Progesterone is a naturally occurring progestin. In the body, it is synthesized in the ovaries, testes, placenta, and adrenal cortex. Progesterone is primarily used to treat amenorrhea, abnormal uterine bleeding, or as a contraceptive. Progesterone is also used to prevent early pregnancy failure in women with corpus luteum insufficiency, including women undergoing assisted reproductive technology (ART). Additionally, the use of progesterone for preterm delivery prophylaxis is being investigated. Preliminary data indicate that progesterone may be effective in preventing preterm delivery in high-risk women, especially those with a history of preterm delivery;1 however, the optimal dosage and route have not been determined. The American College of Obstetricians and Gynecologists (ACOG) Committee recommends that if progesterone is to be used for the prevention of preterm delivery, it should only be used in women with a history of spontaneous birth at < 37 weeks gestation, until more data supporting its use in other high-risk women are available.2 A study in support of the use of vaginal progesterone in women with a short cervix has been published.3 Progesterone is available commercially as an intramuscular injection, an intravaginal gel, an intravaginal insert, oral capsules, or a powder for use in extemporaneous preparations (e.g., vaginal suppositories). A progesterone-releasing IUD (Progestasert®) that was inserted once yearly has been discontinued. Progesterone was approved by the FDA in 1939. In May 1998, micronized progesterone capsules for oral administration were approved for secondary amenorrhea; they received a second indication in December 1998 for the prevention of endometrial hyperplasia in postmenopausal women with an intact uterus taking estrogen replacement therapy. In May 1997, a progesterone vaginal gel (Crinone®) was approved for progesterone supplementation or replacement as part of an Assisted Reproductive Technology (ART) program for infertile women; a second intravaginal gel, Prochieve™, was released to the US market in 2002. A vaginal insert, Endometrin®, for progesterone supplementation as part of an ART program in infertile women was approved in June 2007.