Hydroxyprogesterone caproate is a synthetic progestin. The mechanism by which hydroxyprogesterone prevents preterm delivery is not fully understood, partially because the mechanisms for the onset of preterm labor are not fully understood. Progesterone is thought to be an important hormone in the maintenance of pregnancy both in early gestation as well as later in pregnancy. Progesterone is a smooth muscle relaxant in several organs, including the uterus during pregnancy. Progesterone also inhibits oxytocin's actions on the myometrium, and it prevents the formation of gap junctions between myometrial cells; formation of gap junctions are necessary for labor initiation.