Community Water Fluoridation and Birth Outcomes.

Importance: Community water fluoridation (CWF) is a widely implemented public health intervention aimed at preventing dental caries. However, concerns have emerged about potential unintended birth outcomes of fluoride exposure, particularly birth weight, which is a widely accepted summary measure of infant health and has been found to be associated with later-life health and human capital. Objective: To evaluate the association between prenatal exposure to CWF and birth outcomes. Design, Setting, and Participants: This cohort study was conducted using an event-study analysis with a difference-in-differences (DID) approach that exploited the staggered rollout of CWF across US counties from January 1968 to December 1988. The study included birth outcomes from singleton births obtained from the National Vital Statistics System's Natality Detail Files. The analysis compared within-county changes in birth outcomes before and after the introduction of CWF, using counties that never fluoridated or had not yet fluoridated as the control. Data were analyzed between February 4 and October 28, 2025. Exposure: County-level CWF. Main Outcomes and Measures: The primary outcome was change in mean birth weight, and secondary outcomes were incidence of low birth weight (<2500 g), gestational length (weeks), and incidence of prematurity (gestational age <37 weeks). Community water fluoridation exposure was measured as the population-weighted share of county residents served by fluoridated water based on Centers for Disease Control and Prevention Fluoridation Census data. Results: The study sample included 11 479 922 singleton births (mean [SD] gestational age, 39.5 [0.8] weeks; 51.2% boys; mean [SD] birth weight, 3337.4 [172.8] g; highest mean [SD] maternal age proportions, 0.36 [0.13] aged 20-24 years and 0.27 [0.12] aged 25-29 years) across 677 counties (408 CWF treated [60.3%] and 269 [39.7%] never treated) over the 21-year period, aggregated to the county-month level. No evidence of an association of CWF with birth weight or other birth outcomes was found (DID estimate, -0.53; 95% CI, -4.75 to 3.70). Event-study estimates showed no discernible pretreatment trends and no significant changes following CWF adoption, with estimates small in magnitude across all posttreatment periods, ranging from -8.44 g (95% CI, -20.41 to 3.53 g) to 7.20 g (95% CI, -5.45 to 19.85 g). Findings were supported by sensitivity analyses incorporating state-specific time trends, alternative exposure thresholds, and tests for compositional changes in births. Conclusions and Relevance: This cohort study of more than 11 million births found no association of CWF with adverse birth outcomes. These findings provide reassurance about the safety of CWF during pregnancy and underscore the value of rigorous causal designs in evaluating potential adverse effects of public health interventions.