Electromyographic activity of the nonpregnant and pregnant sheep uterus

Myometrial electromyograms were studied in nonpregnant ewes, pregnant ewes throughout the second half of gestation, and during labor and delivery in normal spontaneous vaginal delivery and premature delivery induced by the continuous infusion of synthetic adrenocorticotropin 1-24, 1 μg . h^-1, to the fetus at 120 and 130 days' gestation. The bursts of electromyographic (EMG) activity during estrus were very similar to those seen in the pregnant ewe in the second half of gestation. In the second half gestation, the bursts of EMG activity lasted a mean of 6.7 minutes and occurred at intervals of 54.7 minutes in utero, without an incision for fetal instrumentation, and were 7.2 minutes in duration, occurring every 46.6 minutes when the uterus had been incised. EMG activity occurred simultaneously at most recording sites within the uterus. There was no evidence to support the view that a uterine pacemaker exists with a fixed location. As delivery approached, the mean duration of each EMG burst began decreasing more than 24 hours before delivery, and the frequency of the bursts increased, demonstrating that the recording of uterine electromyograms provides a precise and sensitive method for the study of the early changes that lead to labor and delivery.