Hematologic effects of phenytoin therapy during pregnancy.

Serum folate and plasma phenytoin concentrations were measured in 10 epileptic gravidas who did not receive folate supplementation. Mean serum folate concentration averaged 5.0 ng/ml during the second and third trimesters of pregnancy but significantly decreased to 2.9 ng/ml during nonpregnant evaluation 1 to 12 months following delivery. No patient exhibited anemia or macrocytosis suggestive of cellular folate deficiency either during or following pregnancy. Although phenytoin dosage remained relatively constant, plasma phenytoin concentrations during pregnancy were significantly lower than nonpregnant concentrations. The improvement in folate status observed during pregnancy may be related to lowered plasma phenytoin concentrations.