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Progesterone helps maintain pregnancy, so in theory it's the obvious prophylactic against preterm birth. Trials have been inconsistent, however, and the latest two are no exception. In the first, vaginal progesterone (200 mg) used daily from 24 to 34 weeks' gestation reduced the risk of preterm birth among high risk women with a short cervix (relative risk 0.56, 95% CI 0.36 to 0.86 among placebo controls). In the second, intramuscular 17 alpha-hydroxyprogesterone had no effect on the risk of birth or fetal death before 35 weeks among women with twins, another high risk group (1.1, 0.9 to 1.3). In both trials, the babies of treated mothers did no better than the babies of controls.
A linked editorial says the discrepancy between the trials is surprising, despite their methodological differences (p 499). If anything, progesterone should have worked better for women with twins. In singleton pregnancies, infection is likely to be a bigger contributor to preterm delivery than hormones. The data so far, along with ongoing worries about the effects of exogenous hormones on the growing fetus, mean that progesterone should probably remain an experimental treatment for women at high risk of preterm delivery, says the editorial's author. We will get a reliable answer sooner or later. At least 16 other trials are in the pipeline.pipeline.