Plasma oxytocin concentrations in men, nonpregnant women, and pregnant women before and during spontaneous labor.
Level 4 - case-series / case-control
Cross-sectional and prospective observational physiological study comparing distinct cohorts.
PubMed 7287862 · doi:10.1210/jcem-53-4-730
What was done
Baseline plasma oxytocin concentrations were measured across several cohorts: 25 healthy men, 102 nonpregnant women (including a subgroup of 20 taking oral contraceptives), and 59 pregnant women from 15 to 42 weeks gestation. In addition, plasma oxytocin was measured during spontaneous labor across different stages: in 6 women during the latent phase and 14 women in the active phase (at the onset, peak, and immediately after a single contraction), in 19 women at initial presentation of the fetal head (+3 station), and in 11 women at the delivery of the fetal head during normal vaginal delivery.
What was found
Baseline plasma oxytocin concentrations showed no significant differences among men (1.5 ± 0.2 µU/ml), nonpregnant women (1.4 ± 0.2 µU/ml), pregnant women before labor (1.3 ± 0.1 µU/ml), or women taking oral contraceptives (1.8 ± 0.7 µU/ml). During labor, oxytocin levels did not correlate with uterine pressure measurements and did not significantly rise above baseline pregnancy levels in the latent (1.3 ± 0.2 µU/ml) or active (1.6 ± 0.2 µU/ml) phases. Levels increased significantly from initial visualization of the fetal head (1.1 ± 0.1 µU/ml) to the delivery of the head (4.2 ± 1.1 µU/ml; P < 0.05).
Why it matters
This study clarifies the physiological profile of circulating oxytocin, demonstrating that systemic levels remain comparable to nonpregnant baselines until late in the second stage of labor.
Limits
Sample sizes for the intrapartum subgroups were small (n = 6 to 19). The study measured circulating plasma concentrations, which may not capture pulsatile release dynamics or local uterine tissue sensitivity and paracrine actions.
Cited by
- contradicts Women get three times as much oxytocin as men.