Trimethadione and its congener paratrimethadione are highly teratogenic. Offspring of mothers treated with either drug during pregnancy are likely to show one or more of a variety of abnormalities. In one review cited of pregnancies in women taking these drugs, about 25% spontaneously aborted and 83% of live births had one or more malformations, several of which were lethal. The disorder is rare (Office of Rare Diseases, 2001), and accurate incidence figures are not available.