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The fetus depends on maternal thyroid hormones during pregnancy for normal brain development. Little is known about the effects of subclinical hypothyroidism and mild hypothyroxinemia during pregnancy on neurodevelopment of the child.We evaluated a population-based birth cohort in Spain. A total of 1761 children and their mothers were included in the main analyses. Serum levels of free thyroxine and thyrotropin were measured in pregnant women. Mental and psychomotor development of their children was assessed using the Bayley Scales of Infant Development during the second year of life.Low free thyroxine levels (<5th percentile) and self-reported prepregnancy thyroid disorder without medical treatment were associated with a decrease of mental scores (Beta = −3.4 [95% confidence interval= −6.7 to −0.2]) and −5.5 [−8.9 to −2.0], respectively). No association between thyrotropin levels and mental scores or psychomotor scores was observed.Low free thyroxine levels in healthy pregnant women are related to a moderate delay in child neurodevelopment. The impairment is similar in magnitude to that observed in mothers with untreated prepregnancy thyroid disorder. The value of screening pregnant women for thyroid function requires further assessment, particularly in mildly iodine-deficient areas.