids were significantly reduced in the most severe cases, in concert with decreased fetal growth. These changes indicate that severe FGR is likely associated with a shift in amino-acid transport capacity and metabolic pathways within the fetoplacental unit. Under normal conditions, after transport across the trophoblast amino acids are actively incorporated into tissue proteins or oxidized. In a sheep growth-retarded fetus, which was hypoxic, hypoglycemic and hypoinsulinemic, there appeared to be net effluxes of amino acids from the liver and skeletal muscle, suggesting changes in amino acid metabolism. Potential changes may be occurring in the insulin/IGF-I signaling pathway, including decreased production and/or activation of specific signaling proteins leading to a reduced protein synthesis in fetal tissues. Such observations in the placental insufficiency model of FGR indicate that this combination decreases fetoplacental amino acid uptake and disrupts insulin/IGF signaling in the liver and muscles in FGR.
FGR is characterized by a reduction in fetal plasma concentrations of a number of essential amino acids. The activity of placental transporters for cationic and neutral amino acids is reduced in FGR: mediated uptake of lysine was reduced by 44% in the basal membrane and uptake of leucine was lower in the both microvillous and basal membranes relative to control vesicles (50). Intravesicular glycine (2 mM) increased the uptake of leucine by 98% in the microvillous membrane. These data suggest that a reduced glycine gradient in the FGR placenta, due to reduction in amino-acid transporter system A activity, impairs leucine transport to the fetus, providing an additional mechanism for the reduced placental transport of leucine in FGR.
The activity of system A has been shown to be reduced in the syncytiotrophoblast microvillous membrane in FGR. However, the impact of these changes on transplacental transport is difficult to assess without information on system A activity in the basal plasma membrane (51). In term of FGR neonates, microvillous membrane system A activity was unaltered compared to controls (52). In preterm FGR neonates (gestational age 28-36 weeks), the system A activity in the microvillous membrane was reduced by 50% as compared to controls (gestational age 28-35 weeks). In all preterm FGR pregnancies, signs of severe fetal compromise were present, whereas term FGR neonates were less affected. These data support the view that the activity of microvillous membrane system A is related to the severity of the compromise in FGR. The markedly reduced system A activity in the microvillous membrane in preterm FGR together with the unaltered activity in the basal membrane is consistent with decreased transplacental transport of neutral amino acids in this pregnancy complication.
Taurine is an essential amino acid during fetal life and appears to be vital for the growth of the fetus and for the development of the fetal central nervous system. In FGR, fetal plasma concentrations of taurine are reduced, due to its altered placental transport (53). Transplacental transfer is the fetus's primary source of taurine. In FGR, the placental transport capacity of taurine is reduced and fetal taurine levels are decreased (54). The taurine transporter (TAUT), detected as a single 70-kDa band, is primarily localized in the syncytiotrophoblast microvillous plasma membrane, and its expression is unaltered in FGR. None of the tested hormones, e.g., leptin and growth hormone, altered TAUT activity significantly. The syncytiotrophoblast TAUT is strongly polarized to the maternal-facing plasma membrane. Microvillous membrane TAUT expression is unaltered in FGR, suggesting that the reduced microvillous membrane taurine transport in this condition is due to changes in the transporter's activity. Nitric oxide plays an important role in downregulating microvillous membrane TAUT activity in FGR.
The placenta plays a critical role in providing an environment that supports optimal fetal growth, by being the site of nutrient transfer from the mother to the fetus and ofwaste secretion from the fetus to the mother, acting as a barrier against pathogens and the maternal immune system, and serving as an active endocrine organ capable of secreting hormones, growth factors, cytokines, and other bioactive products. Among the hormones produced by the placenta are members of the growth hormone/prolactin gene family, the placental lactogens and prolactin-related proteins (55). The placental lactogens are secreted into the maternal and fetal circulations and seem to mediate their effects through unique receptors. The placenta is also primarily responsible for elimination of fetal acid equivalents of respiratory and metabolic origin. Intrauterine sampling of the fetal cord blood has shown that growth-restricted fetuses are more prone to develop acidosis in utero than normally grown fetuses (56). This most probably contributes to the adverse outcome associated with FGR.
Several major acid/base-regulating proteins have been demonstrated in the placental syncytiotrophoblast. Two key transporter proteins, the Na+/H+ exchanger (NHE) (57) and the Cl-/HCO3- exchanger (58) are localized to the microvillous plasma membrane. The main functions of the NHE are maintenance of intracellular pH, vectorial Na+ transport, and regulation of cell volume. This family of proteins consists of at least six isoforms, NHE1 to NHE6, and catalyzes the extrusion of one H+ per Na+ ion entering the cell down its electrochemical gradient. Although NHE-1 is ubiquitously distributed (59), NHE2-6 has a more restricted pattern of expression (60,61). NHE-1 serves housekeeping functions in most cell types, whereas NHE-2 and NHE-3 are epithelial isoforms, implicated in vectorial Na+ transport. NHE isoforms are present in the syncytiotrophoblast plasma membranes of the human placenta (61-64). Inhibition of NHE has been shown to severely impair recovery from an intracellular acid load, both in the first trimester of pregnancy and at term (65). The presence of a pH gradient in isolated microvillous membrane vesicles suggests a role for NHE in regulating intracellular pH, which is critical for the proper functioning of enzymatic and transport functions in the placenta (57,66). These observations also raise the possibility that syncytiotrophoblastic NHEs are involved in the removal of acid equivalents from the fetal compartment. The activity of NHE in microvillous membrane from FGR and control placentas has been investigated in term as well as in preterm preparations, and found to be either unaltered (62 ) or reduced (67).
Regulation of syncytiotrophoblast intracellular pH is critical to optimum enzymatic and transport functions of the placenta (68). The expression of NHE isoforms 1, 2, and 3 was approximately 10-fold greater in the microvillous membrane than in the basal membrane isolated from preterm and term placentas obtained from uncomplicated and FGR pregnancies. NHE-1 and NHE-2 are localized to the microvillous membrane and basal membrane and of NHE-3 to the microvillous membrane, basal membrane, and cytoplasm of the syncytiotrophoblast (63,68). NHE-1 expression in the microvillous membrane from preterm FGR placentas was reduced by 55%, compared to gestational age-matched controls, whereas NHE-1 expression was unaltered in the term FGR placentas relative to their matched controls. The activity of NHE in the microvillous membrane from the FGR preterm placentas was reduced by 48% relative to controls. In contrast, NHE activity in the microvillous membrane of term FGR was unchanged. The reduced activity and expression of NHE in the microvillous membrane of preterm FGR placentas may compromise placental function and may contribute to the development of fetal acidosis in the preterm FGR fetus (68).
NHE-3 also shows intense staining in the cytoplasm. This is in agreement with studies of other epithelia showing storage of this isoform in the plasma-membrane vesicles, termed recycling endosomes (69,70). These endosomes are incorporated into the apical membranes on demand and represent a dynamic and mobile pool of NHE-3 in the renal epithelium, for example. This also appears to be the case in the human syncytiotrophoblast (68). In other polarized transporting epithelia, such as those in the kidneys and intestine, a consistent pattern is observed with regard to the NHE isoform distribution. NHE-2 and NHE-3 are inserted into the apical membranes, and NHE-1 is distributed to the basal membrane (71). This asymmetrical distribution dictates the function of NHE isoforms in these epithelia. The basal localization of NHE-1 is thought to mainly regulate intracellular pH, and the apical NHE-2 and 3 aid in vectorial Na+ transport (72 ). The placental syncytiotrophoblast displays a different pattern of polarization, with the greatest abundance of all three isoforms localized to the microvillous membrane.
The high abundance of NHE isoforms in the microvillous membrane is consistent with protons being transported from the syncytiotrophoblast into the maternal circulation for subsequent elimination by the maternal kidneysw. Besides pH regulation of the syncytiotrophoblast, placental NHE is likely to be involved in vectorial Na+ transport (73). The anion exchanger is also more abundant in the microvillous membrane than in the basal membrane (74) and possibly acts in conjunction with NHE to perform vectorial NaCl transport. Furthermore, these exchangers could be important for creating an acidic microenvironment in the vicinity of the microvillous membrane. This has been shown in the small intestine, where the increased extracellular proton concentration enhances the uphill transport of oligopeptides (75). The presence of co-transporters coupling the uptake of protons to the import of lactate (76), organic cations (77 ), and peptides (78 ) has been described in the chorionic microvillous membrane. The expression of NHE in the basal membrane implies that the syncytiotrophoblast has the ability to transport protons into the fetal compartment. However, NHE activity in the basal membrane, if present, is likely to be low (68). It is possible that NHE localized in the fetal facing plasma membrane is important for optimal function of other transporters by establishing an acidic microenvironment.
A significant reduction in microvillous-membrane NHE activity was found in the FGR preterm-delivery samples in comparison with appropriate-growth-for-gestational age samples (67,68). In the examination of preterm and term FGR, both the microvillous membrane NHE activity and NHE-1 expression were found to be down-regulated in the preterm FGR group, compared to age-matched controls (68). The fact that the preterm FGR microvillous membrane, but not the term FGR microvillous membrane, shows a significant reduction in NHE indicates that both groups are associated with different pathophysiological conditions. The growth retarded fetuses delivered preterm appear to have a decreased capacity to maintain basic homeostatic parameters. The term "growth retarded fetuses" might represent a subgroup of FGR with less severe growth restriction and/or better compensatory mechanisms that allow those pregnancies to progress to term.
The findings in primary villous samples (65), a choriocarcinoma cell line (79), and isolated membrane vesicles (57,66) suggest that NHE represents one of the key mechanisms for intracellular pH regulation in the syncytiotrophoblast. Both NHE activity and NHE-1 expression are reduced in the microvillous membrane from preterm FGR placentas. A decreased capacity to clear the syncytiotrophoblast of protons might adversely affect placental enzymatic and transport functions. A reduction in NHE activity could lead to lower intracellular pH, which may impair nutrient transport and other placental functions. Growth restricted fetuses are prone to developing chronic acidosis in utero (80). It is likely that several factors contribute to this acidosis such as impaired placental blood flow resulting in hypoxemia (56) and the reduced fetal kidney function (81). It is ultimately the placenta that is responsible for removing acid equivalents from the fetal compartment, either by transporting protons to the mother or bicarbonate to the fetus. It is likely that transcellular transport of protons across the syncytiotrophoblast plays a role in the regulation of fetal pH, and NHE in the microvillous plasma membrane of the syncytiotrophoblast represents a key mechanism in this proton-transport route. The decreased microvillous membrane NHE protein expression and activity in preterm FGR has therefore been suggested to contribute to the development of fetal acidosis (68).
Inadequate placental transport of glucose has been implicated as a pathophysiological mechanism in FGR. Glucose transporter (GLUT) protein is normally abundant in the syncytiotrophoblast (82), with GLUT density being approximately threefold higher in the syncytiotrophoblast microvilli than in the basal membranes. In the latter, the density of GLUT is maintained from 16 weeks of gestation, increased twofold in the second trimester and remains unaltered thereafter to term. GLUT densities in term and preterm FGR placentas were unaltered. These data suggest that despite the fact that GLUT is the main glucose transporter protein isoform in the human syncytiotrophoblast, fetal hypoglycemia in FGR is not due to a decrease in placental glucose transporter density.
FGR increases the risk of developing glucose intolerance and cardiovascular disease in adulthood. Fetal exposure to excess glucocorticoids may contribute to FGR. Two glucose transporters, GLUT1 and GLUT3, are expressed in the placenta. In rodent placenta, for example, GLUT1 is replaced by GLUT3 during late gestation (83). An increased placental GLUT1 protein expression may reflect an attempt to increase placental or fetal glucose supply in order to attenuate the reductive effect of excessive exposure to glucocorticoids on fetal growth, whereas suppression of peroxisome proliferator-activated receptor-gamma protein expression during cardiac development may contribute to the increased risk of developing heart disease found in people who had below average birth weight.
10.3) Maternal environment and FGR
FGR presents complex management problems for clinicians. It is estimated that 13.7 million infants are born annually with FGR, comprising 11% of all births in developing countries (84). Failure of a fetus to achieve its growth potential imparts a significantly increased risk of perinatal morbidity and mortality (85). Consequently, the obstetrician must recognize and accurately diagnose inadequate fetal growth and attempt to determine its cause. Growth aberrations, which are the result of intrinsic fetal factors such as aneuploidy, multifactorial congenital malformations and fetal infection, carry a guarded prognosis. However, when FGR is caused by placental abnormalities or maternal disease (see below), the growth aberration is usually the consequence of inadequate substrates for fetal metabolism and, to a greater or lesser extent, decreased oxygen availability. Careful monitoring of fetal growth and well-being, combined with appropriate timing and mode of delivery, can best ensure a favorable outcome.
FGR is multifactorial process. Studies in humans and animals have shown that the maternal environment is the most important determinant of the newborn weight, accounting for more of the similarity in sibling birth weights than genetic affinity (86). In addition to a direct relationship with the degree of maternal plasma volume expansion, many clinical factors are associated with FGR. These factors include multiple gestation, fetal genetic, and chromosomal anomalies (Down's syndrome and Turner's syndrome), infections such as TORCH syndrome (acronym for toxoplasmosis, rubella, cytomegalic disease, and herpes), and various maternal disorders including anemia, severe chronic asthma, chronic renal disease, heart disease and hypertension. Maternal stress factors, including narcotic addiction, cigarette smoking and chronic alcoholism, are also associated with FGR, as are placental anomalies including hemangiomas, placental infarcts, a single umbilical artery, and small placental size. Poor nutritional status of the mother at conception and inadequate energy and protein intake during pregnancy may also result in FGR. Because children born with FGR are not a homogeneous group, they have a broad spectrum of growth, health, and developmental outcomes. In general, they have higher rates of subnormal growth, morbidity, and neurodevelopmental problems. The biomedical mechanisms reflected in nutritional, infection-related, hormonal, and metabolic parameters are not likely to be independent causative factors of FGR, but important mediating factors of a pathological process set in motion by other agents and insults.
10.3.1) Effects of mother's diet
Nutrition is the major intrauterine environmental factor that alters expression of the fetal genome and it may have lifelong consequences. This phenomenon, termed "fetal programming", has led to the recent theory of "fetal origins of adult disease" (87). Namely, alterations in fetal nutrition and endocrine status may result in developmental adaptations that permanently change the structure, physiology, and metabolism of the offspring, thereby predisposing individuals to metabolic, endocrine, and cardiovascular diseases in adult life. Animal studies have shown that both maternal under- and overnutrition reduce the placental-fetal blood flow and stunt fetal growth. Impaired placental synthesis of nitric oxide (a major vasodilator and angiogenesis factor) and polyamines (key regulators of DNA and protein synthesis) may provide a unified explanation for FGR in response to these nutritional extremes that have the same pregnancy outcome. There is growing evidence that maternal nutritional status can alter the epigenetic state (stable alterations of gene expression through DNA methylation and histone modifications) of the fetal genome. This may provide a molecular mechanism for the impact of maternal nutrition on both fetal programming and genomic imprinting. Promoting optimal nutrition does not only ensure optimal fetal development, it also reduces the risk of chronic diseases in adults.
FGR is a result of a complex pathology caused by multiple factors of fetal, placental, and maternal origin. Hormones and growth factors released as a result of maternal-fetal physiological interactions play an important role in fetal well-being and fetal outcome. Both maternal malnutrition and anemia are associated with various degrees of FGR. A relationship between decreasing birth weight percentiles and increasing fetal morbidity and mortality has been demonstrated. Maternal anemia and/or malnutrition are recognized to be the most frequent causes of FGR and small-for- gestational-age births in developing countries, such as
Relationships have been found between blood levels of vitamins and FGR. In a poor population of South Asia (
In Chinese women, elevated homocysteine and suboptimal vitamin B12 and B6 status may increase the risk of preterm birth (89). An elevated homocysteine level was associated with a nearly fourfold higher risk of preterm birth. The risk of preterm birth was 60% lower among women with a vitamin B12 level of 258 pmol/L or higher than among vitamin B12-deficient women, and was 50% lower among women with 30 nmol/L or higher vitamin B6 than among vitamin B6-deficient women.
Marked differences in vitamin A, folate, and iron concentrations in the cord blood between growth-retarded babies and their normal counterparties were established in
Mean folate intake was shown to be correlated with circulating concentrations of folate in the serum of 28-week-pregnant women (91). The women with a low mean daily folate intake (less than 240 μg) had an approximately twofold greater risk of preterm delivery and delivery of low birth weight (LBW) infants after maternal characteristics, energy intake, and other correlated nutrients were controlled for.
Vitamin A was found to be significantly correlated with birth weight, head circumference, length, and gestational period (92). There was also a significant positive relationship between zinc and birth weight. In contrast, copper showed a negative correlation with birth weight and head circumference. Vitamin E and magnesium were not associated with any of the anthropometric measurements, although magnesium showed an increasing trend with birth weight. These data suggested that most of the mothers of the subjects studied may have been marginal with respect to vitamins A and E and zinc. In those with LBW babies. a higher intake would have improved their nutritional status and possibly the outcome of their pregnancy.
An antioxidant nutrient balance is important for pregnant women who are exposed to various oxidants through food, drinking water, or inhaled air. Maternal oxidative stress during pregnancy plays an important role in fetal growth. Maternal serum vitamin C level during the second trimester (24-28 gestational weeks) is positively correlated with birth weight and length in full-term babies (93). An increase of 1 μg/ml in the serum vitamin C level increased the birth weight by 27.2 g and the birth length by 0.17 cm. The birth weight and length were highest when the levels of both vitamins C and E were high.
Serum protein levels are not predictive of birth weight or growth retardation at birth, but are significantly correlated with a number of other measures of nutritional status. Serum albumin levels at 18 weeks of pregnancy were inversely correlated with birth weight (94). This negative correlation was explained by an inverse relationship between albumin concentration and maternal body-mass index. There was no significant correlation between albumin levels at 30 weeks and birth weight, or between birth weight and the concentrations of the other two proteins studied at either gestational age. In individual subjects, the concentration of each protein correlated significantly with the concentration of the other proteins, and the levels at 18 weeks correlated with those at 30 weeks.
Maternal blood levels of trace elements were shown to have a great influence on pregnancy and on fetal growth. In studying the associations between blood zinc concentrations and various measures of pregnancy outcome and neonatal conditions at birth, the plasma zinc concentration was shown to decline as gestation progressed from week 6 to week 34 of gestation (95). After plasma zinc concentration was adjusted for gestational age, it was not significantly associated with any measure of pregnancy outcome or neonatal condition.
During pregnancy, trace elements are indispensable for sustaining life not only the mother's but also the fetus's. Fetal growth has been shown to be associated with altered levels of trace elements in the maternal and fetal blood, and the placental tissue (96). Compared with appropriate-for-gestational-age cases, FGR cases show higher magnesium, copper, and selenium concentrations in the umbilical cord arterial serum, and higher magnesium and selenium concentrations in the placenta tissue, but no significant differences appeared for the elements measured in the maternal and umbilical cord venous serum. The ratio of umbilical cord venous vs. maternal serum concentration was elevated for copper, and the ratios of umbilical cord arterial vs. umbilical cord venous concentration were elevated for copper and zinc, but there were no differences in the placenta tissue vs. maternal serum concentration ratios in FGR. Reduced consumption efficiency of these four essential trace elements may be closely associated with retarded fetal development.
Low levels of zinc in polymorphonuclear white cells were found in women giving birth to small-for-gestational-age babies (97). The maternal plasma zinc and albumin levels 24 to 48 h after delivery were lower than in non-pregnant control women. A combination of smoking and/or low zinc levels was found in 85% of mothers having small-for-gestational-age babies. Despite the commonly accepted facts that zinc and folate are important for the fetal growth, maternal zinc assessed by serum and dietary intake was not associated with birth weight or length of gestation (98). The indirect measures of maternal nutritional status, including maternal pre-pregnancy weight and weight gain during pregnancy, were stronger predictors of adjusted infant birth weight than energy intake or intake of zinc and folate.
10.3.2) Effects of mother's diseases
The most frequent cause of LBW is intrauterine infection associated with intrauterine growth restriction and premature labor. The microorganisms most frequently associated with LBW are Ureaplasma urealyticum, Mycoplasma hominis, Escherichia coli, group B Streptococci and Chlamydia spp.(99). Intrauterine infection by these and other microorganisms are associated with a vigorous inflammatory response at the maternal–fetal interface that is characterized by the increased concentrations of T helper-1 cytokines (100-102), nitric oxide production (103,104), prostaglandin production (105,106), and increased numbers of apoptotic cells (107) and neutrophils in the placenta and amniotic fluid. These alterations ultimately result in decreased fetal and placental development as well as premature activation of the fetal hypothalamic–pituitary–adrenal axis to initiate labor (108).
There are different diseases which in pregnant women can have a detrimental effect on the fetuses. Disorders occurring in early life may underlie abnormal functional development in later life, whereas growth is mainly determined during the second half of pregnancy. For example, in the first trimester of type-1 diabetic pregnancy, the embryo and fetus are often smaller than normal (109), but there is no relationship between the degree of early growth delay and birth weight. Mean growth delay per fetus in early diabetic pregnancy was negatively correlated with the occurrence of no-coincidence between behavioral state parameters at 36 weeks of pregnancy.
Maternal cardiovascular adaptation has to provide the uterine perfusion necessary to meet the requirements of the growing fetus by providing transport of nutrients and oxygen to the placenta and fetus (110). Thus, uterine blood flow is inextricably linked to fetal growth and survival. Reductions in uterine blood flow can occur under acute or chronic conditions or their combination. Chronic reductions in uterine blood flow can be observed in pregnancy-induced hypertension, diabetes mellitus in pregnancy and FGR. Chronic restrictions in uterine blood flow will elicit a placental and fetal response in the form of growth adaptation to the reduced supply of oxygen and nutrients to the conceptus. If compensatory growth restriction reaches its limits, intrauterine fetal distress can ensue.
In mothers with preeclampcia, fetuses often develop the growth retardation. By means of perinatal inquiry, not only was the frequency of preeclamptic toxemia proven, but also the frequency of toxemia in mothers with newborns of different weights (111). In the overweight newborns, toxemia is less often caused by proteinuria than in the underweight babies. These findings can be explained by the hypothesis that preeclamptic toxemia is a compensatory mechanism in FGR. For the fetus, which is insufficiently supplied by the placenta, this regulatory mechanism enhances the placental blood supply. The higher incidence of toxemia in pregnant women with overweight babies is explained by the increased demand on the placenta; developing toxemia entails better placental blood supply to the fetus; in this case, the toxemia is compensated for. In the decompensation stage with FGR, all reserves are mobilized via increased permeability of the vessels, which leads to improved passage through the placenta, but also to proteinuria and increased incidence of edema.
In
Gestational hypertension and/or preeclampsia, the most important gestational diseases that also cause FGR, are characterized biochemically by high blood levels of such glycoproteins as activin A (bA) and inhibin A (ąbA) (113). The serum levels of activin A and inhibin A are higher in preeclampsia, and the presence of FGR does not significantly modify these concentrations. Similarly, inhibin-subunit mRNA levels in placentas from preeclampsia are higher than in controls, and FGR does not significantly affect this expression. It has been suggested that the increased placental expression of inhibin-subunit mRNA is part of the mechanism leading to the increased serum levels of activin A and inhibin A. In a comparative study of women in whom gestation was complicated by preeclampsia and women with isolated FGR, neurokinin B was found to potentially be involved in pregnancy hemodynamic adaptation via the nitric oxide production (114). In pregnancies complicated with preeclampsia and FGR, the increased neurokinin B plasma level correlated well with the increased nitric oxide metabolite level, which may represent a compensatory mechanism for improving blood flow to the uteroplacental unit.
Early fetal growth delay (7-14 weeks of gestation) has been reported in insulin-dependent diabetic pregnancies and in several animal models (115). Macrosomia is a classic feature of infants from such mothers. Early growth delay was greater in the fetuses that subsequently developed macrosomia. Similar results were found for the abdominal circumference measurements. Fetal growth delay occurs in the first half of the insulin-dependent diabetic pregnancy, followed by a phase of increased growth. Although the mechanism governing the early growth delay is unclear, authors speculated that it may be due to a "toxic" effect of glucose or some other metabolite, and that the subsequent increased growth relates to fetal hyperinsulinism which develops from weeks 15 to 20 of gestation.
Recognition of the specific dynamics and characteristics of different morphological patterns can be useful for early detection of the at-risk fetus (116). There is a significantly larger abdominal circumference in the early vs. the late patterns in the fetuses obtained from diabetic women during their third trimester of pregnancy and in gestational diabetes small-for-gestational-age infants. Maternal weight, glycemia after therapy, rates of fetal macrosomia, and a large-for-gestational-age outcome were not significantly different between gestational diabetes mellitus and impaired glucose tolerance (117). In the late second and early third trimester, maternal pregnancy obesity and a previous large-for-gestational-age pregnancy appear to have the strongest influence on the fetus's growth, while later, in the third trimester, during the period of maximum fetal growth, maternal glycemia predominates.
Microalbuminuria and nephropathy in diabetic subjects have been linked to low birth weight or short stature in adulthood. Low birth weight and low pondered index at birth were shown in men and women with microalbuminuria, compared to those who were normal albumin uric (118). The albumin excretion rate of the subjects exposed in utero to maternal starvation was not significantly different from that of subjects exposed in infancy or those who were not exposed. Consistent relationships between short stature and microalbuminuria and nephropathy in non-diabetic and diabetic subjects might suggest that more subtle anthropometric indices relate to the low nephron number at birth, or that postnatal or genetic influences underlie the observed link.
10.3.3) Effects of mother's smoking
Among the etiological factors responsible for FGR, one-third of the variations in birth weight are determined by genetic variables and two-thirds by environmental factors (119). Among the preventable, environmental causes of FGR, mother's smoking during pregnancy is by far the most important one, responsible for more than one- third of all FGR newborns (120,121). The effect of maternal smoking on LBW appears to be attributable to FGR rather than preterm delivery. Smoking is associated with at least a doubling of the risk of FGR, regardless of whether LBW or preterm birth are also present, suggesting that FGR is the key factor mediating the effect of smoking on the birth weight. Social differences in maternal smoking may be responsible for the similarity of the rates between cities with distinct levels of socioeconomic development (122): a wealthier city showed higher rates of maternal smoking, attendance in the private sector, and obstetric intervention than a less developed one.
An association between the intensity and duration of cigarette smoking during pregnancy and the frequencies of LBW, preterm births and FGR was shown in a cohort performed of 5166 live births in
An association was described between reductions in adjusted birth weight among women who smoked throughout pregnancy and the amount of cigarettes they smoked per day (124). For low/moderate smokers (15 cigarettes per day or less), infant birth weights were reduced by up to 168 g relative to infants of non-smoking women. In heavy smokers (more than 15 cigarettes per day), the reduction in body weight reached 288 g. A decrease in birth weight (179 g) was found among smokers who reported quitting early in pregnancy. Heavy maternal smoking can affect any point in the pregnancy, including solely in the early months, resulting in LBW.
Different statistical methods were used to estimate the effect of smoking on infant birth weight independent of gestational age and maternal weight gain during pregnancy (125). After adjustment for non modifiable factors, smoking accounted for 1.5 to 3.1% of the variance in gestational age at delivery. It accounted for 5.3 to 7.7% of the variance in net maternal weight gain after adjustment for non modifiable factors and gestational age. After adjustment for gestational age, net maternal weight gain and the non modifiable factors, smoking accounted for 2.7 to 5.2% of the variance in infant birth weight. Most of the gain in the infant birth weight upon quitting smoking was due to the independent effect of smoking on FGR, with a much smaller increase related to maternal weight gain and a slightly longer gestational age.
A synergistic effect has been suggested when smoking is combined with preeclampsia, causing a lower birth weight than would be expected from their additive effects (120). In smoking women suffering from preeclampsia, the fetuses often develop growth retardation. Nicotine, a vasoconstrictive substance, increases blood pressure, thereby causing a higher incidence of preeclamptic toxemia in smoking pregnant women (111). By means of perinatal inquiry, not only was the frequency of preeclamptic toxemia proven in smoking women, so was the frequency of toxemia in mothers with different weights of newborns. However, results from other researchers do not support a synergistic effect of smoking and preeclampsia on fetal growth (126). Birth weight reductions related to maternal smoking appear to be added to those caused by preeclampsia, suggesting that there is no synergism between smoking and preeclampsia effects on growth restriction (112).
Possible interference with the placentation or implantation has been suggested by the observed increased frequency of spontaneous abortions of a chromosomally normal conceptus in smoking women (127). On average, infants born to women who smoke during pregnancy are 200 g lighter than those born to comparable women who do not smoke. The finding of antepartum bleeding of unknown cause is found consistently been found more often in smokers, relative to nonsmokers. Sudden infant death syndrome is closely associated with both the frequency and level of maternal smoking during pregnancy.
10.3.4) Effects of mother's alcohol consumption
In humans, fetal alcohol syndrom (FAS) has been estimated to occur in between 1 in 600 and 1 in 1000 live births in the
One of the most common and serious defects associated with ethanol teratogenicity is mental retardation (127). Recent evidence supports the concept of a prenatal origin to the problem. At birth, in infants with FAS, there is a deficiency in both their length and weight, usually at or below the 3rd percentile for both parameters. Growth and mental deficiency are seen in many conditions, but the rather striking facial appearance of children with FAS confirms the diagnosis. The characteristic face in small children includes short palpebral fissures, a short upturned nose, a hypoplastic philtrum and maxilla, and a thinned upper vermilion.
Increasing prenatal alcohol exposure was shown to be associated with LBW and gestational age, higher lead levels, higher maternal age, and lower education level, prenatal exposure to cocaine and smoking, custody changes, lower socioeconomic status, and paternal drinking and drug use at the time of pregnancy (128). Children with any prenatal alcohol exposure were more likely to have higher child behavior checklist scores on externalizing (aggressive and delinquent) and internalizing (anxious/depressed and withdrawn) syndrome scales and total problem score.
Heavy drinking during pregnancy is an established risk factor for FAS and other adverse perinatal outcomes. In a prospective investigation based on 2,714 singleton live births at Yale-New Haven Hospital in the USA, performed from 1988 to 1992, mild drinking (defined as more than 0.10-0.25 ounce of absolute alcohol per day) during the first month of pregnancy, was found to be associated with an effect on FGR (129). Overall, drinking during the first month of pregnancy suggested a curvilinear effect on growth retardation, with consumption of more than I oz. of absolute alcohol per day showing increased risk. Drinking during the seventh month of pregnancy was associated with a uniform increase in the odds of a preterm delivery. Differences in the risk estimates for FGR and preterm delivery were thought to indicate etiological differences between these outcomes and critical periods of exposure. LBW is not a useful neonatal parameter for this exposure because it is a heterogeneous mix of preterm delivery and FGR. Despite the observed protective effects of mild drinking on FGR, the increased risk of preterm delivery with alcohol use supports a policy of abstinence during pregnancy.
Among women having live births, alcohol consumption during pregnancy was significantly related to having a LBW baby (less than 2500 g), similar to the other factors, such as race, age, mother's education, prenatal care, prematurity, gestational age, and smoking (130). Women who drank more during pregnancy also smoked more, and were younger and less educated than women who drank in lower amounts or not at all. The effect of alcohol is significant for the occurrence of LBW, fetal death and infant death, analyzed by multivariable logistic regressions. Results indicated that alcohol has an important relationship with birth outcome, but that the alcohol effect on mean birth weight is small relative to that of other risk factors, accounting for the non-significant result in the multiple linear regression. First- trimester alcohol consumption (average: four drinks per week) was associated with a 155 g reduction in fetal growth (124). The association, observed with all types of alcohol consumption, was stronger among smokers (-270 g) but was also present in nonsmokers (-115 g). It seems that even moderate alcohol drinking may serve as a cause of LBW.
Although heavy maternal alcohol consumption during pregnancy has been associated with FGR, paternal alcohol consumption was not associated with any of the fetal growth measures after adjustment for other variables (131). The adjusted odds ratio for moderate consumption of three or more drinks per week was 2.6 for LBW, and 2.3 for FGR. Examining the combined effect of smoking and alcohol consumption revealed a synergistic effect was found for LBW but not for FGR. Moderate alcohol consumers had an average birth weight decrement of 143 g, which varied with smoking. There was little association of alcohol consumption with preterm delivery (less than 37 weeks).
Studies with animal models of alcohol-related birth defects suggest that reductions in circulating thyroid hormones, including thyroxine, may be a persistent postnatal effect of fetal alcohol exposure. The few clinical reports of children with FAS describe thyroid hormone levels that are within normal limits. It was shown that alcohol intake and smoking each had a substantial negative impact on birth weight, gestational age at birth, and fetal growth, assessed as birth weight corrected for gestational age (132). Infant thyroxine levels were positively related to the birth weight and gestational age and were more strongly related to the fetal growth. Infant thyroxine levels were not significantly influenced by either maternal smoking or alcohol consumption. Smoking- and alcohol-related reductions in birth weight, gestational age, or fetal growth were not significantly associated with variations in infant thyroxine levels. Interesting questions remain regarding species differences and the influences of maternal alcohol consumption on thyroxine metabolism as a mechanism for alcohol-related birth defects. However, the current data do not support the hypothesis that maternal alcohol consumption, or smoking, during pregnancy leads to compromised thyroid system function in human newborns.
10.3.5) Effects of drinking water
There are many uncertainties regarding the risk of adverse pregnancy outcomes associated with exposure to by-products of drinking water disinfection. In
A great risk for FGR was found in communities whose drinking water contains elevated levels of herbicides. Communities in southern
10.3.6) Effects of socioeconomic factors
Embryonic and fetal growth depends on genetic and environmental factors, and the process is a result of the interaction between these factors. About 7% to 9% of live-born infants have below-normal birth weight (below the 10 percentiles). The rate and extent of FGR varies by ethnicity and socioeconomic status (135). Maternal factors such as inadequate or severe malnutrition, chronic maternal diseases, birth order, multiple births, and parental genetic factors are suspected causes of FGR. Chronic maternal malnutrition and other poverty-related factors are likely to be important determinants of FGR in developing countries (136). A study of relationships between birth weight, sociodemographic variables and maternal anthropometry showed that in an inner urban area of Dhaka, Bangladesh, about 21% of the babies were of LBW, less than 2,500 g of the mean birth weight (137). LBW was more common in younger (younger than 20 years) and older (older than 30 years) mothers, in the low-income group and in women with little or no education. The mean birth weights among a higher-educated, higher-income group and of male children were on average 290, 260 and 120 g, respectively, higher than in uneducated, lower-income groups and female children. The sensitivity (69%) and specificity (68%) were best for maternal weight. A regression analysis found that mother's weight at term was the best predictor of LBW, while maternal weight combined with age, educational level, and income group correctly predicted slightly more than 35% of LBW.
LBW for gestational age is associated with increased risk of developing cardiovascular and diabetic diseases later in life (138 ). These associations are the main basis for the 'Fetal origins hypothesis' (the 'Barker hypothesis') that malnutrition-related FGR has long-lasting physiological and structural effects that predispose the individual to diseases later in life (139). Fetal malnutrition has two main causes, in particular, poor maternal nutrition and placental insufficiency.
Some authors are of the opinion that maternal malnutrition was not prevalent in the majority of the populations in which the fetal origins hypothesis was tested (140). These authors suggested that there is no reason to believe that poor maternal nutrition was present in the populations used to study associations between birth weight and hypertension in children (141). Socioeconomic conditions are claimed not to confound the association between fetal weight and life-long health risks. This implies that significant maternal undernutrition was present in the upper social classes, for example in
Higher rates of FGR are not necessarily be observed in poorer areas relative to wealthier ones (122). In
Maternal hypertension is a risk factor of placental insufficiency measured as prevalence of small-for-gestational-age infants (146). Coagulation disorders and dyslipidemia are more often found in preeclamptic women, among whom placental insufficiency and FGR are more prevalent (147,148). Thus, established risk factors for cardiovascular diseases and diabetes are present with a higher prevalence in pregnant women with placental insufficiency. Associations between maternal blood pressure, LBW and hypertension in the offspring have been described (149,150). The association between LBW and the risk of adult cardiovascular diseases may therefore indicate that placentation in women with cardiovascular risk factors is impaired, resulting in placental insufficiency and small infants (140). Pathogenically, it is conceivable that 'hypertensive' spiral arteries are more resistant to transformation, that dyslipidemia may induce acute atherosis and that coagulation disorders result in local thrombosis and placental infarctions.
It was suggested that in adequately nourished populations, maternal cardiovascular risk factors may increase the risk of adult diseases in the offspring via two mechanisms (140): inheritance (151), and impairment of the normal placentation process resulting in placental insufficiency (147,149). The latter may result in FGR which itself can cause adult disease. Alternatively, the association between LBW and adult disease may also be an epiphenomenon, leaving inheritance as the main explanation for the fetal origin hypothesis, in well-nourished populations.
A significant association between low serum folate levels and FGR was found in pregnant women at 30 weeks gestational age, each of whom had been provided with folate supplementation at enrollment in prenatal care (152). Because high folate levels are most likely explained by recent folic acid intake, it has been speculated that the decreased fetal growth associated with a low folate level may be related to a combination of psychological and behavioral characteristics for which the low serum folate level is only a surrogate measure. Poor psychological scores, including measures of depression, anxiety, self-esteem, mastery, stress, and social support, were significantly related to the lower serum folate levels. In women with both good and poor psychosocial scores, the high folate level was significantly associated with increased birth weight, a relationship that persisted even after adjusting for maternal race, body mass index, smoking, history of a LBW infant, and infant gender. It has been suggested that women with good psychosocial scores are more likely to take folate, but that the use of folate itself is related to a lower risk of FGR and increased birth weight.
10.3.7) Genetic predisposition to FGR
FGR is an important cause of small stature in children presenting to pediatric endocrinologists (153). FGR has to be differentiated from familial ('constitutional') short stature, where the growth deficit is genetically determined and/or induced by smallness of the mother (maternal constraint). Intrinsic fetal anomalies such as chromosomal abnormalities, primary growth failure syndromes, congenital infections and congenital anomalies are of equal importance with maternal disorders, in particular chronic use of alcohol, tobacco and narcotics, and pregnancy complications such as hypertension and preeclampsia, in causing FGR. The relative importance of placental abnormalities and environmental factors (with the exception of malnutrition) appears to be small. Genetic predisposition and abnormal trophoblastic function are thought to contribute to the development of preeclampsia. Multiparous women developed severe preeclampsia and subsequently delivered a live growth-retarded infant with trisomy 13 (154).
LBW in healthy term neonates is associated with fetal inherited prothrombotic risk factors (155). The proportion of children in the LBW quartile increased from 23.7% to 30.5% to 48.0% for children with no defect, only a single heterozygous defect and multiple or homozygous defects, respectively. The respective adjusted odds ratios (95% confidence intervals) of thrombophilia for birth weight in the lowest quartile (lowest deciles) were 1.53 (0.76-3.08) in carriers of one prothrombotic risk factor and 4.01 (1.48-10.84) in subjects carrying multiple or homozygous defects.
Genetic factors may interact with altered intrauterine growth causing the risk of cardiovascular and renal diseases (156). Subjects homozygous for the D allele of the ACE gene are predisposed to both cardiovascular complications and nephrosclerosis. Altered intrauterine growth is associated with a reduced number of nephrons at birth, has a negative influence on the development of the cardiovascular system and favors the occurrence of hypertension, insulin resistance, hypercholesterolemia and hyperuricemia in adult life.
Frequencies of ACE, Apo-E gene polymorphisms, apolipoprotein-B (Apo-B) mutation and lipid compositions were determined in full-term newborn infants with FGR (157). An insertion/deletion polymorphism with a significantly increased frequency was observed in the FGR group (65%) as compared to the control group (33%). When the distribution of the Apo-E gene polymorphism (E2, E3 and E4) was studied, no difference was found between the FGR and control groups with respect to frequency. No Apo-B gene mutation was identified in the study groups. An insertion/deletion polymorphism is responsible, at least in part, for the etiology of intrauterine growth restriction (IUGR). Levels of total cholesterol and Apo-B are elevated in FGR infants, suggesting a linkage between LBW and atherosclerosis.
A number of genetic and environmental factors are taken into account in relation to fetal pathology and as responsible for FGR. It is likely that a gene polymorphism or mutation that is susceptible to reproductive failure has a beneficial effect on the process of human reproduction with or without the environmental interaction. Pregnancies with a mosaic trisomy for chromosomes 7, 2 and 14 resulted in pre- and postnatal growth restriction (158). These chromosomes are known or suspected to harbor imprinted genes, so that an unbalanced gene dosage in a subset of cells during embryonic development could lead to early impairment of placental function. Inherited thrombophilia, such as factor V Leiden, prothrombin, and methylenetetrahydrofolate reductase mutations, gene polymorphisms of detoxification enzyme (CYP1A1), growth factors (insulin-like growth factor-I), and hormones such as angiotensinogen and CYP17 are involved in the pathogenesis of FGR (159). The inherited thrombophilia, gene polymorphisms of coagulation and anticoagulation factor such as thrombomodulin, endothelial protein C receptor, plasminogen activator inhibitor 1, and factor XIII, human lymphocyte antigen (HLA-G), detoxification enzymes (glutathione-S-transferase M1), cytokines such as IL-1 and IL-6, hormones (CYP17), vasodilators (nitric oxide synthase 3), and vitamins (transcobalamin) are involved in the pathogenesis of sporadic and recurrent miscarriage. The factor V Leiden mutation has genetic advantages that are believed to confer improved implantation rate in in vitro fertilization and a reduction in maternal intrapartum blood loss. The CYP17 A2 allele has bidirectional effects on human reproduction, including increases in susceptibility to recurrent miscarriage and fetal growth enhancement.
10.3.8) Racial effects on FGR
Marked racial variation in birth-weight percentiles by gestational age has been described (160). Compared with extremely-low-risk white mothers, the risk of a small-for-gestational-age infant was 2.64 times greater for extremely low-risk African American mothers, and the risk of infant mortality was 1.61 times greater for the latter. For the extremely low-risk group, the infant mortality rates of African American and white infants at or below the 10th percentile of birth weight for gestational age of their respective maternal race group were essentially identical after controlling for gestational age. In the other words, race differences in fetal growth patterns remained after controlling for risk status. Efforts to remove racial disparities in infant mortality need the development of etiological pathways that can explain why African Americans have relatively higher rates of preterm births and higher infant mortality rates among term and non-small-for-gestational-age infants.
Maternal serum ą2-macroglobulin was elevated at as early as 18 weeks of gestation in women destined to have a growth-retarded infant, and this elevation persisted through 30 weeks of gestation (161). Furthermore, levels were higher in white vs. black women, in smokers vs. non-smokers, and in thin vs. heavier women. When the effect of ą2-macroglobulin on birth weight was evaluated in a multiple regression analysis adjusting for gestational age, race, body size, smoking, fetal sex, and a history of a LBW infant, high ą2-macroglobulin levels were associated with a statistically significant decrease in birth weight. The effect was greater in smoking women. This relationship did not appear to be associated with differences in serum zinc or hematocrit levels.
Low nephron number has been related to LBW and hypertension. In the southeastern
Fundamental differences in the etiology of IUGR have been found between European and Asian women (163). Fetoplacental function tests discriminated well between poorly grown and normally grown fetuses in European mothers. In contrast, apart from human placental lactogen, the individual values of poorly grown Asian fetuses fell within the normally grown range. Hb values rose in the second trimester in European pregnancies associated with poor growth, whereas the values fell in the Asian pregnancies for both normally grown and poorly grown fetuses.
Fetal growth during pregnancy was shown to be related to ethnic factors, such as African and European origin (164). Bi-parietal diameter was significantly smaller among African fetuses mainly during late pregnancy, and that of the femur was greater during the entire pregnancy, whereas the transverse diameter of the fetal abdomen did not differ between the two groups. Ethnic factor remained significant when taking into account confounding factors. Observed differences appeared to be more likely related to a racial factor than to hypotrophy.
Racial differences in hematocrit levels and the relationship between low and high hematocrit were found to be connected with FGR and preterm delivery (165). In black women, hematocrits of 27% to 30% were associated with lower reductions in the rates of FGR and preterm delivery. At 31 to 34 weeks, 40% or more of the hematocrits were associated with significantly higher odds ratios for FGR in both blacks and whites.
Research based on hospital records demonstrated that many births classified as normal according to conventional demographic measurements are FGR when evaluated clinically. Moreover, in analyses of birth outcomes, one must focus on a third dimension, maturity, in addition to birth weight and gestational age. Although clinical studies allow more precise classification, the small number of cases tends to result in unreliable estimates of rates and in loss of generalizability. The fetal growth ratio, a measure recently shown to be a valid proxy for maturity, was used to develop a classification system based on combinations of weight, gestational age, and maturity, which were applied in a comparative analysis of a large data set. The results evaluated large differences in the distribution of compromised births across racial and ethnic groups, as well as significant race/ethnic differentials in the risk of infant mortality associated with adverse outcomes (166). Considerable racial/ethnic variations were found across birth outcome categories; differences persisted in the adjusted parameter estimates, and the effects of other risk factors on birth outcomes were similar in direction, but varied somewhat in magnitude (167). The odds of compromised birth outcomes were much higher among African Americans than among Mexican Americans or non-Hispanic whites.
Ethnicity and birth place affect prenatal care and birth outcomes but are probably not as significant as racial differences. Poor outcomes without elevated newborn costs may indicate less access to high-quality neonatal care among some ethnic groups. With the influx of Latin American immigrants to the
In studying the perinatal outcomes in Hispanic, black, and white non-Hispanic women in the
Fetal growth patterns among different
Maternal nativity status, along with ethnicity, may serve as an important axis of differentiation in birth-outcome studies. This conclusion was derived from the finding of substantial maternal nativity differences in risks of infant mortality and LBW between US- and foreign-born women, with the magnitude of the nativity effect varying significantly across racial/ethnic groups (171). Overall, foreign-born status was associated with 7% and 20% lower risks of LBW and infant mortality, respectively. However, the reduced risk of adverse pregnancy outcome associated with immigrant status tended to be substantially higher for blacks, Cubans, Mexicans, and Chinese than for other ethnic groups.
Maternal characteristics and birth outcomes of Mexican-born vs. native-born mothers in the
Associations were examined between obesity, diabetes, and three adverse pregnancy outcomes, such as primary cesarean delivery, preterm birth, and LBW, in different racial/ethnic groups, and it was shown that obesity and diabetes are independently associated with adverse pregnancy outcomes (173). Chronic and gestational diabetes were significant risks for a primary cesarean and for preterm birth in all women. Diabetes as a risk for LBW varied by group. For example, whereas chronic diabetes increased the risk for LBW among Asians, Hispanics, and whites (adjusted odds ratios were 2.28, 1.69, and 1.59, respectively), it was not a significant predictor of LBW among blacks.
Although a decline in neonatal mortality has been widely publicized in the United States, research suggests that clinicians may still be underestimating the chances of survival of an infant who is born too early or too small and may overestimating the eventuality of serious disability. Clinicians may have the current and needed ethnic- and race-specific estimates of the "chances" of early survival for newborn infants. With this in mind, birth weight/gestational age-specific neonatal mortality rates for the three largest ethnic/racial groups in the
Race differences in the proportion of LBW attributable to maternal cigarette smoking in a low-income population were described (175). Non-Hispanic whites had a much higher prevalence of smoking and were heavier smokers than African Americans. For both moderately LBW and very LBW, the population attributable risk percentages for smoking were twice as high for non-Hispanic whites as for African Americans. Overall, after adjustment, 30.7% of low birth weight births among non-Hispanic whites and 14.4% of low birth weight births among African-Americans were attributable to smoking.
The relationship between maternal birth weight, infant IUGR, and prematurity was determined (176). Race-specific rates of small-for-gestational-age (weight for gestational age in the lower 10th percentile) and preterm (less than 37 weeks) infants rose as maternal birth weight declined. The adjusted (controlling for maternal age, education, marital status, parity, prenatal care utilization, and cigarette smoking) odds ratio (95% confidence interval) of small-for-gestational-age for maternal LBW (less than 2,500 g) among African Americans and whites was 1.7 and 1.8, respectively. The adjusted odds ratio (95% confidence interval) of prematurity for maternal LBW (less than 2,500 g) among African Americans and whites was 1.6 and 1.3, respectively. The racial disparity in the rates of small-for-gestational-age and prematurity persisted independent of maternal birth weight: adjusted odds ratio equaled 2.2 and 1.5, respectively. Maternal LBW is a risk factor for infant IUGR and prematurity among African Americans independent of maternal risk status during pregnancy; it is a risk factor for infant IUG among whites. Maternal LBW fails to explain the racial disparity in the rates of small-for-gestational-age and premature infants.
10.4) Effects of FGR on postnatal growth and development
FGR is a frequent cause of perinatal morbidity as well as of impaired growth during childhood, and is associated with significant perinatal and childhood morbidity (85). Epidemiological studies suggest that FGR is a significant risk factor for the subsequent development of chronic hypertension, ischemic heart disease, diabetes, and obstructive lung disease in adult life. FGR contributes disproportionately to neonatal mortality and morbidity in both preterm and term babies, and is a predisposing factor to major psychiatric sequelae, such as depression, suicide and suicide attempts (177). FGR is usually due to maternal effects, fetal factors, or placental insufficiency (178). Newborns with FGR are at increased risk of developing a metabolic syndrome later in life, namely obesity, arterial hypertension, hypercholesterolemia, cardiovascular disease, impaired glucose tolerance, or diabetes mellitus type 2. This association is the result of the adaptational changes in the fetal endocrine-metabolic mechanisms to the impaired intrauterine milieu that occur to ensure survival in the short term. The persistence of these changes after birth can be detrimental in adult life. Furthermore, premature adrenarche, as well as ovarian hyperandrogenism, seem to be associated with FGR in girls, demonstrating that FGR may have long-lasting effects on both somatic health and reproductive functions. Finally, intrauterine exposure of the fetus to stressors may affect the individual's ability to face stress in postnatal life. Therefore, if optimization of the individual somatic and psychosocial well-being is the golden goal of medicine, special attention should be paid to maintaining an optimal intrauterine milieu - devoid of any stressors and with adequate nutrient supply, and to reserve ideal psychosocial support for the pregnant woman. Some catch-up growth has been observed in about 70% of children with FGR during the first year of life (153). Many FGR children show major or minor birth defects which may be predisposing factors or may also coexist because of common underlying factors producing both small stature and structural anomalies.
IUGR fetuses are at risk for the development of adult hypertension and related cardiovascular diseases. IUGR may be linked to congenital oligonephropathy and potentially to hypertension in later life (179). IUGR appears to be associated with a decrease in fetal renal volume: renal volume in the IUGR fetuses was 31% (95% CI, 20%-40%), which was less than that in the group of fetuses that were not IUGR after an adjustment was made for gestational age The ratio of renal volume to estimated fetal weight was 15%, which was less than the same ratio in the fetuses that were not IUGR.
IUGR has an effect on the clinical course and prognosis of IgA glomerulonephritis (IgA GN) in children (180). A significantly higher mean percentage of sclerotic glomeruli was found in children with IUGR than in those without IUGR (33 vs. 13%). At the end of the follow-up period, a significantly higher incidence of arterial hypertension was observed in children with IUGR than in those without IUGR (50 vs. 11 %). An increased risk of developing arterial hypertension and glomerulosclerosis was demonstrated in children with IgA GN who had suffered from IUGR with a birth weight below the 10th percentile for gestational age. IUGR may therefore help in identifying early in the course of IgA GN those children who are at higher risk of an unfavorable course.
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