 The recently completed trial by the National Institute of Child Health and Human Development NICHD-sponsored Maternal-Fetal Medicine Units MFMU Network has demonstrated that intramuscular 17-alpha-hydroxyprogesterone caproate 17-OHPC substantially reduces the rate of preterm birth in women at high risk for preterm delivery because of a prior spontaneous preterm birth. No other strategy or treatment for prevention of preterm birth has proven to be effective. Consequently, the American College of Obstetricians and Gynecologists has cautiously supported this treatment but points out that much more information about this therapy and alternative therapies is required. Although a large body of evidence exists about the safety of this treatment, almost nothing is known about the pharmacology of this agent, especially in pregnancy. The purpose of this study is to define the pharmacology of 17-hydroxyprogesterone caproate in pregnancy. This protocol will focus on pharmacokinetics and placental transport and provide preliminary data on the pharmacoepidemiology of 17-OHPC. The primary research question of this study is: Do the pharmacokinetics of 17-OHPC as represented by area under the concentration vs. time curve after IM injection of 250 mg 17-OHPC differ between the second and third trimesters of pregnancy? We will obtain blood samples prior to and daily for one week after injection of 17-OHPC 8 samples total for each of two parts of the study, with an optional third part for eligible subjects. Additionally, blood samples will be collected prior to each weekly injection of the study drug and at time of delivery. Approximately 60 subjects ages 18-45 will be accrued at one of the Obstetrical Fetal Pharmacology Research Units OPRU Network sites, with 15 at Magee-Womens Hospital of the University of Pittsburgh Medical Center. Study treatment will be administered until delivery. The total duration of this multi-center study is 2-3 years. 