Delivery mode, birth order, and sex impact neonatal microbial colonization.

The initial microbial colonization of the infant gut during birth plays a critical role in shaping both immediate and long-term health outcomes. While mode of delivery is a known determinant of this colonization process, the potential impacts of infant sex and birth order remain underexplored. This study investigates the influence of delivery mode, infant sex, and birth order (maternal parity) on the microbial communities in first-pass meconium samples from neonates, using 16S rRNA gene sequencing. We found that delivery mode impacted the presence of detectable microbial communities. Specifically, only 17% of samples from neonates delivered by elective Cesarean section showed any microbial presence, compared to approximately two-thirds of samples from neonates exposed to maternal vaginal microbes (emergency C-section or vaginal delivery). Among vaginally delivered neonates without antibiotic exposure, birth order was associated with taxonomic shifts. Neonates born to primiparous mothers had a lower abundance of Bifidobacterium, a keystone species in the infant gut microbiome. Unexpectedly, the gut microbiota differed by infant sex, with males having lower alpha diversity and shifts in microbial community composition (PERMANOVA = 0.008), characterized by elevated levels of Enterobacteriales, which was both less prevalent and less abundant in female neonates. These findings highlight the intricate interplay between delivery mode, infant sex, and birth order in shaping the early gut microbiome.