DISCUSSION
Our study is the first double-blind, randomized, placebo-controlled trial to determine whether vaginal seeding causes differential engraftment of maternal bacteria in the skin and stool of neonates. The study has important limitations, such as a small sample size and only 2 time points until 30 days of life. Despite these limitations, we observed significant effects of vaginal seeding on the neonatal microbiota. Vaginal seeding of C-section-delivered neonates increased mother-to-neonate microbial transfer and changed the neonatal microbiota in different body sites, urging further studies to determine whether these changes provide health benefits to the more than 1 million babies annually born by C-section in the United States (The maturation of the stool microbiota in mammals progresses first with a postpartum reduction of the stool diversity—demonstrated in mice (C-section-born neonates have, in relation to their vaginally born counterparts, a higher proportion of potential pathobionts, such as from the Consistent with infants born vaginally showing stool bacterial communities resembling their mother’s vaginal microbiota and C-section-delivered infants showing skin-like microbiota (Our findings also support the growing recognition that microbiota from different maternal body sites are important for infant microbiome maturation (In conclusion, vaginal seeding caused changes in the neonatal skin and stool microbiota, leading to a pattern of reduced bacterial diversity that is characteristic of the microbiota of vaginally delivered and breastfed infants. There is now a critical need to evaluate the health benefits and safety of vaginal seeding in large randomized controlled trials.