According to the American College of Obstetricians and Gynecologists and the American Academy of Pediatrics (2007), the goals of antepartum fetal surveillance include prevention of fetal death and avoidance of unnecessary interventions. Current techniques employed to forecast fetal well-being focus on fetal physical activities, including heart rate, movement, breathing, and amnionic fluid production. In most cases, a negative, that is, normal 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, the widespread use of antepartum fetal surveillance is primarily based on circumstantial evidence because there have been no definitive randomized clinical trials.
Passive unstimulated fetal activity commences as early as 7 weeks and becomes more sophisticated and coordinated by the end of pregnancy (Vindla and James, 1995). Indeed, beyond 8 menstrual weeks, fetal body movements are never absent for periods exceeding 13 minutes (DeVries and co-workers, 1985). Between 20 and 30 weeks, general body movements become organized, and the fetus starts to show rest-activity cycles (Sorokin and co-workers, 1982). In the third trimester, fetal movement maturation continues until approximately 36 weeks, when behavioral states are established in most normal fetuses. Nijhuis and colleagues (1982) studied fetal heart rate patterns, general body movements, and eye movements and 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 and James, 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 infants.
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 (Nijhuis and colleagues, 1982).
These behavioral states—particularly 1F and 2F, which correspond to quiet sleep and active sleep—have been used to develop an increasingly sophisticated understanding of fetal behavior. Oosterhof and co-workers (1993) studied fetal urine production in normal pregnancies in states 1F or 2F. As shown in Figure 15-1, bladder volumes increased during state 1F quiet sleep. During state 2F, the fetal heart rate baseline bandwidth increased appreciably, and bladder volume was significantly diminished. The latter was due to fetal voiding as well as decreased urine production. These phenomena were interpreted to represent reduced renal blood ...