Phenylketonuria (PKU) is a recessive hereditary disorder, meaning that two copies of the mutated gene implicated in PKU disease processes must be inherited in order for an individual to display symptoms of this disorder. It’s one of the most common inherited metabolic disorders. The mutated genes cannot produce the enzyme that normally metabolizes the compound phenylalanine, found in certain food products, into tyrosine. The build-up of phenylalanine in the brain can become toxic, leading to intellectual disability when the build-up interferes with myelination in the central nervous system. If a baby with PKU ingests food with phenylalanine, the build-up during the post-birh period stunts brain development. Because of its preventable nature, and because the test is inexpensive and accurate, many governments have passed laws for mandatory PKU testing shortly after birth, and this has prevented many cases of intellectual disability. PKU can be prevented by regulating the level of phenylalanine intake, especially early in life. After the post-birth brain development period has passed, the low phenylalanine diet can be relaxed somewhat since the danger to brain development has passed. For pregnant women with PKU, however, this isn’t the case. When a fetus is in utero, it’s protected by its mother’s metabolism. For non-PKU having mothers, this means the baby is especially protected during this time. However, if the mother has PKU and ingests a diet with typical or high levels of phenylalanine - while it won’t damage HER brain - it will cause improper brain development in the fetus.