Toxic metabolites of phenylalanine that accumulate in phenylketonuria (PKU) can be teratogenic to a developing fetus.

Effects on the fetus may include:

(1) congenital heart defects

(2) microcephaly

(3) developmental delay and mental retardation

(4) facial anomalies (wide nasal bridge, anteverted nares, hypoplastic mandible, cleft palate)

(5) growth retardation


The teratogenic effects can be prevented by a low phenylalanine diet, but this may need to be started prior to conception (Rouse et al, 1997). Starting the diet at 16 weeks gestation may not prevent malformations (Scott et al).

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