The development of ultradian rhythms in the human fetus

Objective: Our objective was to determine the normal sequence of neurobehavioral development in the human fetus between 14 weeks' gestation and delivery. Study Design: The study was performed by longitudinal ultrasonographic observation of 45 low-risk singleton fetuses. Results: During the first half of the midtrimester there was a high rate of spontaneous movement that appeared randomly distributed. By the end of that trimester an increase in the duration of intervals of quiescence resulted in activity appearing cyclically distributed, with the duration of quiet cycles progressively increasing to term. Fetal mouthing and breathing were linked with cyclic behavior from the time of their emergence. Fetal heart rate pattern A could be recognized from around 32 weeks, due to a reduction in baseline variability in quiet cycles after 30 weeks, whereas pattern B could be recognized several weeks earlier. From the time cyclic behavior and heart rate patterns could be recognized, intervals of coincidence of the fetal behavioral state variables increased progressively with advancing gestation. © 1992, Mosby. All rights reserved.