Long-term exposure to prenatal fluoride and social-emotional and behavioural outcomes in children in the New Hampshire Birth Cohort Study

Background and Objectives Prenatal fluoride exposure may pose risks for maladaptive social-emotional and behavioural development, but evidence using biomarkers that integrate long-term prenatal exposure is limited. This study examined associations between maternal toenail fluoride at multiple time points and water fluoride on preschool social-emotional and behavioural outcomes and evaluated potential differences by child sex and exposure window. Methods Participants included 358 mother-child dyads from the New Hampshire Birth Cohort Study using private, unregulated household water systems. Fluoride concentrations were measured in maternal toenails collected at ∼28 weeks’ gestation (reflecting early-pregnancy exposure) and 5 weeks postpartum (reflecting mid-to-late-pregnancy exposure), and in household tap water samples collected at ∼24-28 weeks’ gestation. Children’s social-emotional and behavioural functioning was assessed at approximately 5 years old using the Behavior Assessment System for Children–Second Edition, Parent Rating Scale-Preschool Form . Primary composite scores were dichotomized as at-risk (T-score ≥ 60). Covariate-adjusted generalized estimating equations estimated associations between toenail fluoride and at-risk outcomes, and multiple logistic regressions estimated associations between water fluoride and at-risk outcomes, testing sex and exposure-window interactions. Results A 1-µg/g increase in toenail fluoride concentration reflecting early pregnancy exposures, was associated with higher odds of at-risk Externalizing Problems (OR=1.39; 95% CI: 1.15-1.67). Toenail fluoride concentrations reflecting late-pregnancy exposures, showed a similar association (OR=1.30; 95% CI: 0.99-1.71). No associations were observed for other behavioural composites, or with water fluoride concentrations, with no apparent differences by sex or exposure window for the behavioural composites . Conclusions Higher maternal toenail fluoride concentrations, but not water fluoride concentrations, were associated with greater odds of at-risk externalizing problems in preschoolers in this U.S. cohort. Generalizability may be limited because participants were predominantly White, college-educated families from rural New England using private water systems. Further research incorporating both individual-level biomarkers and population-level exposure measures, together with information on major fluoride sources (e.g., drinking water), is needed to clarify the relationship between prenatal fluoride exposure and child neurobehavioural development.