BACKGROUND AND OBJECTIVE: Congenital hypothyroidism due to iodine deficiency is the most common cause of preventable mental retardation in the world. The lowest level of iodine deficiency during pregnancy could result in disorders such as miscarriage, premature delivery, intrauterine fetal death, low birth IQ, mental retardation, hearing difficulty and speech impediments. This study aimed to evaluate urinary iodine concentrations during the first, second and third trimesters of pregnancy. METHODS: This cross-sectional study was conducted on 354 pregnant women referring to prenatal care clinic of Akbar Abadi Hospital in Tehran. Demographic questionnaires were completed by an expert, and random urine samples were obtained to measure urinary iodine concentrations. In addition, venous blood samples were provided to determine the levels of thyroxine (T4) and thyroid stimulating hormone (TSH) during the first, second and third trimesters. Analysis of iodine status was based on the urinary iodine excretion in the patients. Urinary iodine deficiency was defined as concentrations of ≤100 micrograms/liter. FINDINGS: In this study, 285 patients (80.5%) had urinary iodine levels of ≤100 µg/l with a mean of 62.35±67.7 µg/l. The mean urinary iodine concentrations during the first, second and third trimesters were 65.83±72.4, 50.34±41.5 and 62.67±68.3 µg/l, respectively. No significant difference was observed in the mean of urinary iodine between the first and second trimesters, second and third trimesters, and first and third trimesters. Moreover, no significant differences were observed between patients with urinary iodine levels of <100 µg/l and ≥100 µg/l in terms of the mean of maternal age, age at pregnancy and TSH level. CONCLUSION: According to the results of this study, despite the consumption of iodized salt, urinary iodine concentrations were below the standard limits in the studied pregnant women. It could be inferred that use of iodized salt may not be a proper solution for iodine deficiency in pregnant women. Therefore, it is recommended that  iodine supplements be used before and during pregnancy, and iodine content of salt be increased as well