Introduction: Pregnancy induces profound changes in thyroid function and iodine metabolism leading to thyroid stimulation. Low iodine intake during pregnancy may cause thyroid dysfunction in pregnant women and their newborn. Objective: To determine iodine status among Belgian pregnant women during the first and third trimester of pregnancy and to asses the determinants of iodine status one year after the introduction of bread fortification with iodised salt.Methods: Women in the first and third trimester of pregnancy were selected according to a multi-stage proportionate-to-size stratified sampling design. Urine samples were collected and a general questionnaire was completed face-to-face with the study nurse. Median urinary iodine concentration (UIC) in µg/L and in µg per g creatinine were determined.Results: The median UIC among the pregnant women (n=1311) was 124.1 µg/L and 122.6 µg/g Cr when corrected by urinary creatinine. The median UIC in the first trimester (118.3 µg/L) was significantly lower than in third trimester (131.0 µg/L) but significantly higher than among women of child-bearing age (84.8 µg/L). Intake of iodine-containing supplements was reported by 60.8% of the women and 57.4% of the women took this supplement daily. The risk of iodine deficiency was significantly higher in younger women, in women not taking iodine-containing supplements, in women with low consumption of dairy products and during autumn. Pregnant women with higher BMI had a higher risk of iodine deficiency but the risk was lower in women who reported alcohol consumption during pregnancy.Conclusion: The median UIC during pregnancy indicates iodine deficiency in Belgium. The low iodine intake in women of child-bearing age precludes the correction of iodine deficiency in pregnant women supplemented with multivitamins containing 150 µg of iodine as currently recommended in Belgium.