Level 2: Likely reliable evidence

The risk of preterm birth is increased in women with a short cervix. Intramuscular or vaginal progesterone appears effective for reducing premature delivery, but data on neonatal outcomes have been limited (Cochrane Database Syst Rev. 2009;1:CD004947).

A recent randomized trial evaluated a new progesterone gel in 465 women at 19 to 23 6/7 weeks’ gestational age (Ultrasound Obstet Gynecol. 2011;28:18-31). Women with singleton pregnancy and cervical length 10-20 mm on sonography were randomized to a vaginal gel with progesterone 8% vs. placebo until 36 6/7 weeks, rupture of membranes or delivery.

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The progesterone group had significantly reduced rates of preterm birth before 28 weeks (5.1% vs. 10.3%; P=0.04; number needed to treat=20), before 33 weeks (8.9% vs. 16.1%; P=0.02; NNT=14), and before 35 weeks (14.5% vs. 23.3%; P=0.02; NNT=12).

Babies born to mothers in the progesterone group had reduced risk of neonatal respiratory distress syndrome (3% vs. 7.6%; P=0.03; NNT=22) and reduced incidence of birth weight <1,500 g (6.4% vs. 13.6%; P=0.01; NNT=14).

There were no significant differences in neonatal mortality (1.3% vs. 2.2%), in days in neonatal intensive care unit, or in birth weight <2,500 g. There were also no significant differences in adverse events.

Alan Ehrlich, MD, is an assistant clinical professor in family medicine at the University of Massachusetts Medical School in Worcester.