Effects of untreated maternal phenylketonuria and hyperphenylalaninemia on the fetus.

We studied the effects of maternal phenylketonuria and hyperphenylalaninemia on 53 offspring from untreated pregnancies in 22 mothers who were identified by routine screening of umbilical-cord blood. The IQ of the offspring was significantly correlated with both maternal IQ (r = 0.83, P less than 0.001) and maternal blood phenylalanine level (r = 0.82, P less than 0.001), but with one exception, mental retardation in offspring was present only when the maternal blood phenylalanine level exceeded 1100 mumol per liter (18 mg per deciliter). Microcephaly in offspring was consistently present only when the maternal blood phenylalanine level exceeded 1200 mumol per liter (20 mg per deciliter). Congenital heart disease and other congenital anomalies were rare among offspring. The data suggest that severe atypical or classic phenylketonuria (blood phenylalanine level, greater than 1100 mumol per liter) in the mother has a substantial cognitive effect on her offspring but that the effect of mild hyperphenylalaninemia may have been overstated in the past.