Fig. 1

^a,bPower curves demonstrating projected statistical power. ^a In the da Fonseca trial of vaginal progesterone for preterm birth, among the 72 women who received this intervention, 2 (2.8%) delivered before 34 weeks and 10 (13.9%) before 37 weeks; by comparison, among 70 who received placebo, 13 (18.6%) delivered before 34 weeks and 20 (28.5%) before 37 weeks. The corresponding odds ratio estimates were 0.13 (95% confidence interval [CI] 0.03–0.58) and 0.40 (95% CI 0.17–0.94). ^b In the NICHD Meis et al. trial of 17-OHPC, among 306 women receiving 17-OHPC, 35 (11.4%) delivered before 32 weeks, and 63 (20.6%) delivered before 35 weeks; by comparison, among 153 who received the placebo, 30 (19.6%) delivered before 32 weeks and 47 (30.7%) delivered before 35 weeks. The corresponding odds ratio estimates were 0.53 (95% confidence interval [CI] 0.31–0.90) and 0.58 (95% CI 0.38–0.91)