Maternal phenylketonuria: abnormal baby despite low phenylalanine diet during pregnancy.

During a screening programme of 10000 pregnant women by the Guthrie test, a previously unrecognised phenylketonuric woman was detected. A low phenylalanine diet introduced from the 16th week of gestation failed to prevent fetal abnormality and mental retardation. Maternal phenylketonuria requires earlier diagnosis than can be achieved at the initial antenatal clinic visit if its teratogenic effects are to be prevented.