Progesterone and Preterm Labor — Still No Definite Answers

In 1930, Willard Allen and George Corner named the steroid hormone that they had extracted from rabbit ovaries “progestin” because they believed, rightly, that it helped maintain pregnancy.1 Removal of progesterone, or blockage of its receptors, reliably ends pregnancy at all gestational ages, and in many nonhuman species, falling levels precipitate natural labor.2 Progestins thus have been an obvious candidate treatment for the major health problem of preterm labor. Therapeutic studies in pregnancy have been limited because natural progesterone given orally is rapidly metabolized in the liver, and synthetic progestins that are not rapidly metabolized have androgenic effects, giving rise . . .