The mechanisms of action of 17-alpha Hydroxyprogesterone Caproate in prolonging gestation are not entirely known. They include relaxation of myometrial smooth muscle, blocking of the action of oxytocin, and inhibition of the formation of gap junctions. There is also evidence that local changes in the progesterone level or the ratio of progesterone to estrogen in the placenta, decidua, or fetal membranes may be important in the initiation of labor in humans. As for its safety profile, aside from local injection-site reactions like soreness, swelling, itching and bruising, 17-alpha Hydroxyprogesterone Caproate appears to be safe during pregnancy with absence of teratogenic effects. The safety of 17-alpha Hydroxyprogesterone Caproate administration in pregnancy is well documented by animal and clinical studies. Reviews of this topic by knowledgeable authors have uniformly concluded that no evidence exists that administration of 17-alpha Hydroxyprogesterone Caproate in pregnancy represents a significant risk to mother, fetus or newborn.