Washington, DC -- The hormone progesterone may be used as treatment to help prevent preterm birth but should be restricted to pregnant women with a documented history of preterm birth before 37 weeks gestation, according to a committee opinion issued today by The American College of Obstetricians and Gynecologists' (ACOG) Committee on Obstetric Practice.

ACOG's new opinion is based on two recent randomized placebo controlled trials. One large study compared birth outcomes of a select group of high-risk pregnant women who were given either a weekly intramuscular progesterone (17 alpha hydroxyprogesterone caproate) injection beginning at 16-20 weeks of gestation through 36 weeks or until delivery, or a weekly placebo injection. All of the women had a history of previous spontaneous preterm birth at less than 37 weeks gestation. The study was halted early because the results showed a significant protection against preterm birth. Pregnant women who received the progesterone injection had up to 42 percent fewer premature births than women given the placebo. A smaller study showed that pregnant women with histories of preterm deliveries who received vaginal progesterone suppositories also had a significantly lower preterm birth rate (2.7 percent) compared to a placebo group (18.6 percent).

Prematurity, accounting for 12 percent of all births in the US, is the major cause of newborn illness and death. Premature births are those occurring anytime before the 37th week of gestation. Women who have had a prior preterm delivery, who are African American, who have vaginal bleeding, who have concurrent sexually transmitted diseases, or who weigh less than 110 pounds are at an increased risk for preterm birth. To date, there have been no preventive interventions that have reliably decreased the preterm delivery rate.

Despite the apparent benefit of this formulation of progesterone in a specific subset of high-risk women, the ideal progesterone formulation remains unknown until further research is done. According to ACOG, more research is needed to determine if women with other high-risk factors including multiple gestations, short cervical length, or a positive fetal fibronectin test can benefit from progesterone treatment. Because the optimal route of drug delivery and the long-term safety of the treatment remain unknown, it is important to restrict this treatment to women with a documented history of spontaneous preterm birth at less than 37 weeks gestation.

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