Sleep-wake cycles in normal fetuses

To study the normal ranges of sleep-wake cycles in normal fetuses. A total of 600 fetal heart rate (FHR) tracings were studied from uncomplicated singleton pregnancies at a gestational age between 30 and 40 weeks with a 1-h recording. Two obstetricians interpreted all the FHR recordings independently. The mean baseline FHR was 141.6 (7.6) beats/min. The median relative percentage time spent in a quiet state was 26% (0-88.6). The median duration of time spent in a quiet state was 15.7 min (0-53.2) within a 1-h recording. Both quiet and active states were established in 84% of cases. The number of FHR accelerations per hour in an active state increased with advancing gestational age, whereas the baseline FHR gradually decreased. Normal ranges of fetal sleep-wake cycles were established. Fetuses between 30 and 40 weeks had wide variations in the percentage of time spent in sleep-wake states.