Thyroid dysfunction is frequent in pregnant women and is often associated with an increased risk of adverse maternal and fetal outcome. In the present work, thyroid function of pregnant women from Benin republic was studied. Two hundred forty (240) pregnant women, without thyroid disease history, have been included in the study. A blood sample was drawn for measurement of TSH, free T3 and free T4 serum levels. From the first to the third trimester, plasma levels of free T3 and free T4 decreased when plasma levels of TSH increased. Using recommendations of the 2011 American Thyroid Association (ATA) guidelines, thyroid dysfunction was observed in 24.17% of subjects. Hypothyroidism was present in 22.50% of subjects with 19.17% of subclinical hypothyroidism, 0.83% of overt hypothyroidism and 2.50% of hypothyroxinemia and hyperthyroidism was observed in 1.67% of subjects. An increase from 15.52% to 28.07% was observed in the frequency of hypothyroidism from the first to the third trimester of pregnancy. When an upper cut-off value of 4 mU/L was used for TSH, as recommended in the 2017 ATA guidelines, prevalence of thyroid disorders was 14.58% with 12.91% of hypothyroidism but no hypothyroidism was observed in women in the first trimester of pregnancy. A rise in hypothyroidism frequency was observed when pregnant women age increased. Hypothyroidism was very common in pregnant women in Benin. To allow accurate assessment of thyroid status in pregnant women in Benin, pregnancy specific range for plasma level of TSH and thyroid hormones should be established.