There is growing interest in the use of microbial-seeding interventions to mitigate the impacts of prenatal antibiotics, C-section, and lack of breastfeeding on mother-child microbe sharing. However, the relative importance of maternal vaginal vs. fecal microbiota in this process is unclear. Analyzing 16S rRNA sequences from five US birth cohorts, we found that maternal vaginal and fecal microbiota became more similar as pregnancy progressed, and both niches influenced the child's fecal microbiota. The relative contribution of maternal vaginal microbiota increased when vaginal sampling occurred later in gestation. As children aged from birth to 5 years, their fecal microbiota increasingly resembled their mother's fecal microbiota as compared to vaginal microbiota. Patterns of sharing appeared to differ by prenatal antibiotic use, birth mode (C-section vs. vaginal), and breastfeeding. Our findings enhance understanding of niche-specific mother-child microbe sharing and may inform microbial-seeding interventions. Metagenomic studies are needed to identify specific shared strains.