The primary purpose of this project is to determine the best method of managing women during pregnancy who have hyperphenylalaninemia (HPA) including: classical phenylketonuria (PKU) - blood phenylalanine concentrations of more than 20 mg/dl while on a normal diet; variant PKU - 10-20 mg/dl; or benign HPA - 4-10 mg/dl. Fetal morbidity, with subsequent problems in growth and development, associated with maternal PKU will be assessed in relation to maternal phenylalanine levels.