In the United States, the Food and Drug Administration (FDA) assigns each medication to a Pregnancy Category according to whether it has been proven to be harmful in pregnancy. Phenytoin is listed in Pregnancy Category D. This means that there is a risk to the baby, but the benefits may outweigh the risk for some women.

In fact, a large majority of women who use phenytoin during pregnancy have normal, healthy babies. Certain types of defects are increased when phenytoin is taken during the first 3 months of pregnancy, but they are still relatively uncommon. Some of these are growth delay, mental retardation, mild facial defects, and short fingers with small nails. The risk of defects is higher for women who take more than one seizure medicine. Women with a family history of birth defects also have a higher risk.

All women who are capable of becoming pregnant should take 0.4 mg (400 mcg) of the vitamin called folic acid every day because it helps to prevent one type of birth defect. (The most well-known of these is spina bifida, in which the spinal cord is not completely enclosed.) These defects are more common in the babies of women who take phenytoin during the first 4 to 6 weeks of pregnancy. If the doctor thinks a woman is at especially high risk, a much larger dose of folic acid—4 mg (4000 mcg) per day—may be recommended.

Women with epilepsy who are pregnant or thinking about becoming pregnant should talk to their doctor about their seizure medicines. Taking more than one seizure medicine may increase the risk of birth defects, so doctors sometimes gradually reduce the number or amount of seizure medicines taken by women planning for pregnancy. This is not done routinely, however, because it increases the risk of seizures. Some kinds of seizures can injure the baby, so women should not stop using seizure medicines or reduce the amount without the doctor’s OK.

About 20% to 35% of women have seizures more often during pregnancy because of changes in hormones or changes in how their seizure medicine is handled by the body. It is helpful for the doctor to check the levels of medicine in the blood regularly during pregnancy so that the dosage can be adjusted if necessary.

Some babies born to mothers taking phenytoin have had inadequate blood clotting within the first 24 hours after birth. It is recommended that the mother be given about 10 mg of vitamin K per day during the last month of pregnancy to prevent this problem.

If you are taking phenytoin, breast-feeding is generally safe, although a small amount of the medication will appear in your milk. The level in breast milk is about 10% of the level in the mother's blood.