Fetal hydantoin syndrome (FHS) is caused by the anticonvulsant drug (AED) phenytoin (Dilantin). Phenytoin is a prenatal teratogen, causing a variety of physical defects, including infant failure to thrive, dysmorphic facies and other physical abnormalities, growth deficiency, and mental retardation, usually mild. The effects, particularly craniofacial, of prenatal exposure to phenytoin are so similar to those of several other AEDs, including carbamazepine, valproic acid, mysoline, and phenobarbital, as to have led to the general descriptive term fetal antiepileptic drug syndrome. As with most teratogens, major damage occurs from exposure early in prenatal development.