Abstract
Progestin supplementation appears to be a promising approach to both preventing initiation of preterm labor and treating it once it is already established, given progesterone’s role in maintaining pregnancy as well as support from basic and clinical research. Progesterone and 17- alpha-hydroxyprogesterone-acetate (17-OH P) slow the process of cervical ripening and this is the rationale for prophylactic long-term progestin supplementation mostly studied so far. However, progesterone (but not 17-OH P) also inhibits myometrial activity even after the cervix is already ripened. Moreover, these effects depend greatly on the vehicle used and the route of administration. Understanding different mechanisms of action, as well as the importance of progestin formulation, vehicle, and route of administration is the key to finding the optimal progestin treatment for prevention of preterm birth.