[Development of the activity-rest cycle in the human fetus].

Some features of natural fetal behavior (general motor activity, including respiratory activity, heart rate and the motor-cardiac reflex as their correlated function) were studied by means of ultrasound scanning, monitoring and dynamic cardiography in 96 women at 20 to 36 weeks of pregnancy under continuous recording of physiological parameters for 2-3 hours. Computer analysis of heart rate variation in correlation with body movements has revealed that there is only one functional state in the fetal behavior, called "nondifferentiated state" (NDS), by the 20th week of pregnancy. Against this background, there appear transitory episodes of active state beginning from the 21st week, and those of quiet state, from the 23rd week; duration of the episodes increases with the progress of pregnancy. In the intervals between the episodes of active and quiet states, NDS gradually transforms into an intermediate state by the 28th week, which shortens in the course of ontogenesis; the alternation of active and quiet states assumes distinct cyclicity by the 32nd week that indicates the establishment of the rest-activity cycle, becoming apparent in all fetuses in the 36th week of gestation when it reaches 50-70 min in duration. On the basis of the comparison of somato-vegetative patterns of quiet state in human fetuses with the ones, seen earlier in rabbit fetuses, the homology of these states is postulated.