During late pregnancy the uterine electromyographs (EMGs) consisted of irregular episodes (<3.5/h) of prolonged phases of activity (>3.5 mm duration) which occurred close to electrodes that had been implanted on uterine segments containing a fetus. Empty parts of the uterus remained relatively inactive. There were no significant changes in the mean plasma steroid concentrations and myometrial activity over the last 3 weeks of gestation. Plasma oxytocin concentrations remained below 1.2 µU/ml in 6 animals that were sampled between 12 and 2 days prior to parturition.

Between -24 and -10 h before expulsion of the first piglet, mean concentrations of progesterone had significantly fallen and those of estrogens had increased. This coincided with behavioral changes of the sow (nest-building) but myometrial activity at this stage was similar to that recorded on the previous days. It was only between -9 and -4 h before the first piglet was born that the pattern of the EMG5 changed and regular phases of electrical discharges appeared with increasing frequency (7-12/h) in all part of the uterus. At the same time oxytocin concentrations in peripheral plasma were elevated. During delivery of the piglets the mean frequency of the phases of electrical discharges increased further (18-20/h), their duration being significantly shorter than in the previous few hours. Oxytocin concentrations reached highest values at this stage, estrogen concentrations were already falling and progesterone had reached values below 4 ng/ml in all sows.