Birth defects can be prevented if the mother educates herself on what the inherent risks are, for her particular case. In some cases a medicine change or decrease is possible, in others it is not. Hormone fluctuations can complicate matters by affecting the way her medication is absorbed. Dilantin slightly increases cleft palate, heart disease and Fetal Hydanton Syndrome. Phenobarbital's risks are fewer than for Dilantin, but there are risks worthy of her doctor's discussion.
All anticonvulsants have their degree of risks, needless to say.
The more milligrams daily she takes, the more risks there are. Seizure control plays the key role in whether her medicine can be reduced to risks. A woman with seizures has a difficult time even when not pregnant. The goal is not to sacrifice a woman's health for the sake of her child. If a medicine reduction caused her to have increased seizures, she'd have less strength and endurance to endure the birth process ahead of her.