AbstractThe vaginal microbiota refers to the microorganisms that reside in the vagina. These microorganisms contribute significantly to a woman’s reproductive and general health. A healthy vaginal microbiota is typically a low-diversity environment with a predominance of lactic acid-producing Lactobacillus species. Factors such as antibiotic use, sexual activity, and hormonal changes can disrupt the balance of the vaginal microbiota, leading to conditions such as bacterial vaginosis. The composition of the vaginal microbiota changes and takes on added importance during pregnancy, serving as a barrier against infection for both mother and fetus. Despite the importance of the microorganisms that colonize the vagina, details of how changes in composition and diversity can impact pregnancy outcomes is poorly understood. This is especially true for woman with a high prevalence of Gardnerella vaginalis. Here we report on a diverse cohort of 749 women, enrolled in the InSPIRe cohort, during their final trimester of pregnancy. We show that Lactobacilli, including L. crispatus are important in maintaining low diversity, and that depletion in this critical community is linked with preterm delivery. We further demonstrate that it is overall diversity of the vaginal microbiota, not specific species, which provides the best indicator of risk.