Maternal immunity, cesarean delivery, and childhood neuropsychiatric risk in 1.18 million births

Maternal infection, immune disease, and delivery mode are plausible influences on early brain development. We analyzed 1,179,611 US Merative MarketScan mother-child pairs (2003–2024), including 259,339 non-twin siblings in 123,926 families. Population models screened 18 perinatal exposures against 13 childhood psychiatric/neurodevelopmental diagnosis-count outcomes; sibling fixed effects tested robustness to stable family-level confounding. Cesarean delivery was associated with higher composite neurodevelopmental diagnosis counts in pairs (23.4%) and siblings (25.0%) and with ADHD in siblings (38.8%; FDR q = 0.025). Autism was elevated in pairs (20.0%) but not supported within families (5.0%; p = 0.87). Claims-defined no-labor/no-repeat cesarean showed stronger lower-risk-birth associations for composite neurodevelopmental burden (48.0%), autism (44.9%), speech/language disorders (41.0%), and ADHD (24.1%). Maternal infection/immune-mediated disease, preterm birth, and advanced maternal age were additional population signals.