Preterm delivery remains one of the most important issues facing perinatal medicine today. In 1999, prematurity/low birthweight accounted for 4,304 neonatal deaths, reflecting a rate of neonatal mortality due to prematurity of 23.0 per 100,000 live births. Despite the extent of the problem, the exact etiology of preterm delivery is not completely understood. It is clear that many pathways are involved in preterm delivery, and that ultimately these must converge upon one final endpoint, which is likely related to progesterone. In the animal model progesterone withdrawal is clearly directly (rodent, rabbit) or indirectly (sheep) involved in the initiation of parturition, however the exact way in which progesterone works in humans is unclear. There has been a resurgence of interest in the association between progesterone and preterm delivery. Two recent trials have looked at the utility of progesterone in the prevention of preterm delivery in high-risk patients. In a multicenter trial reported in the New England Journal of Medicine in 2003, Meis et al, recruited 463 patients with a history of spontaneous preterm delivery and randomized them in a 2:1 ratio to intramuscular 17-hydroxyprogesterone vs. placebo from 16-20 weeks until 36 weeks. Treatment with 17P significantly reduced the risk of delivery at <37 weeks, <35 weeks, and <32 weeks.