The influence of artificial rupture of the membranes on plasma levels of 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM) and oxytocin was examined in 23 pregnant women at term. Serial blood samples were collected before and 15 minutes, 2 hours, 5 hours, and 8 hours after artificial rupture of the membranes. A significant rise in the concentration of plasma PGFM was observed at 15 minutes in the majority of women (20 of 23), but the magnitude of this early rise or the lack thereof was not related to the subsequent course of labor. The concentration of plasma PGFM at 2 hours was, on the other hand, significantly correlated with the induction-delivery interval. Amniotomy, by itself, induced labor and delivery when the increased PGFM levels were maintained from 2 to 5 hours after the procedure (n = 16). In those cases where Pitocin stimulation was required for adequate uterine contractions, it was found that plasma PGFM levels had declined to initial values at 2 hours. Pitocin infusions then partially reversed this decline. In one patient, the cervix failed to dilate in spite of prolonged Pitocin infusion which did not induce significant uterine contractions, and the infusion did not reverse the marked fall in plasma PGFM after the early but transient rise. Mean plasma oxytocin levels did not rise significantly during labor induced by artificial rupture of the membranes and were, on the average, similar to the levels observed during the first stage of spontaneous or oxytocin-induced labor. Considering the previously demonstrated maximal levels of uterine oxytocin receptors in early labor, the absence of a rise in the plasma oxytocin levels does not negate a role for oxytocin in working synergistically with prostaglandins in the mechanism of labor.
Amnion Cervix Uteri