Iodine has long been recognized as an essential micronutrient for maternal thyroid function, as well as fetal growth and development during pregnancy. The current study aimed to evaluate thyroid hormone status, urinary iodine concentration (UIC), thyroid volume, and nodularity in pregnant women, throughout trimesters, in a borderline iodine sufficient, urban area with mandatory table salt iodization. Two-hundred-sixty-five pregnant women ranging from 17 to 45 years participated in this prospective longitudinal study. Thyroid function tests, thyroid volume, nodule growth, and UIC were recorded throughout the first, second, and third trimesters with no intervention. Median UIC was 96, 78, and 60 mu g/L in the first, second, and third trimester of pregnancy, respectively (p < 0.001). Mean TSH values increased significantly (i.e. 0.65 mIU/ml, 1.1 mIU/ml, and 1.3 mIU/ml in the first, second, and third trimesters, respectively) (p < 0.001). Mean +/- s.d. thyroid volume was significantly higher in the third trimester (14.72 +/- 6.8 ml) compared with the first trimester (13.69 +/- 5.31 ml) (p < 0.001). An intensifying iodine deficiency (ID) was reported throughout trimesters in this cohort of pregnant women from Ankara. A significant percentage of pregnant women from a borderline iodine sufficient, urban area in Turkey were iodine deficient during all trimesters, and the deficiency increased throughout the pregnancy. Pregnant women should receive iodine supplementation, besides consuming iodized salt in borderline iodine sufficient areas.