Preterm birth (PTB) is the leading cause of infant mortality and incurs billions of dollars per year in avoidable health costs. The causes of preterm birth are not well understood but are thought to originate from complex demographic, biological and behavioral factors. More recently vaginal dysbiosis - disruptions in the vaginal microbiota - have been shown to be associated with PTB. Studies have also shown that the composition and the frequency and duration of dysbiosis in the vaginal microbiota may be affected by the same demographic, biological and behavioral factors that are known predictors of preterm birth. This proposed study will examine the contribution that social and bio-behavioral factors known to be associated with PTB have on the composition of the vaginal microbiota as well as the frequency and duration of vaginal dysbiosis. The overall goal of this study is to elucidate the relationship between social and bio-behavioral factors, the vaginal microbiota and birth gestation. Specific aims include characterizing the effect of social and bio-behavioral factors on birth gestation (Aim 1) and on the vaginal microbiota (Aim 3), characterizing the effect of the microbiota on birth gestation (Aim 2) and quantifying the role of the vaginal microbiota in the relationship between social and bio-behavioral factors and birth gestation (Aim 4). In this prospective cohort study, 400 women will be followed from 20 weeks gestation through to birth. By recruiting pregnant women from neighborhoods in Baltimore City with higher than average PTB rates we expect that at least 16% of the cohort will experience premature birth. Weekly self-collected mid-level vaginal swabs will be obtained using a validated protocol. Diet information, vaginal and prenatal health behavior data will be collected via a web- based application specifically designed for this study. Structural equation modeling and mixed effects models will be used to test the hypotheses. The sample for Aims 1, 3, and 4 will include the full cohort while Aims 2-4 will use a subsample of cases that deliver preterm and age-matched controls. Vaginal swabs and pH for the women that experience PTB and the age-matched control will be analyzed using r16S culture-independent techniques. Findings will provide essential knowledge about the influence that modifiable social and bio- behavioral factors have on the composition and frequency and duration of vaginal dysbiosis during pregnancy the PTB.