This is a prospective cohort, enriched with women with a prior term birth, of singleton pregnancies who will be followed for the outcome of preterm birth. The main exposure of interest is the characterization of the cervico-vaginal microbiota. Women will be recruited from a population seeking routine obstetrical care at obstetrical practices at Penn. They will have three research visits during pregnancy to complete questionnaires and collect samples of cervico-vaginal fluid and cervical cells. Outcome data will be collected about the prenatal events and timing of delivery.