The U.S. Food and Drug Administration (FDA) lists phenytoin in Pregnancy Category D. This indicates that there is clear evidence of risk to the human fetus, but the benefits may outweigh the risk for pregnant women who have a serious condition that cannot be treated effectively with a safer drug.

The babies of women taking phenytoin have a greater than usual number of major birth defects like cleft lip, cleft palate, and heart malformations. Defects like these occur in 2-3% of all pregnancies but affect 4-7% of the babies of women taking only phenytoin. (This rate is similar to that for women taking other seizure medicines.) There is also an increased chance of minor birth defects such as short fingers and widely spaced eyes. When these babies are re-examined after several years, however, these minor defects are often undetectable or very subtle.

Women taking phenytoin used to be warned about a disorder called "fetal hydantoin syndrome," but this concept is no longer accepted. Phenytoin does not clearly differ from other seizure medicines in the type of defects that may occur.

The risk of defects is higher for women who take more than one AED and for women with a family history of birth defects.

Women who are capable of becoming pregnant should take 400 mcg (0.4 mg) of folic acid (folate) daily to help prevent a type of birth defect called a neural tube defect. (The best-known of these is spina bifida, in which the spinal cord is not completely enclosed.) Women at high risk, such as those with a history of this kind of defect in a previous pregnancy, should take 4000 mcg (4 mg) daily, beginning before they become pregnant.

About 20% to 35% of women have seizures more often during pregnancy because of changes in hormones or changes in how phenytoin is handled by the body. The doctor may recommend checking the level of medication in the blood regularly during pregnancy so that the dosage can be adjusted as needed.

No studies have been performed to demonstrate the effect of specific seizure medicines during labor and delivery. Possible causes of seizures include:

• failure or inability to take medication
• sleep deprivation
• hyperventilation
• stress
• pain

Some babies born to mothers taking phenytoin have had inadequate blood clotting within the first 24 hours after birth. Taking 10 mg of vitamin K per day during the last month of pregnancy may prevent this problem.

Breast-feeding by mothers taking phenytoin should be safe for healthy, full-term newborns, although a small amount of the medication will appear in the milk. The level in breast milk is about 10% of the level in the mother's blood.