The antithyroid drugs like carbimazole, methimazole crosses the placenta (transplacental passage) and produce fetal hypothyroidism and goiter if the maternal dose is excessive. However, oral propylthiouracil would be the most appropriate therapy in early gestation at low doses as its greater protein binding allows less transfer to the foetus. Carbimazole and methimazole are also associated with rare cases of fetal aplasia cutis and other defects, such as choanal atresia. Thyroidectomy and Radioactive Iodine I131 are contraindicated during pregnancy. Reference : page 254 Essentials of Medical Pharmacology K D Tripathi 7th edition and page 2297 Harrison's Principles of Internal Medicine 19th edition