1. Vaginal progesterone reduces preterm birth in women with an asymptomatic sonographic short cervix in the midtrimester. The meta-analysis found vaginal progesterone once daily from identification of a short cervix <25mm until 37 weeks decreases preterm birth <33 weeks by 45% and decreases neonatal morbidity and mortality. 2. Treatment with vaginal progesterone was associated with significant reductions in preterm birth before 28 weeks, 33 weeks, and 35 weeks as well as composite neonatal morbidity and mortality. 3. There were no significant differences in adverse maternal events or congenital anomalies between the vaginal progesterone and placebo groups.