Several types. Major congenital malformations include: spina bifida or heart defects. Other important ones include cleft lip or cleft palate. More subtle, but still important ones include effects on learning and cognition during childhood. Not all anticonvulsants do theses things to the same degree. The total daily dose often helps to determine the frequency of these defects.
But realize this. Most congenital malformations occur without presence of epilepsy drugs which may increase risk by 50-100% depending on agent. Watch out for depakote, which has especial risk of spina bifida and mental retardation in kids, and Dilantin which does cause a specific birth syndrome. Newer drugs may be far safer.
Birth defects. Birth defects with anticonvulsants can vary, but generally speaking there are increases in craniofacial abnormalities and cardiac defects. There are also neural tube defects such as spina bifida. Amy woman with a seizure disorder who is contemplating pregnancy or who is pregnant should seek counseling from their obstetrician, neurologist, and primary care physician.