Timing of the switch from myometrial contractures to contractions in late-gestation pregnant rhesus monkeys as recorded by myometrial electromyogram during spontaneous term and androstenedione-induced labor

Pregnant rhesus monkeys were studied to determine the precise time in relation to photoperiod of the onset, and the nature, of the switch in myometrial activity patterns from contractures to contractions. We investigated both spontaneous term labor and androstenedione-induced preterm labor. Under general anesthesia at 127 ± 2 days gestation (dGA) (mean ± SEM), 16 pregnant rhesus monkeys were instrumented with maternal femoral arterial and venous catheters and myometrial electromyogram electrodes. Eight animals (group I) received continuous i.v. infusion of intralipid (n = 7) or saline (n = 1) that was started at 143.3 ± 2 dGA and maintained until the spontaneous onset of term labor. Nine animals (group II) received continuous i.v. infusion of androstenedione that was started at 139 ± 0.4 dGA and maintained until the onset of prematurely induced labor. Myometrial activity was recorded continuously. All monkeys in both groups demonstrated nocturnal switches in myometrial activity from contractures to contractions. The mean time of onset of the switch in group I and group II monkeys was similar, occurring at 0.7 ± 0.4 h or 0.8 ± 0.5 h, respectively, after the onset of darkness. Group II monkeys demonstrated greater regularity in both the time of onset and the repetitive occurrence each night once the switch occurred, as well as greater consistency in duration in their switch patterns, than did group I monkeys.