Plasma oxytocin concentrations in men, nonpregnant women, and pregnant women before and during spontaneous labor.

Baseline plasma oxytocin (OT) concentrations were measured in 25 healthy men, 102 nonpregnant women, and 59 pregnant women from 15-42 weeks gestation. In addition, plasma OT levels were measured at the onset, peak, and immediately after a single uterine contraction in 6 women in the latent phase and 14 women in the active phases of labor, as well as in 19 women at initial presentation of the fetal head on the perineum (+3 station) and 11 women at the time of delivery of the head during a normal vaginal delivery. Baseline plasma OT concentrations did not vary significantly among men (1.5 +/- 0.2 microunits/ml), nonpregnant women (1.4 +/- 0.2 microunits/ml), or pregnant women before labor (1.3 +/- 0.1 microunits/ml) and did not differ in an additional subgroup of 20 women receiving oral contraceptive medication (1.8 +/- 0.7 microunits/ml). In studies conducted during labor, plasma OT concentrations did not correlate with uterine pressure measurements and did not increase significantly over baseline pregnancy concentrations during the latent (1.3 +/- 0.2 microunits/ml) or active (1.6 +/- 0.2 microunits/ml) phases of labor. There was a significant increase in plasma OT levels from the time of initial visualization of the fetal head to the time of delivery of the head (1.1 +/- 0.1 to 4.2 +/- 1.1 microunits/ml, respectively; P less than 0.05). These data support the view that maternal plasma OT levels remain low during pregnancy until late in the second stage of labor.