Mechanisms of the Oral–Gut Microbiota Axis in Adverse Pregnancy Outcomes

Adverse pregnancy outcomes (APOs), including preterm birth, preeclampsia, low birth weight, recurrent miscarriage, gestational diabetes mellitus, and fetal growth restriction, remain major threats to maternal and offspring health. Increasing evidence links the maternal microbiome to pregnancy health, but most studies have examined individual microbial niches rather than their interactions. The oral cavity and gut are anatomically and immunologically connected and form a bidirectional oral–gut microbiota axis through microbial trafficking, immune signaling, and metabolite-mediated feedback. Emerging studies suggested that oral dysbiosis, periodontal inflammation, and gut microbial remodeling were associated with APOs, although direct causal evidence in human pregnancy remains limited. This review summarizes pregnancy-related remodeling of the oral–gut microbiota axis, evaluates clinical and experimental evidence linking oral and gut dysbiosis to APOs, and discusses potential mechanisms, including microbial translocation, immune and inflammatory activation, metabolic remodeling, epigenetic regulation, and outer membrane vesicle-mediated signaling. Candidate biomarkers, probiotic and dietary intervention strategies, and current translational limitations are also discussed. Overall, the oral–gut microbiota axis offers a useful framework for understanding microbiome-associated APOs, but standardized sampling, longitudinal cohorts, and mechanistic validation are required before clinical application.