Objective: We sought to evaluate whether 17 alpha-hydroxyprogesterone caproate (17-OHP) reduces preterm birth (PTB) in nulliparous women with a midtrimester cervical length (CL) <30 mm. Study Design: In this multicenter randomized controlled trial, nulliparous women with a singleton gestation between 16 and 22 3/7 weeks with an endovaginal CL <30 mm (<10th percentile in this population) were randomized to weekly intramuscular 17-OHP (250 mg) or placebo through 36 weeks. The primary outcome was PTB <37 weeks. Results: The frequency of PTB did not differ between the 17-OHP (n = 327) and placebo (n = 330) groups (25.1% vs 24.2%; relative risk, 1.03; 95% confidence interval, 0.79-1.35). There also was no difference in the composite adverse neonatal outcome (7.0% vs 9.1%; relative risk, 0.77; 95% confidence interval, 0.46-1.30). Conclusion: Weekly 17-OHP does not reduce the frequency of PTB in nulliparous women with a midtrimester CL <30 mm. © 2012 Mosby, Inc.