What is the main risk associated with the use of methimazole during the second and third trimesters of pregnancy?

The use of methimazole during the second and third trimesters of pregnancy is primarily associated with the risk of aplasia cutis. Aplasia cutis is a congenital defect characterized by the absence of skin, typically on the scalp, which can occur due to exposure to certain medications during pregnancy, including methimazole.

This drug, which is used to manage hyperthyroidism, can cross the placenta and, if used during these later trimesters, it has been linked to various fetal development issues, particularly skin-related defects. The mechanism is believed to involve methimazole's potential to affect the fetal skin formation processes, leading to skin defects like aplasia cutis.

Understanding this risk is crucial for healthcare providers when considering treatment options for hyperthyroidism in pregnant women. Proper counseling is essential to weigh the benefits and risks of continuing methimazole versus switching to another medication such as propylthiouracil, especially during the first trimester when the risks associated with certain medications are generally more pronounced.