To evaluate whether daily oral micronized progesterone is effective in preventing recurrent spontaneous preterm birth (RSPB) and whether micronized progesterone use increases maternal serum progesterone levels.

To evaluate whether 400 mg daily oral micronized progesterone from 16 to 34 weeks' is effective in preventing recurrent spontaneous preterm birth (RSPB) and whether micronized progesterone use increases maternal serum progesterone levels.

Inclusion Criteria:

• pregnant women less than 20 weeks' gestation who had at least one prior spontaneous preterm birth of a liveborn baby between 20 & 0/7 weeks' and 36 & 6/7 weeks' gestation.

Exclusion Criteria:

• multiple gestations, the presence of major fetal anomalies, progesterone use in the current pregnancy (ongoing or past), the presence of a cervical cerclage, and the presence of a placenta previa.

From trial entry until delivery of the infant, average (+/- std dev) gestational age at randomization was 18.2 +/- 2.7 weeks for placebo group and 16.9 +/- 2.6 weeks for progesterone group; Average gestational age at delivery was 35.9 +/- 3.8 weeks for placebo group, and 37.0 +/- 2.7 weeks for progesterone group. The average duration adverse events were collected was 17.7 weeks for the placebo group, and 20.1 weeks for the progesterone group.

All subjects were queried by an investigator at every prenatal visit regarding adverse events, and the electronic medical record was searched for any occurrences.