RCT of Progesterone to Prevent Preterm Birth in Nulliparous Women With a Short Cervix

Inclusion Criteria: * Nulliparous * Cervical length as measured on transvaginal examination \< 30mm between gestational ages of 16 weeks 0 days to 22 weeks 3 days * Gestational age 16 weeks 3 days to 22 weeks 6 days at time of randomization Exclusion Criteria: * Multifetal gestation * Progesterone treatment after 14 weeks 6 days during current pregnancy * Vaginal bleeding, heavier than spotting, after 15 weeks 6 days * Amniotic membranes prolapsed beyond external os * Preterm rupture of membranes * Fetal anomaly * Pregnancy without a viable fetus * Current or planned cervical cerclage * Congenital Mullerian abnormality of the uterus * Contraindication to intra-muscular injections * Hypertension requiring medication * Diabetes managed with insulin or oral hypoglycemic agents * DES exposure * Cervical surgery such as cold knife conization * Planned indicated preterm delivery * Participation in another interventional study that influences age at delivery * Participation in this trial in a previous pregnancy * Prenatal care or delivery planned outside a MFMU Network center