Thionamides prevent the pregnancy risks associated with hyperthyroidism, but because they cross the placenta, they inhibit fetal thyroid function as well and may cause transient fetal hypothyroidism. Propylthiouracil (PTU) reportedly is not associated with embryofetal anomalies, but anomalies have been reported in infants whose mothers took methimazole (MMI) while pregnant. A condition termed "MMI embryopathy" consists of choanal atresia and/or esophageal atresia, minor dysmorphic features, and delayed development. The investigators examined the outcomes of 241 pregnancies attending 10 Teratology Information Services in Europe because of MMI exposure. Control subjects were 1089 pregnant women referred because of exposure to nonteratogenic drugs. The participating units had comparable pregnancy outcomes. The proportion of spontaneous abortions in the study group did not differ appreciably from the control rate (6.3% vs. 6.6%). There also were no differences in gestational age at delivery, neonatal weight, or head circumference. Eight of 204 exposed live-stillborns had a major malformation, as did 23 children in the control group. Two infants exposed to MMI had one of the malformations that make up MMI embryopathy; one had choanal atresia, and one, esophageal atresia. The respective exposure periods were 4 to 7 weeks' and 0 to 16 weeks' gestation. These findings indicate that choanal atresia and esophageal atresia may be more frequent than expected n fetuses exposed early in pregnancy to MMI. It seems best, at present, to treat maternal thyrotoxicosis with PTU instead.