Birth defects are probably a bit more common in the babies of women who take phenobarbital during pregnancy than in others. A large majority of women who take phenobarbital do have healthy, normal babies, however. 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.

There also is a danger to the baby from withdrawal symptoms if the mother takes phenobarbital in the last few months of pregnancy, since it crosses into the baby.

All women who are capable of becoming pregnant should take at least 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 phenobarbital 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.

Talk with your doctor about the potential effects of phenobarbital if you are pregnant or plan to become pregnant. 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 always done, however, because it increases the risk of seizures. Some kinds of seizures can injure the baby, so do not stop using seizure medicines or reduce the amount without the doctor's OK.

Because phenobarbital reduces the effectiveness of birth control pills, women who take it need to be especially cautious about pregnancy.

Reviewed February 2004 by Steven C. Schachter, MD, epilepsy.com Editorial Board.