Effect of vaginal progesterone, administered to prevent preterm birth, on impedance to blood flow in fetal and uterine circulation

Objective To evaluate the effect on the maternal and fetal circulation of progesterone administered to prevent preterm birth. Methods We used an observational cohort study design. The study group included 44 women at 18-32 weeks' gestation who presented with an episode of preterm labor, with or without history of delivery before 34 weeks' gestation, or an incidental finding of short cervix (≤ 25 mm). Doppler flow assessment of the umbilical artery, fetal middle cerebral artery and uterine arteries was performed before and 24 h after vaginal administration of progesterone. Results Seventeen (38.6%) women gave birth before term, but only nine (20.4%) did so before 34 weeks' gestation. Following progesterone treatment, there was a statistically significant decrease in the pulsatility index of the fetal middle cerebral artery (mean reduction, 18.2%; mean change in pulsatility index, 0.44 (95% CI, 0.25-0.63), P < 0.001), with no changes in the other vessels. Comparison of the women who gave birth before with those who delivered at term yielded no significant differences in Doppler flow parameters in any vessel examined, either before or after progesterone treatment. Conclusion Treatment with vaginal progesterone is associated with a lower pulsatility index in the fetal middle cerebral artery, suggesting a vasodilatory effect on the fetal circulation.