The effects of oral administration of progesterone for premature labor

The potential tocolytic effect of natural progesterone administration on premature labor was investigated in a double-blind study. An oral progesterone formulation was used because its ability to increase both plasma and myometrial concentration of progesterone in pregnant women had been previously demonstrated. Furthermore, no commercial intravenous or intramuscular natural progesterone formulation is currently available in France. Fifty-seven patients in two obstetric clinics, admitted because of the risk of premature delivery, were included in the study, and uterine contractility and fetal cardiac rhythm were monitored in all of them. At random and after 30 minutes' rest, 29 women absorbed four capsules of 100 mg of progesterone each and 28 women absorbed four capsules of a placebo. Plasma progesterone levels were evaluated in all cases after 30 minutes' rest and 1 hour after absorption of the capsules. The results showed that bed rest and placebo administration decrease uterine activity in 42% of the cases and oral progesterone decreases activity in 75% to 88% of cases, depending on the initial severity of the menace of premature delivery. The difference between the effects of progesterone and of placebo is significant. The tocolytic effect of oral progesterone is not as intense or as rapid as the effect of intravenous beta-mimetics but is sufficient in 80% of cases, on the average, to stop the premature labor without any detectable side effects. This tocolytic effect of oral progesterone is related not just to an increase in plasma progesterone levels but probably to an increase in myometrial progesterone concentration.