n important determinant of fetal activity appears to be sleep-awake cycles, which are independent of maternal ones. Fetal sleep cyclicity varies from approximately 20 minutes to as much as 75 minutes. In one study, the mean length of the quiet or inactive state for term fetuses was 23 minutes (TimorTritsch, 1978). Patrick and associates (1982) measured gross fetal body movements with real-time sonography for 24-hour periods in 31 normal pregnancies and found the longest period ofinactivity to be 75 minutes. Amnionicluid volume is another important determinant of fetal activity. Sherer and colleagues (1996) assessed the number of fetal movements in 465 pregnancies during biophysical profile testing in relation to amnionic luid volume. hey observed decreased fetal activity with diminished amnionic volumes and suggested that a restricted uterine space might physically limit fetal movements.
with fetal heart rate (FHR) variation recorded in relation to 1 F or 2F behavior states. State 1 F fetal heart rate has a narrow bandwidth consistent with quiet sleep. State 2F heart rate shows wide oscillation ofthe baseline consistent with active sleep. (Modified with permission from Oosterhof H, vd Stege JG, Lander M, et al: Urine production rate is related to behavioural states in the near term human fetus, Br J Obstet Gynaecol. 1993 Oct;l 00(1 0):920-922.) �E > 400 ..
._ 100
FIGURE 17-2 Graph depicts averages offetal movements counted during 12-hour periods (mean ± SEM). (Data from Sadovsky, 1979a.)
Sadovsky and coworkers (1979b) classiied fetal movements into three categories according to both maternal perceptions and independent recordings using piezoelectric sensors. Weak, strong, and rolling movements were described, and their relative contributions to total weekly movements throughout the last half of pregnancy were quantiied. As pregnancy advances, the rate of weak movements decreases, more vigorous movements increase for severalweeks, and then rates of these subside at term. Presumably, declining amnionic luid and space account for diminished activity at term.
Figure 1i7-2 shows fetal movements during the last half ofgestation in 127 pregnancies with normal outcomes. he mean number of weekly movements calculated from 12-hour daily recording periods rose from approximately 200 at 20 weeks' gestation to a maximum of 575 movements at 32 weeks. Fetal movements then declined to an average of 282 at 40 weeks. Normal weekly maternal counts of fetal movements ranged between 50 and 950. Count showed large daily variations, with included counts as low as 4 to 10 per 12-hour period in normal pregnancies.
Diminished fetal activity may be a harbinger ofimpending fetal death (Sadovsky, 1973). To quantiy fetal movement, clinical methods include use of a uterine contraction tocodynamometer, visualization with sonography, and maternal subjective perceptions.