Vaginal Progesterone to Reduce the Risk of Another Preterm Birth

This research study is being conducted at over 12 pregnancy research centers in the US. The study will compare an investigational treatment with a placebo (a treatment without medication). Neither the investigators nor the patients in the trial will know which treatment has been assigned. All study medications will be given vaginally once a day. Treatment will begin before pregnancy week 23 and will continue until the end of pregnancy week 36.