http://www.who.int/ionizing_radiation/pub_meet/Iodine_Prophylaxis_guide.pdf Pregnant women During pregnancy, the maternal thyroid gland is stimulated, especially during the first trimester. The fraction of radioactive iodine taken up by the thyroid is increased as compared to other adults. Thus, there is a greater need to protect the thyroid gland of the pregnant woman. During the second and third trimesters, the thyroid gland of the developing fetus takes up and stores iodine in increasing amounts. Iodine passes readily across the placenta, and thus, after the first trimester, the fetal thyroid gland can be exposed to radioactive iodine through the placenta, but it can also be protected by stable iodine taken by the mother. However, the risk of blocking the fetal thyroid function by a prolonged overload of stable iodine must be kept in mind, especially in areas with inherent dietary iodine deficiency.