If a pregnant woman has an elevated blood phenylalanine level, it is likely that she will give birth to a microcephalic, small-for-dates baby with multiple congenital anomalies and future mental retardation. The elevatedmaternalphenylalanine has affected the fetus; after delivery the newborn's phenylalanine level will be normal. At present, only newborns are routinely screened for hyperphenylalaninemia. The obstetrician needs to be aware of the danger of undetected maternal hyperphenylalaninemia as well as maternal phenylketonuria. Blood phenylalanine should be monitored in all women at risk for pregnancy.