Thyroid diseases are more frequent in women. We made a longitudinal study at 112 healthy pregnant women from Iasi county, Romania. The aim of study was to determine incidence and evolution of autoimmune thyroiditis in pregnancy and postpartum. The proceedings of study imposed to verify clinical aspect of thyroid, the volume of thyroid measured echographic, echostructure of thyroid, function of thyroid (TSH, FT4) and autoimmune modification of thyroid (anti-thyroid peroxidase autoantibodies - AAT-anti-TPO). Knowing that in postpartum it exists a risk to develop a subclinical autoimmune thyroiditis, even in normal women, we tried to find a specific element that can be use like signal to identify such disease risk during pregnancy. In these conditions we followed in progress the echostructure of thyroid during pregnancy and in the first 3 months after delivery. We observed an hypoechogenity of thyroid correlated with the levels of AAT-anti-TPO, TSH and FT4. The prevalence of hypoechogenity was 12.5% in I trimester, 16% in 2nd trimester, 23% in the end of pregnancy and 25% after delivery. Majority of cases with thyroid hypoechogenity (accentuated in 3rd trimester) presented some degree of autoimmunity, despite of reduction of AAT anti-TPO level in the end of pregnancy. The conclusion of our study is that the echography of thyroid can represent a screening method for detection of subclinical autoimmune thyroiditis during the pregnancy.