ORCiD numbers: 0000-0002-9164-4869 (D. Levie). Context: Although the consequences of severe iodine deficiency are beyond doubt, the effects of mild to moderate iodine deficiency in pregnancy on child neurodevelopment are less well established. Objective: To study the association between maternal iodine status during pregnancy and child IQ and identify vulnerable time windows of exposure to suboptimal iodine availability.

IMPORTANCE Air pollutants interact with estrogen nuclear receptors, but their effect on thyroid signaling is less clear. Thyroid function is of particular importance for pregnant women because of the thyroid's role in fetal brain development.OBJECTIVE To determine the short-term association of exposure to air pollution in the first trimester with thyroid function throughout pregnancy. DESIGN, SETTING, AND PARTICIPANTS In this cohort study, 9931 pregnant women from 4European cohorts (the Amsterdam Born Children and Their Development Study, the Generation R Study, Infancia y Medio Ambiente, and Rhea) and 1 US cohort (Project Viva) with data on air pollution exposure and thyroid function during pregnancy were included. The recruitment period for the