PKU is a rare inherited disorder that causes an amino acid called phenylalanine to build up in your body.
A woman who has PKU and becomes pregnant is at risk of another form of the condition called maternal PKU. If a woman doesn't follow the special PKU diet before and during pregnancy, blood phenylalanine levels can become high and harm the developing fetus or cause a miscarriage.
Women with PKU can prevent birth defects by sticking to (or returning to) a low-phenylalanine diet before becoming pregnant.
When two carriers conceive a child, there is a one in four or 25% chance for each pregnancy that the baby will have PKU.