Diurnal and other cyclic variations in human fetal heart rate near term.

In 10 healthy women near term, fetal heart rate, as monitored from an abdominal electrocardiogram, and its variation were analyzed over 24 hours. During the daytime, this was combined with real-time scanning for fetal breathing and movements. There was an episodic change in high and low pulse (R-R) interval variation with a mean cycle length of 92 minutes. The changes in cycle length were large; around midnight, episodes of high variation lasted up to 6 hours. Episodes of high variation coincided with greater fetal movement (measured only in daytime). Neither low nor high episodes were consistently related to fetal breathing. Both fetal heart rate and its variability showed a diurnal variation, by 11% and 50%, respectively; these were not statistically related. There was a particularly large change in pulse interval variation with peak values around midnight; concomitantly, there was a diurnal variation in the incidence of accelerations of defined size. The implications of these findings are discussed.