We tested the hypothesis that administration of vaginal progesterone in women with arrested preterm labor would result in lower rates of preterm birth <37 weeks compared to placebo. We performed a randomized, placebo-controlled trial comparing vaginal progesterone to placebo in women with arrested preterm labor. Our trial included women with a singleton or twin gestation at 240/7–336/7 weeks’ gestation who presented with preterm labor with cervical dilation ≥1 centimeter but remained undelivered. Participants were randomized...