Fetal damage despite low-phenylalanine diet after conception in a phenylketonuric woman.

A woman with phenylketonuria gave birth to an infant with a cardiac defect and microcephaly despite introduction of a low-phenylalanine diet 5 weeks after conception (3 weeks after the first missed period). In maternal phenylketonuria fetal damage leading to reduced brain growth and cardiac malformations probably occurs within a few weeks of conception.