The rate of the foetal heart is subject to considerable variations, which affords us a fairly reliable means of judging as to the well-being of the child. As a general rule, its life should be considered in danger when the heart-beats fall below 100 or exceed 160.
—J. Whitridge Williams (1903)
More than 100 years ago, the approach to fetal assessment was rather primitive. Since that time, and especially since the 1970s, technology to evaluate the health of the fetus has advanced remarkably. Techniques employed today to forecast fetal well-being focus on fetal biophysical findings that include heart rate, movement, breathing, and amnionic fluid production. These findings aid antepartum fetal surveillance to prevent fetal death and avoid unnecessary interventions, which are stated goals of the American College of Obstetricians and Gynecologists and the American Academy of Pediatrics (2017).
Most fetuses will be healthy, and usually a negative—that is, normal—antepartum test result is highly reassuring, because fetal deaths within 1 week of a normal test are rare. Indeed, negative-predictive values—a true negative test—for most of the tests described are 99.8 percent or higher. In contrast, estimates of the positive-predictive values—a true positive test—for abnormal test results are low and range between 10 and 40 percent. Importantly, fetal surveillance is primarily based on circumstantial evidence. No definitive randomized clinical trials have been conducted for obvious ethical reasons (American College of Obstetricians and Gynecologists, 2016).
Passive unstimulated fetal activity commences as early as 7 weeks’ gestation and becomes more sophisticated and coordinated by the end of pregnancy (Sajapala, 2017; Vindla, 1995). Indeed, beyond 8 menstrual weeks, fetal body movements are never absent for periods exceeding 13 minutes (DeVries, 1985). Between 20 and 30 weeks’ gestation, general body movements become organized, and the fetus starts to show rest-activity cycles (Sorokin, 1982). Fetal movement maturation continues until approximately 36 weeks, when behavioral states are established in most normal fetuses. Nijhuis and colleagues (1982) described four fetal behavioral states:
State 1F is a quiescent state—quiet sleep—with a narrow oscillatory bandwidth of the fetal heart rate.
State 2F includes frequent gross body movements, continuous eye movements, and wider oscillation of the fetal heart rate. This state is analogous to rapid eye movement (REM) or active sleep in the neonate.
State 3F includes continuous eye movements in the absence of body movements and no heart rate accelerations. The existence of this state is disputed (Pillai, 1990a).
State 4F is one of vigorous body movement with continuous eye movements and heart rate accelerations. This state corresponds to the awake state in newborns.
Fetuses spend most of their time in states 1F and 2F. For example, at 38 weeks, 75 percent of time is spent in these two states. These behavioral states—particularly 1F and 2F, ...