We sought to evaluate the effectiveness of daily oral micronized progesterone (MP) in preventing recurrent spontaneous preterm birth (RSPB) and whether MP increases maternal serum progesterone. We performed a pilot, single-center, randomized, double-blind, placebo-controlled trial in women with a prior preterm birth and current singleton gestation at 16 to 20 weeks ( N = 33). The primary outcome was the rate of RSPB. Subjects were given either daily MP (400 mg) or placebo from 16 to 34 weeks. Serum progesterone was obtained at enrollment and in the late second/early third trimester. Pregnancy outcome data were collected. RSPB occurred in 5/19 (26.3%) in the MP group versus 8/14 (57.1%) in placebo group ( P = 0.15). The mean age at delivery was 37.0 ± 2.7 weeks for the MP group versus 35.9 ± 2.6 weeks for the placebo ( P = 0.3). Mean serum progesterone at 28 weeks was 122.6 ± 61.8 pg/mL for MP group versus 90.1 ± 38.7 pg/mL for placebo ( P = 0.19). MP was associated with a trend toward a reduction in RSPB and an increase in the maternal serum progesterone. Although the primary outcome in this pilot study did not reach statistical significance, the results suggest a favorable trend meriting further investigation.
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