17-alpha hydroxyprogesterone caproate (17-OHPC) 250 mg weekly reduces recurrent spontaneous preterm birth (SPTB) in women with a prior SPTB by 33%. The dose is not based on pharmacological considerations. A therapeutic concentration has not been determined hampering any attempt to optimize treatment. This study evaluated the relationship between 17-OHPC plasma concentrations and the rate of SPTB in women with singleton gestation
