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.