Oxytocin secretion and human parturition: Pulse frequency and duration increase during spontaneous labor in women

The secretory pattern of oxytocin was determined in blood samples taken at 1 -minute intervals for 30 minutes from 32 parturient women. The samples were collected in a manner that minimized degradation by plasma oxytocinase, and a highly specific antibody was used for the radioimmunoassay. The results indicated that oxytocin is secreted in discrete pulses of short duration. The frequency of the pulses was significantly higher during spontaneous labor than before the onset of labor. The mean pulse frequencies per 30 minutes were 1.2 ± 0.54 before labor, 4.2 ± 0.45 during the first stage, and 6.7 ± 0.49 during the second and third stages of labor. The mean pulse durations in these three groups were 1.2 ± 0.20, 1.9 ± 0.28, and 2.0 ± 0.26 minutes, respectively. The amplitude of the pulses was variable with no significant differences between the groups, the majority being around 1.0 μU/ml. The spontaneous pulses were of similar magnitude as those measured in 18 women after intravenous injections of 4 to 16 mU of oxytocin, which doses stimulated uterine contractions. We therefore conclude that the pulses of oxytocin observed at increasing frequency during spontaneous labor are of physiologic significance and provide evidence for the participation of oxytocin in the onset and maintenance of spontaneous labor.