Fetal ultrasonography in maternal PKU

Maternal phenylketonuria (PKU) is teratogenic and results in birth defects that include microcephaly, mental retardation, congenital heart disease, and intrauterine growth retardation. Treatment with a low phenylalanine diet can prevent or reduce the severity of the complications. Optimal benefit, however, requires frequent monitoring with fetal ultrasonography as a critical element. We have studied ultrasonography in 39 pregnancies enrolled in the Maternal PKU Collaborative Study and followed at our centre. First-trimester examinations in 24 pregnancies resulted in the discovery of non-viability in five. In each, this led to discontinuation of the difficult and expensive diet. Among the 33 pregnancies with second-trimester evaluation, congenital heart disease was identified in five. Two of these pregnancies were terminated. Microcephaly as determined by biparietal diameter (BPD) was identified in the second trimester in only one of nine fetuses who had microcephaly at birth. Among 20 pregnancies with third-trimester ultrasound, fetal microcephaly was identified by BPD in three of seven who had birth microcephaly. We conclude that fetal ultrasonography in maternal PKU is valuable during the first trimester in identifying non-viable pregnancies and determining gestational age and is also valuable during the second trimester in identifying congenital heart disease and perhaps other major anomalies, but not in identifying fetal microcephaly. Third-trimester ultrasound seems to be of limited usefulness.