Aplasia cutis congenita after methimazole exposure in utero.

We describe a patient who was exposed to the antithyroid drug methimazole during the first 6 weeks of gestation and was born prematurely with scalp and skull defects associated with facial asymmetry. A review of the literature seems to support the hypothesis that methimazole is a potential teratogen. Although the risk of birth defects is low with clinically applied doses of the drug, it cannot be regarded as safe and should therefore be avoided in the treatment of pregnant women.