Key PointsQuestionTo what extent does prenatal folate status modify associations between early pregnancy perfluoroalkyl and polyfluoroalkyl substance (PFAS) exposure and birth outcomes?FindingsIn this cohort study of 1400 mother-singleton pairs in the US, higher early pregnancy plasma PFAS concentrations were associated with lower birth weight and gestational age and a higher risk of low birth weight. These findings were observed only among mothers in the lowest quartile of prenatal folate status measured by dietary intake or plasma biomarker.MeaningThe findings of this study suggest that PFAS exposure may be associated with adverse birth outcomes only among mothers with low folate status in early pregnancy.