Antepartum Fetal Evaluation

Characteristics of Fetal Death

Another way to consider the effect of fetal events on PMR is to look at the infant mortality rate ( Fig. 27.3 ). Although the infant mortality rate includes all deaths of infants younger than 1 year of age, 50% of all infant deaths occur in the first week of life, and 50% of these losses result during the first day of life. In 2017, the leading causes of infant mortality were congenital malformations (21%), disorders related to short gestation and low birthweight (17%), maternal complications (6%), sudden infant death syndrome (6%), and unintentional injuries (6%). Clearly, antepartum and perinatal events play an important role in infant mortality.

Causes of Fetal Death

In addition to declining frequency in PMR over time, the overall pattern of perinatal deaths in the United States has changed considerably during the past 50+ years. Investigators in the 1980s–90s who studied longitudinal cohorts from the mid- to late 20th century suggested that etiologies for antepartum fetal deaths could be categorized as chronic asphyxia/placental insufficiency, congenital malformations, superimposed complications of pregnancy (such as Rh isoimmunization, placental abruption, and fetal infection—many as a consequence of preterm premature rupture of membranes), and unexplained cause. Decline in the fetal death rates in these cohorts over time was attributed to the prevention of Rh sensitization, antepartum and intrapartum fetal surveillance, improved detection of intrauterine growth restriction (IUGR) and fetal anomalies with ultrasound, and improved care of maternal diabetes mellitus and preeclampsia. The role of antenatal diagnosis and management of congenital malformations and aneuploidy is obviously critical to a goal of reducing perinatal morbidity and mortality and will be discussed separately (see Chapters 9 and 10 ).

With regard to undiagnosed IUGR in particular, this has been identified by many investigators as a potentially preventable contributor to fetal death rates, especially in the gestational period between 28 and 36 weeks (Fretts ). Despite a marked fall in unexplained fetal deaths in the cohorts studied during the second half of the last century, this category was used for more than 25% of all stillbirths and is still the categorization used to describe 30% of fetal deaths in the most recent US National Vital Statistic Report on the subject. Numerous more recent population-based analyses of the causes of fetal death in the United States and other similarly developed nations have been performed, including serial evaluations by the Stillbirth Collaborative Research Network, which utilized rigorous methods to classify as many “unexplained” fetal deaths as possible. This group and others concluded unspecified causes and conditions of placenta/cord/membranes are the leading classifications for fetal death utilized in a modern high-resource population at 25% to 30% each, followed by maternal complications of pregnancy and congenital anomalies at 10% to 15% each ( Fig. 27.4 ; in addition, fetal deaths late in gestation are more likely than earlier gestation fetal deaths to have no identifiable etiology . Regarding our ability to whittle away at the number of unexplained fetal deaths, and thus possibly increase the proportion of deaths that are potentially preventable, many leaders in the field have put out a call to action for standardized and rigorous methodology in clinical and research settings when classifying and cataloguing each fetal death that occurs in high-resource settings.

The ability of our current methods of surveillance to make an impact on perinatal mortality will depend on the ability of available tests to predict and predate injury and on use of obstetric interventions to prevent adverse outcomes. Since the decline in fetal deaths attributable to factors largely considered identifiable and modifiable during routine modern prenatal care, including congenital malformations and Rh isoimmunization, the concept of further progress toward identifying and reducing other causes of potentially preventable fetal deaths has emerged as an important goal. In one British series, obstetric and pediatric assessors reviewed the circumstances surrounding each case of perinatal death to identify any “avoidable” factors that may have contributed to the death. Of the 309 perinatal deaths in this population (half fetal and half in the first week of life), 59% were considered to have had avoidable factors, including 74% of normal-birthweight infants with no fetal abnormalities and no maternal complications. Most avoidable factors were found to be obstetric rather than pediatric, maternal, or social. The failure to respond appropriately to abnormalities during pregnancy and labor—including results from the monitoring of fetal growth or intrapartum fetal well-being, significant maternal weight loss, or reported reductions in fetal movement—constituted the largest groups of avoidable factors. This characterization of avoidable factors that contribute to perinatal death has been confirmed in additional studies. Most recently a study by the US Stillbirth Collaborative Research Network, including a comprehensive analysis of 512 fetal deaths, characterized 22% as potentially preventable. This “potentially preventable” classification was made on the basis of features that could theoretically be detected in an otherwise-viable (>24 weeks and 500 g, no lethal anomalies) fetus based on maternal and fetal risk factors and testing. Of those fetal deaths categorized as potentially preventable, the majority shared a common feature of impaired placental function—a condition theoretically amenable to detection via fetal growth surveillance and perhaps antepartum fetal testing.

Timing of Fetal Death

Another way to classify fetal deaths may be to differentiate those that occur during the antepartum period and those that occur during labor, or intrapartum deaths. Antepartum fetal death is much more common than intrapartum fetal death, and unexplained fetal death occurs far more commonly than unexplained infant death. In a population-based study in the United States in 2007, the antepartum fetal death rate was 3.7 per 1000, compared with 0.6 per 1000 intrapartum fetal deaths. Although most fetal deaths occur before 32 weeks’ gestation, in planning a strategy for antepartum fetal monitoring, the risk for fetal death must be examined in the population of women who are still pregnant at that point in pregnancy. The understanding that all pregnancies at a given gestational age and at each subsequent point until delivery are at risk for fetal death has led to the development of a metric with which to examine fetal death rates per gestational age, the prospective fetal mortality rate . Rather than compute the fetal mortality rate as number of fetal deaths per 1000 live births or fetal deaths at a given gestational age, the prospective fetal mortality rate is defined as the number of fetal deaths per 1000 live births or fetal deaths at a given gestational age or greater ( Fig. 27.5 ) . When this approach is taken, the data would suggest that fetuses at 40 to 41 weeks are at a threefold greater risk and those at 42 or more weeks are at a 12-fold greater risk for intrauterine death than fetuses at 28 to 31 weeks. The risks are even higher in multiple gestations as pregnancy progresses. For twin gestations, the optimal time for delivery to prevent late-gestation perinatal deaths is before 39 weeks, and for triplets, 36 weeks. The issue of timing is also illustrated by a recent cohort study of over 75,000 singleton pregnancies with fetal growth restriction, the focus of which was to find the point at which the competing risks of fetal death and neonatal death were in balance, in order to inform delivery decisions. In this cohort, the balance point was 32 to 34 weeks.

Identifying Those at Risk

Some risk factors have a clear etiologic relationship to fetal compromise and death, such as exposures to teratogens or maternal conditions that alter the fetal environment or blood supply or content. Other risk factors—such as epidemiologic factors that include maternal age, race, and body habitus—have a perhaps more complex and less well-understood link to fetal death risk ( Fig. 27.6 ). Common risk factors for fetal death in the United States are listed in Table 27.1 . Many of these conditions can coexist in individual patients, which makes assessment of the contribution of each factor to perinatal mortality a challenge. A recent concept of looking at individual risk factors as part of a “triple risk model” may prove useful in making sense of this challenge, similar to that used to understand contributors to sudden infant death syndrome (SIDS). In this model, proposed by Warland and Mitchell, an interplay exists among maternal, fetal, and placental factors and a stressor. They posit that whereas these factors in isolation may be insufficient to cause fetal death, they may prove lethal in combination ( Fig. 27.7 ). Also necessary to consider is the contribution of these conditions to fetal injury that results in liveborn children with permanent neurologic compromise; this has yet to be determined but is an important alternative outcome to perinatal mortality that deserves further study.

Maternal Characteristics

Maternal Race and Ethnicity

The variation in fetal death risk in the United States by maternal race is complex, which makes ascertainment of biologic risk factors related to race difficult. Factors that contribute to increased rates of fetal death among black women compared with white women include disparities in socioeconomic status, access to health care, and preexisting medical conditions. A 2009 population-based study of more than 5 million US births demonstrated that the greatest black-and-white disparity is in preterm perinatal death, with a hazard ratio at 20 to 23 weeks of 2.75, which decreases to 1.57 at 39 to 40 weeks. Associations with fetal death in black women include lower education levels and higher rates of medical, pregnancy, and labor complications, with congenital anomalies more contributory in white women. The study of racial and ethnic disparities in fetal death rates worldwide, and how these intersect with sociodemographic features and what remediating steps could be taken, has been identified as a critically overdue initiative ( Fig. 27.8 ).

Maternal Comorbidities

Obstetric Factors