Patients receive a phenylalanine restricted diet based on a medical food such as Phenyl-free, Lofenalac, PKU 3, or Maximum XP (may be supplemented with tyrosine as needed) to achieve a target blood phenylalanine level of 2-6 mg/dL. The diet begins approximately 3 months prior to conception and continues until delivery of the baby. Patients are evaluated once a month before conception and weekly during pregnancy. Patients also undergo sonographic examination at 8, 20, 28, and 34 weeks gestation. At birth, the cord blood is evaluated for plasma amino acids. If the baby is found to have an elevated phenylalanine level, blood and urine are obtained to determine the baby's genetic status. Phenylalanine and tyrosine levels are checked in the baby daily for three days after birth.