Dodd JM et al. (2006) Prenatal administration of progesterone for preventing preterm birth (Review). The Cochrane Database of Systematic Reviews, Issue 1, Art. No CD004947.pub2

Preterm infants have a high risk of death and other complications, and it has been suggested that preventing or delaying early labor should lead to improved infant outcomes. PROGESTERONE therapy has shown some efficacy in reducing the risk of preterm birth, but little information exists on adverse effects and optimal timing, dosage, and route of administration.

A Cochrane meta-analysis of six randomized, placebo-controlled trials, involving 988 women, found that there was insufficient evidence for giving progesterone therapy to women at an elevated risk of preterm labor, because of the lack of data on the benefits and harm of such therapy. Progesterone therapy (five trials used intramuscular and one used intravaginal progesterone) did have beneficial effects, including prolonging the pregnancy: the relative risk of giving birth before 37 weeks of gestation for women given progesterone was 0.65 (95% CI 0.54–0.79). Infants of women given progesterone were less likely than those of women given placebo to have a birthweight below 2.50 kg (four trials, 763 infants; relative risk 0.63, 95% CI 0.49–0.81). It is not clear, however, whether prolonged pregnancy translates into better health outcomes.

Almost all the information on infant outcomes came from a single trial involving 459 babies, which was underpowered to detect clinically significant differences in neonatal outcomes. Additional trials that should help to address these questions are ongoing.