Preterm birth affects 12% of all births in the United States. Recent studies support the hypothesis that progesterone supplementation reduces preterm birth in a select group of women (ie, those with a prior spontaneous birth at <37 weeks of gestation). Despite the apparent benefits of progesterone in this high-risk population, the ideal progesterone formulation is unknown. The American College of Obstetricians and Gynecologists Committee on Obstetric Practice believes that further studies are needed to evaluate the use of progesterone in patients with other high-risk obstetric factors, such as multiple gestations, short cervical length, or positive test results for cervicovaginal fetal fibronectin. When progesterone is used, it is important to restrict its use only to women with a documented history of a previous spontaneous birth at less than 37 weeks of gestation because unresolved issues remain, such as optimal route of drug delivery and long-term safety of the drug.