People with PKU have a deficiency of an enzyme, which is necessary for the proper metabolism of an amino acid called phenylalanine. Serum phenylalanine level at conception is critical to the organ development of the fetus.

Elevated maternal blood levels of phenylalanine may cause spontaneous abortion, brain and heart defects, poor fetal growth, as well as mental retardation in the baby.

If maternal blood levels of phenylalanine are optimised by special diet prior to conception and during pregnancy, this risk to the foetus is markedly reduced. It is important for any woman with PKU who is contemplating pregnancy to understand this.

Reference: Healthhub