Progesterone therapy given to pregnant women with a history of preterm birth could reduce the national preterm birth rate by approximately 2% (or about 10,000 births) annually,
These results are according to a new study by the National Centers for Disease Control and Prevention, the National Institute of Child Health and Human Development and the March of Dimes, as published in the February 2005 issue of Obstetrics & Gynecology.

Preterm birth -- defined as birth occurring before 37 weeks gestation -- affects approximately 12% of all live births in the US. It is a leading cause of infant death and long-term disability. Nationally, the preterm birth rate increased 27% between 1982 and 2002. While the cause of spontaneous preterm birth is unknown, a history of spontaneous preterm birth is one of the strongest predictors for a preterm birth in a subsequent pregnancy. To date, there are no widely established treatments to prevent preterm birth. However, some studies have shown that administering weekly progesterone (17 alpha-hydroxyprogesterone caproate or '17P') injections beginning between 16 and 20 weeks of gestation to women with a history of preterm birth reduces the chance of preterm births in subsequent pregnancies by as much as 33%.

Researchers analyzed 2002 national birth certificate data, augmented by vital statistics from Missouri and New Jersey. Using these data, they estimated that approximately 30,000 recurrent preterm births occurred in the US to pregnant women who would have been eligible for 17P therapy. Women were classified as eligible for 17P if they had a history of preterm birth, were carrying a single fetus, and received prenatal care within the first four months of pregnancy which is when weekly therapy must begin. The researchers applied the reported estimate that a third of these preterm births could have been prevented if these women had received 17P. The researchers conservatively project that nearly 133,000 pregnant women may be eligible for the treatment, and this number would likely be higher if more women received prenatal care in the first trimester.

Although the overall effect of 17P therapy on the US preterm birth rate is likely to be modest, 17P is a promising intervention for women at high risk of recurrent preterm birth. The researchers point out that there needs to be long-term follow-up of women and infants exposed to progesterone treatment during pregnancy to ensure its safety. They also note that there is no commercial production of 17P that is licensed for use during pregnancy and that it is primarily available only through a limited number of compounding pharmacies, thus limiting its potential use.

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