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Can Progesterone Prevent Preterm Birth in At-Risk Singleton Pregnancies?

Vaginal progesterone lowered the rates of preterm birth in women with a short cervix.

Preterm birth is the leading cause of neonatal mortality and morbidity worldwide, and previous findings have suggested that administration of progesterone reduces the likelihood of preterm birth. To identify a population at risk for preterm delivery, investigators assessed cervical length in consenting women with singleton or twin pregnancies scheduled for routine ultrasonography at 20 to 25 weeks’ gestation. Women with cervical length 15 mm were asked to participate in a randomized international trial of vaginal progesterone. Participants (median age, 29; 56% nulliparous; 55% black) inserted micronized progesterone (200 mg) or placebo capsules vaginally at bedtime from 24 through nearly 34 weeks’ gestation; 226 had singleton pregnancies, and 24 had twin pregnancies.

Significantly fewer spontaneous deliveries occurred before 34 weeks’ gestation with progesterone than with placebo (19.2% vs. 34.4%). Neonatal morbidity was lower with progesterone (8.1% vs. 13.8%), but this reduction did not reach statistical significance. With progesterone, a nonsignificant reduction in preterm delivery was observed in the small number of twin pregnancies. Overall, use of progesterone was not associated with increased incidence of adverse maternal effects.

Comment: These results indicate that in women with a sonographically identified short cervix, vaginally administered progesterone can reduce the risk for preterm birth. Although epidemiologic and animal studies have not identified significant associations between prenatal exposure to progestational drugs and congenital anomalies, more data on long-term safety would be welcome. The authors state that their findings support administration of progesterone to those with a short cervix. The American College of Obstetricians and Gynecologists Committee on Obstetric Practice recommends that progesterone be considered for women at high risk for preterm birth but also notes that the ideal formulation, optimal route of delivery, and long-term safety are unknown in the context of such risk. An editorialist suggests that high-risk women should enroll in a trial rather than receive progesterone as an unproven treatment, given the "remaining uncertainties about both efficacy and fetal safety."

— Andrew M. Kaunitz, MD

Published in Journal Watch Women's Health August 1, 2007

Citation(s):

Fonseca EB et al. Progesterone and the risk of preterm birth among women with a short cervix. N Engl J Med 2007 Aug 2; 357:462-9.

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Thornton JG. Progesterone and preterm labor — Still no definite answers. N Engl J Med 2007 Aug 2; 357:499-501.

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