Placental and Fetal Circulatory and Metabolic Changes Accompanying Fetal Growth Restriction

Introduction

During fetal life, factors associated with maternal, placental, and fetal environments interact to ensure optimal fetal metabolism and growth. However, in 3% to 7% of pregnancies these interactions become suboptimal and growth of the fetus does not align with its in utero genetic growth potential, resulting in a reduced growth trajectory and the outcome at birth often termed fetal growth restriction (FGR) . To ensure survival, preservation of fetal development and growth, albeit reduced, lead to significant changes to the fetal circulatory and metabolic systems. The etiology of FGR is multifactorial, and initial insults, responses, and fetal adaptations likely differ across the FGR outcome spectrum, depending on specific causes and adaptations.This chapter summarizes some of the work that has been directed at describing circulatory differences and changes in fetoplacental oxygen and nutrient supply and metabolism in FGR pregnancy, independent of fetal genetic abnormalities or maternal environment (e.g., maternal anemia, respiratory conditions, or altitude), where placental insufficiency arises. This FGR occurs where there is a reduction in the density and size of fetal blood vessels in the placenta, promoting increased vascular resistance in association with reduced oxygen and nutrient exchange. Early foundational work using the sheep model is presented alongside other animal model systems and new and emerging human data. Discussion also focuses on detection of FGR and the clinical implications of abnormal placental development and fetal oxygenation, as well as advances in screening and assessment of abnormal fetoplacental function in FGR.

Defining Fetal Growth Restriction

FGR is an important clinical problem in obstetrics and has stimulated considerable clinical research that not only has elucidated aspects of the pathophysiology of growth restriction but also has led to a better understanding of normal human biology. A plethora of terms have been used in describing FGR. In part, this potentially confusing terminology has a historical foundation. The first attempts to recognize growth restriction in newborn infants came from studies using gestational age and birth weight information. Infants were classified as being small-for-gestational-age (SGA) if their birth weights fell below the 10th percentile. Later reports used terms such as small for dates, basing the definition on standard deviations (SDs) for the birth weight distribution at each gestational age. Many studies identified differences in birth weight–gestational age distribution data among different populations. All of these attempts to define FGR were based on birth weight information that was recorded to establish the norms for populations. However, it was clear from the beginning that preterm birth weights could hardly be considered to represent “normal” in utero growth for a population. It also was clear that growth-restricted infants were not a homogeneous group but instead included some infants who were small but normally grown. The term intrauterine growth retardation (IUGR) was used initially to identify SGA infants who had truly grown more slowly in utero because of one or another disease process that usually represented abnormal placental development and function. Later, because of concerns that parents might associate the term retardation with mental retardation, this term was modified to fetal growth restriction. This latter designation has come to be used more and more widely and interchangeably with IUGR. This chapter will use SGA for fetuses that have growth <10th percentile and FGR to describe the pathologic process of placentally mediated fetal growth restriction whereby genetic growth potential is not obtained. Further to this classification it is important to note that FGR fetuses have often been classified as having an either symmetric or asymmetric body pattern based on the ratio between abdominal circumference (AC) and another reference biometric index (e.g., brain). The rationale underlying this classification is the hypothesis that this pattern may provide information on the etiology of FGR (i.e., constitutional, placental, or intrinsic fetal abnormalities), timing of onset of FGR (early vs. late), duration of FGR, and risk of adverse outcome.

The approach to defining FGR in human pregnancies changed fundamentally once ultrasound techniques permitted in utero determination of fetal body size, a measurement that could be made repeatedly during the pregnancy. This capability removed the necessity to use only birth weight data at the completion of a pregnancy. It also permitted examination of the rate of fetal growth, that is, the change in fetal size with time in normal pregnancy, rather than establishing growth curves based on birth weights of pregnancies that are by definition pathologic because they ended prematurely. This wide application of ultrasound-derived fetal growth curves made possible the early detection of fetuses who were SGA. Currently, the common practice is to use equations derived from population data to calculate an estimated fetal weight, with equations usually relying on ultrasound measurements of head circumference (HC), abdominal circumference (AC), and femur length. Using the 10th percentile as the cutoff to identify SGA, sensitivity has varied among studies between 0% and 10% in low-risk to 72% and 95% in a high-risk populations, while specificity of ultrasound to detect small fetuses ranges from 50% to 95%, with large variability attributed to differences in methodology and the use of either estimated fetal weight (EFW) <10th percentile or only AC <10th percentile as the means to identify SGA. ^, High false-positive rates and a measurement inaccuracy of up to 10% have limited the use of ultrasound in normal pregnancy, and there are no guidelines recommending the incorporation of ultrasound for routine screening for FGR in low-risk populations. ^,

Distinguishing fetuses who are small but are fulfilling their growth potential (i.e., those who are normally grown) from those with FGR (i.e., those who are growth-restricted as a result of some pathology) is important for a number of reasons: (1) intensive obstetric surveillance can be applied more effectively to the poorly growing fetus, (2) neonatal intensive care can better anticipate problems for affected infants who had FGR, and (3) FGR has important long-term implications for development of the infant, not only through childhood but also into adult life.

However, variability in presentation and difficulties in distinguishing normal fetuses from those with FGR who are at risk for adverse perinatal outcomes has limited the ability of clinicians to prevent all FGR-associated adverse perinatal outcomes. As a result of this inability to detect fetuses most at risk, FGR is still the attributable cause of one-third of stillbirths and perinatal mortality, and is one of the most common causes of spontaneous and iatrogenic preterm births and perinatal morbidity related to birth asphyxia. ^,

With only 30% to 50% of SGA fetuses with EFW <10th percentile pathologically growth restricted (FGR) and the remainder of fetuses growing appropriately for maternal ethnicity, parity, and weight, additional measures are needed to distinguish between FGR and the constitutionally SGA fetuses. The addition of measures of asymmetric growth restriction (a small AC to HC ratio) or AC growth velocity may help improve the distinction between FGR and SGA. However, none of these measures has resulted in clinically useful information, and both SGA and FGR remain defined as an EFW and/or AC <10th percentile. Newer modalities such as magnetic resonance imaging (MRI) ^, , fetal volume, determinations ^, and fetal and umbilical blood flow relationships, and the addition of three-dimensional (3D) ultrasound of soft tissue volumes may have the potential to yield better estimates of fetal growth.

Attempts also have been made to sharpen the diagnosis of FGR by the use of standards that incorporate maternal and paternal size. Essentially, this approach seeks to distinguish fetuses that are small because the parents are small from those with growth restriction. Focusing on this approach, Gardosi and colleagues have reviewed this subject extensively, and nomograms for particular countries are available. ^, However, there is only limited improvement when maternal factors such as parity, obesity, and ethnic background are accounted for, and all EFW formulas perform less well in fetuses at the extremes of fetal growth and in late gestation. ^,

Placental Development AND Oxygen Supply

The human placenta is classified as a discoid haemomonochorial placenta that is dually perfused from the maternal circulation and a separate fetal circulatory system. During pregnancy, the human placenta undergoes two major developmental stages, primarily to expand its surface area and metabolism to support optimal fetal development and growth. The first stage of placental development extends to approximately the 23rd week of gestation. During this stage, maternal blood in the intervillous space (IVS) entering through the uterine arteries and fetal blood are separated by a thick placental endo-epithelial membrane. This layer consists of a monolayer of terminally differentiated trophoblast cells, the syncytialized trophoblast, the underlying layer of progenitor cells, the cytotrophoblast (CTB), and the endothelium of the fetal vasculature. An oxygen gradient exists across this membrane that promotes oxygen diffusion between the uterine and umbilical circulations. This membrane collectively forms what are termed villous trees, and several villous trees might occupy a single placental lobule. The villous trees are extensively branched, commencing with stem villi, and dividing over gestation to form intermediate villi, with the mature type branching off to form the terminal villi in the later part of pregnancy.

Given the placenta’s essential roles of hormone production, immunologic protection, waste removal, nutrient production, and nutrient transportation for the developing fetus, there is high oxygen utilization for oxidative metabolism. In fact, the placenta has an extraordinarily high metabolic rate, consuming approximately 40% of the oxygen used by the entire conceptus, and studies in the near-term placenta show that approximately 70% to 75% of the O ₂ consumed is used to generate ATP by mitochondrial oxidative phosphorylation to support placental metabolism and subsequent functions. Indeed, it is interesting to note that oxygen consumption rates calculated per kilogram of placenta are of the same order of magnitude in different species with an epitheliochorial placenta and are similar to those of the hemochorial human placenta.

This oxygen consumption by the trophoblast and the stroma of the placental villi is a hindrance to fetal oxygenation, preventing transplacental P o ₂ equilibration and enlarging the transplacental P o ₂ difference. This hindrance is compensated for by rapid, early growth of uterine blood flow supplying the maternal facing placental membrane. Studies in sheep pregnancies highlight that at mid-gestation, uterine blood flow has already increased to approximately 40% of its near-term value or upwards of 470 mL/min. This high flow is used to oxygenate a fetus whose body weight is only 7% of its near-term value (i.e., 200 vs. 3000 g). The function of a high uterine flow in early pregnancy is to maintain a high P o ₂ in the IVS and to counteract the adverse effect of placental structure and oxidative metabolism on the transplacental diffusion of O ₂ . Attempts to measure IVS P o ₂ in early pregnancy indicate a mean value of ∼60 mm Hg at 20 weeks gestation. At a P o ₂ of 60 mm Hg, human adult blood has an O ₂ saturation of approximately 89%. Because maternal arterial O ₂ saturation at sea level is ∼97%, this observation implies an extremely low (8%) level of uteroplacental O ₂ extraction.

The second stage of placental development is from 23 weeks to term, during which there is an exponential growth of fetal O ₂ demand that requires the placenta to become more efficient in extracting O ₂ from maternal blood. This requirement is met by the exponential growth of the terminal villi and their volume. ^{, ,} These villi grow as short side branches of the villous tree (more exactly, as side branches of the mature intermediate villi) through increased angiogenesis, and each contains a dense capillary network. Some capillaries of this network bulge against a very thin segment of the trophoblastic membrane to form local sites of blood flow limited O ₂ transport. These structural adaptations reduce the placental membrane to a diffusion distance of 2 to 3 μm. Accompanying these changes in villous development, there is a synchronous exponential increase of umbilical blood flow, most of which goes to perfuse the expanding capillary bed of the terminal villi.

It is physiologically significant that the intermediate villi continue to be perfused by umbilical blood throughout pregnancy. Even in the term placenta, intermediate villous capillary volume has been estimated to be 18% of the total volume of capillaries in the placental villous tree. This is a manifestation of the fact that respiratory gas exchange is only one of several placental functions, some of which require a substantial fraction of umbilical flow to perfuse the thick, O ₂ consuming membrane that maintains a transplacental P o ₂ difference. As a consequence, in its last developmental stage, the human placenta fails to attain the maximum level of performance of a venous equilibrator, which would allow umbilical venous P o ₂ to become equal to intervillous P o ₂ . In the last trimester of a normal pregnancy, the P o ₂ of maternal blood in the IVS and the uterine veins is approximately 10 mm Hg higher than umbilical venous P o ₂ (approximately, 45 vs. 35 mm Hg). ^, Additional factors that underscore this imperfect equilibration have come from foundational work using the pregnant sheep. These studies highlight that the umbilical uptake of oxygen is approximately 55% of the total uterine oxygen uptake. The large difference between uterine and umbilical uptakes is due primarily to the large utilization rate of oxygen by the placenta.

The relationships between abnormal placental development and FGR are complex and well-reviewed elsewhere. FGR has many causes, but the majority of cases that are not associated with fetal congenital malformations, fetal genetic anomalies, or infectious etiology are understood to arise from compromise of the uterine circulation to the placenta. The resultant malperfusion induces cell stress within the placental tissues, leading to changes in transcript expression, selective suppression of protein synthesis and reduced cell proliferation and, in more severe cases, infarction, increased fibrin deposition and calcification are observed. ^, Correspondingly, within this smaller placenta, there is a reduction in volume, surface area, and vascularization of the intermediate and terminal villi that mediate maternal-fetal exchange, ^, and their normal branching to form a dense capillary network is restricted. This maldevelopment of the terminal villi prevents the decrease in resistance to umbilical flow that characterizes normal placental development. Simultaneously, it creates a condition in which the venous return from villous capillaries that carry deoxygenated blood from thick, O ₂ consuming portions of the placental membrane becomes an abnormally large fraction of the total umbilical venous effluent. This condition enlarges the P o ₂ difference between the uterine and umbilical circulations. It decreases umbilical venous P o ₂ , as demonstrated in Table 21.1 , and increases uterine venous P o ₂ . ^, This increase in the uterine-umbilical oxygen gradient in human FGR pregnancy together with increased placental mitochondrial DNA content and changes in the mitochondrial function of CTB and mesenchymal stromal cells in FGR pregnancy confirm that placental oxygen consumption plays a limiting role in the delivery of oxygen to the fetus, and possibly more so in FGR. However, it should be noted that not all FGR and placental mitochondrial reports display an increased mitochondrial DNA content, ^, highlighting the need to understand the etiology of the FGR.

In the FGR condition, the increased uterine venous P o ₂ represents the failure of the placenta to become more efficient in extracting O ₂ from the uterine circulation. Uterine blood flow to the smaller fetoplacental mass is low in absolute terms but is relatively high with respect to O ₂ uptake. It has been suggested that a high P o ₂ in the IVS of the FGR placenta could inhibit the growth of the terminal villi, that is to say that the initial failure of these villi to grow sets up a self-inhibiting mechanism. Correspondingly, mean umbilical blood flow is significantly lower and increases with FGR severity relative to control pregnancies. However, it is important to note that when umbilical blood flow is normalized to fetal weight, there are no differences between control and FGR umbilical flow rates or relative to the severity of the FGR.

Fetal Oxygenation

The chronically catheterized pregnant sheep model has permitted the estimation of fetal oxygen consumption by measuring the rate of umbilical blood flow together with the umbilical venous-arterial difference in oxygen content. A unique aspect of prenatal life is that even under normal physiologic conditions, all organs are perfused by blood with a lower level of oxygenation than occurs in the normal neonate after birth. Owing to the structure of the fetal circulation, fetal arterial blood is formed by mixing of oxygenated blood flowing to the fetus via the umbilical vein with deoxygenated blood flowing through the superior and inferior vena cava. In the third trimester, the normal O ₂ saturation and P o ₂ of the blood perfusing the fetal upper body via the ascending aorta are approximately 65% and 27 mm Hg, respectively. The blood perfusing the lower body via the abdominal aorta is approximately 50% saturated with O ₂ and has a P o ₂ of 21 mm Hg. The difference between upper and lower body oxygenation is due to preferential streaming of oxygenated blood from the umbilical vein to the left ventricle via the ductus venosus (DV) in the liver and the foramen ovale between the right and left atria of the heart.

In the third trimester, the fetal oxygen consumption rate is approximately 315 μmol/min/kg body weight. This rate is twice that of the maternal body at rest, and virtually equal to that of a newborn infant in a thermo-neutral environment. The main compensatory mechanism that allows the fetus to maintain a high rate of oxidative metabolism in the presence of a low level of oxygenation is a relatively high cardiac output. In the third-trimester fetus, the combined output of the two cardiac ventricles is approximately 450 mL/min/kg, giving a biventricular output/O ₂ consumption ratio of approximately 1.4 mL/μmol O ₂ . This ratio is approximately 70% higher than that of an adult mammal of equal body weight at rest.

The most important function of fetal cardiac output is to allow the fetus to maintain a rate of oxidative metabolism that is independent of the O ₂ supply rate. The rate at which umbilical venous blood supplies O ₂ to the fetus is defined as the product of umbilical blood flow × umbilical venous O ₂ content. In the fetal lamb, the normal umbilical venous O ₂ supply/O ₂ consumption ratio is approximately 3. This ratio is approximately twice the critical value at which O ₂ supply begins to limit O ₂ consumption. This defines two degrees of fetal hypoxia: mild hypoxia, in which O ₂ supply is less than normal but high enough to allow a supply-independent rate of fetal oxidative metabolism, and severe hypoxia in which supply restricts consumption.

Animal and human FGR studies in which the fetus is hypoxic highlight suboptimal placental oxygen diffusion parameters and likely changes in fetal oxygen utilization. The adequate transfer of oxygen to the fetus is dependent upon the development of both the uteroplacental and fetoplacental circulations, and as such three categories of fetal hypoxia have been proposed depending on maternal physiologic status and environment and/or adaption to pregnancy. One category is that of preplacental hypoxia . This is a situation where both placenta and fetus become hypoxic because of a reduced oxygen content within maternal blood, such as observed with pregnancy at high altitude, maternal anemia, and asthma. The second situation, termed uteroplacental hypoxia, is where normally oxygenated maternal blood has a restricted entry into the uteroplacental tissues due either to occlusion or failed trophoblast invasion of uteroplacental arterioles, such as in preeclampsia. The third situation termed post-placental hypoxia describes the case were normally oxygenated maternal blood enters the IVS, either at a normal or reduced rate, but there is a major defect in fetoplacental perfusion that prevents the fetus from receiving sufficient oxygen, such as in situations of reduced membrane area for exchange and nutrient transporter activity. Being aware of the type of fetal hypoxia is important as differences in fetoplacental adaptative processes likely exist and these may impact differentially upon fetal circulatory and metabolic outcomes, depending on the etiology of the FGR.

Indeed, in an ovine placental insufficiency FGR (PI-FGR) that produces prolonged moderate to severe fetal hypoxia independent of maternal oxygenation, there is a 25% reduction in fetal oxygen uptake, in what is understood to be a post-placental fetal hypoxia situation. Placental oxygen utilization may represent a limiting step in FGR by restricting oxygen delivery to the fetus. In agreement with these animal studies, the human PI-FGR situation displays a similar increase in the uterine-umbilical or transplacental oxygen gradient along with lower rates of umbilical oxygen delivery and uptake, both in absolute values and normalized for fetal body weight.

The development of the placenta as the organ of fetal oxygenation is an autonomous process to which the fetus must adapt. This adaptation is mediated by the fetal adrenal glands. Blood flow to the adrenals and adrenal norepinephrine (NE) output are inversely related to fetal arterial O ₂ content. ^, The increase in fetal NE blood concentration inhibits pancreatic insulin output and decreases fetal growth rate. ^, When sustained over time, the decrease in growth rate generates a “small-for-gestational-age” fetus. In addition, it results in a decrease in the growth of fetal O ₂ consumption because in fetal life O ₂ consumption is proportional to body weight, and it is clear that this deceleration is the main target of this regulatory mechanism. The low blood O ₂ content that stimulates NE output is a signal that the placenta is incapable of generating its normal O ₂ transport capacity. It is essential for fetal survival to match the growth of fetal O ₂ demand to the growth of placental O ₂ diffusion. A widening gap between O ₂ demand and the ability of the placenta to satisfy that demand leads to a progressive decrease in fetal blood O ₂ content. Fetal oxygenation therefore plays an important role in the pathogenesis of all placentally mediated FGR, potentially including those in which there is no evidence of severe hypoxia. The reduced oxygen delivery in FGR fetuses indicates impaired placental oxygen diffusion, whereas reduced oxygen consumption presumably reflects metabolic adaptation to diminished substrate delivery, resulting in slower fetal growth. ^,

Nutrient Transport and Metabolism