The fetus determines circadian oscillation of myometrial electromyographic activity in the pregnant rhesus monkey

Uterine electromyographic activity in the pregnant rhesus monkey showed two types of episodic electrical events, defined on the basis of duration of the episodes. One of these (designated type I in this report) was characterized by frequent bursts of electrical activity that lasted from 0.5 to 1.2 minutes (mean duration, 0.64 ± 0.007 [SE] minutes) and occurred at frequencies of 10 to 45 per hour. The other type of electromyographic activity observed (type II) consisted of fragmented series of discharges that lasted 2 to 15 minutes (mean duration, 5.45 ± 0.12 [SE] minutes) and occurred at frequencies of 0 to 6 per hour. The frequency of type I electromyographic events showed a circadian pattern of low amplitude with a maximum at night in animals with minimal uterine manipulation (electrodes only). This daily pattern was amplified during the 8 to 10 days that preceded delivery. Hysterotomy and fetal catheterization induced a circadian increase in the frequency of type I electromyographic events akin to that observed during the predelivery period. Fetal death abolished the high-amplitude circadian variation of type I electromyographic events both postoperatively and in the predelivery period. These data indicate that the fetus plays a role in the regulation of uterine type I electromyographic activity in the rhesus monkey.