Relationship between 17-alpha hydroxyprogesterone caproate concentration and spontaneous preterm birth - Caritis, S N; Venkataramanan, R; Thom, E; Eunice Kennedy Shriver National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network and Obstetric-Fetal Pharmacology Research Units Network (Am J Obstet Gynecol, 2014)