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.

Advise women who are capable of becoming pregnant to take 400 mcg (0.4 mg) of folic acid (folate) daily to help prevent neural tube defects. Women at high risk, such as those with a history of a neural tube 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. Check the blood levels of phenytoin regularly during pregnancy so that the dosage can be adjusted as needed.

No studies have been performed to demonstrate the effect of specific AEDs 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. 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. The vitamin K given to babies when they are born may be too late to prevent this disorder.

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. Since 90% of phenytoin is bound to plasma proteins in the mother's blood, the level in breast milk is about 10% of the level in the mother's blood.