Why is a fetus not affected with PKU until birth?

A fetus is not affected by phenylketonuria (PKU) until birth because the mother’s body regulates phenylalanine levels through her metabolism. During pregnancy, the placenta allows for the exchange of nutrients and waste products, but it protects the fetus from high levels of phenylalanine, as long as the mother maintains a low-phenylalanine diet. After birth, the infant's liver must process phenylalanine without the same protective mechanisms, making it crucial to screen for PKU and manage dietary intake immediately to prevent harmful effects.