Serum progesterone, estradiol, and estriol before and during induced labor.

To investigate the association of serum levels of progesterone (P), estradiol (E2), and estriol (E3) with the initiation of regular contractions, venous blood samples were taken prior to and 3 hours after the successful induction of labor in 83 parturients by means of low amniotomy and intravenous oxytocin infusion. The serum P level and P/E2 ratio decreased and serum E2 level increased after induction in healthy primigravidas and in parturients with an initial P dominance (serum P/E2 ratio more than 5). There was also a decrease in the serum P level and P/E2 ratio in postterm patients and parturients with a ripe cervical state. Cholestasis of pregnancy was associated with a rise in the serum level of E2 and a decrease in the P/E2 ratio. In an unfavorable cervical state there was a rise in the serum E2 level; patients with an initial E2 dominance (serum P/E2 ratio 5 or less) showed a rise in the serum P level and P/E2 ratio. Healthy multigravidas and patients with pre-eclampsia did not show any hormonal changes. The onset of induced labor seems to be associated with a rise in serum E2 concentration and/or with a drop in P concentration. The increase in serum E2 level is thought to be due to the activation of the anterior pituitary--adrenal axis. The lower P level is supposedly a result of diminished uteroplacental circulation.