1 Joseph P. Kennedy, Jr. Laboratories, University of Wisconsin Medical Center, Madison, Wisconsin 53706
Stevenson and Huntley (Pediatrics, 40:33, 1967) suggest that high maternal plasma phenylalanine levels do not invariably cause intrauterine damage to the developing fetus, but they do not suggest how high the level can be without causing damage. In pregnant heterozygotes, Kang and Paine found fasting levels as high as 6 mg/100 ml (modification of Udenfriend and Cooper decarboxylation method) and 13 mg/100 ml (LaDu method), and these levels were no higher when the infant was PKU than when the infant was normal.