Daily vaginal progesterone initiated at 16 weeks was a reasonable alternative to weekly 17-alpha-hydroxy-progesterone caproate (17-OHPC) for the prevention of spontaneous preterm birth in women with singleton gestations and prior spontaneous preterm birth, a systematic review in Obstetrics & Gynecology reported.

There have been recent randomized controlled trials comparing intramuscular 17-OHPC vs. vaginal progesterone for reducing the risk of spontaneous preterm birth in singletons with prior spontaneous preterm birth. Researchers sought out all trials of asymptomatic singleton gestations with prior spontaneous preterm birth who were randomized to prophylactic vaginal progesterone (intervention group) or intramuscular (IM) 17-OHPC (comparison group). The primary outcome was spontaneous preterm birth <34 weeks. Other outcomes included spontaneous preterm birth <37 weeks, <32 weeks, <28 weeks, and <24 weeks, maternal adverse drug reaction and neonatal outcomes. 

Three randomized controlled trials enrolling 680 women were included for the review. Women were administered progesterone until 36 weeks or delivery. One study used 90mg progesterone vaginal gel daily; one 100mg vaginal suppository daily; and the other one 200mg suppository daily. The 3 trials used 250mg 17-OHPC weekly as the comparison group. 

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The analysis found women who were given vaginal progesterone had a significantly reduced rate of spontaneous preterm birth <34 weeks (17.5% vs. 25.0%; relative risk [RR] 0.71, 95% CI: 0.53-0.95) and <32 weeks (8.9% vs. 14.5%; RR 0.62, 95% CI: 0.40-0.94) vs. women who received 17-OHPC. 

No significant differences were seen in the rate of spontaneous preterm birth <37 weeks, <28 weeks, and <24 weeks. 

There was a significantly lower rate of women who experienced adverse drug reactions in the vaginal group vs. 17-OHPC group (7.1% vs. 13.2%; RR 0.53, 95% CI: 0.31-0.91). In addition, use of vaginal progesterone was associated with a lower rate of neonatal intensive care unit admission vs. 17-OHPC (18.7% vs. 23.5%; RR 0.63, 95% CI: 0.47-0.83). 

Due to imprecision and indirectness, the quality of evidence was downgraded for all outcomes by at least 1 degree. As the evidence estimates were low or very low as assessed by GRADE, it indicates that the true effect may be substantially different from the estimate of the effect, authors concluded.

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