It is well documented that women with robust, Lactobacillus-dominated vaginal microbiotas are less likely to develop a range of adverse reproductive health outcomes. The birthing process is thought to be a critical event in the initial seeding, or colonization, of the human microbiome, and the transfer of microbiota from mother to baby during delivery is associated with long-term health. We recruited 88 adult women to a cross-sectional study to evaluate the relationship between their vaginal microbiota and the mode of their birth (self-reported as vaginal delivery or Cesarean section (C-section)). In a multivariable analysis, women who had a less protective, low-Lactobacillus community state type had 3-fold increased odds of having been born via C-section, indicating that C-section is related to vaginal dysbiosis in adulthood (adjusted OR: 3.73, p=0.01, 95% CI: 1.08-12.77). Although the cross-sectional analysis does not account for fluctuations in microbial composition, the significant point estimate suggests that birth mode may play a role in vaginal seeding and colonization outcomes in adult women.