The Yale Progesterone Study is a randomized, placebo-controlled trial of the use of 17 hydroxyprogesterone for the treatment of preterm labor. The design is similar to the Meis NEJM trial, except that the patients will be symptomatic with preterm labor, rather than asymptomatic with a history of preterm delivery. In addition to the therapeutic intervention planned, the researchers intend to collect specimens to assess for markers of PTD, both before and after treatment. In this way, the researchers can analyze which pathway of PTD is involved, and finally, the effect of progesterone on these markers can be assessed.