Abstract

Objectives: To document the time relationship between the maternal plasma concentrations of 17?-estradiol and progesterone and term and preterm delivery. Study design: Plasma 17?-estradiol and progesterone were measured in pregnant women divided into a control group (delivery at 39.2 ± 0.4 weeks (mean ± SEM); n = 13), and a group (n = 6) delivered preterm (delivery at 35.0 ± 1.0 weeks). Blood samples were drawn at 2-hour intervals for 24 h, every 2 weeks from 28 weeks' gestation until delivery. Results: A clear increase (23% over the preceding interval, p < 0.01) in the mean 24-hour 17?-estradiol was observed 9 days before the onset of labor, in both groups. No differences in plasma progesterone were observed. Conclusion: There is a 20% surge in maternal 17?-estradiol before the onset of preterm and term labor. This change can be mediated by an activation of the estrogen synthesis pathways or, alternatively, by a decrease in estrogen clearance.