Maternal PKU

Phenylketonuria (also called PKU) is a condition in which your body can’t break down an amino acid called phenylalanine. Amino acids help build protein in your body. Without treatment, phenylalanine builds up in the blood and causes health problems.

Maternal PKU means that a woman who has PKU is pregnant. About 3,000 women of childbearing age in the United States have PKU.

What causes PKU?

PKU is inherited. This means it’s passed from parent to child through genes. A gene is a part of your body’s cells that stores instructions for the way your body grows and works. Genes come in pairs — you get one of each pair from each parent.

Sometimes the instructions in genes change. This is called a gene change or a mutation. Parents can pass gene changes to their children. Sometimes a gene change can cause a gene to not work correctly. Sometimes it can cause birth defects or other health conditions. A birth defect is a health condition that is present in a baby at birth.

You have to inherit a gene change for PKU from both parents to have PKU. If you inherit the gene from just one parent, you have the gene change for PKU, but you don’t have PKU. When this happens, you’re called a PKU carrier. A PKU carrier has the gene change but doesn’t have PKU.

Can you pass PKU to your children?

Yes. But it depends on both you and your partner. If you and your partner both have PKU, your baby will have PKU.

If you and your partner are both PKU carriers, there’s a:

  • 3-in-4 chance (75 percent) that your baby won’t have PKU
  • 1-in-2 chance (50 percent) that your baby won’t have PKU but will be a PKU carrier
  • 1-in-4 chance (25 percent) that your baby will have PKU
  • 1-in-4 chance (25 percent) that your baby will not have PKU and not be a carrier

A genetic counselor can help you understand your chances of passing PKU to your baby. A genetic counselor is a person who is trained to help you understand about how genes, birth defects and other medical conditions run in families, and how they can affect your health and your baby's health.

Ask your health care provider if you need help finding a genetic counselor. Or contact the National Society of Genetic Counselors.

Can maternal PKU cause problems for your baby?

Yes. When a pregnant woman’s phenylalanine levels get too high, they can cause serious problems in her baby, including:

  • Intellectual disabilities
  • Microcephaly. Microcephaly is a birth defect in which a baby’s head is smaller than expected, compared to babies of the same sex and age.
  • Heart defects
  • Low birthweight 

How can you find out if you’re a PKU carrier?

You may not know if you’re a PKU carrier. If you’re a carrier, you don’t have any signs or symptoms of PKU and you do not have PKU.

There are two types of tests that can tell you if you’re a PKU carrier. Both are safe to take during pregnancy. Your partner can have the tests, too.

  1. A blood test
  2. A swab of cells inside your mouth. This means your provider rubs a cotton swab against the inside of your cheek to get some cells.

You and your partner may want to be tested if PKU runs in either of your families. To help you find out, take your family health history. This is a record of any health conditions and treatments that you, your partner and everyone in both of your families have had. Use this family health history form and share it with your health care provider.

Can you find out during pregnancy if your baby has PKU or is a PKU carrier?

Yes. If you or your partner has PKU or is a PKU carrier, you can have a prenatal test to find out if your baby has PKU or is a carrier. You can have either of these tests:

  • Chorionic villus sampling (also called CVS). This test checks tissue from the placenta for birth defects and genetic conditions. You can get CVS at 10 to 13 weeks of pregnancy.
  • Amniocentesis (also called amnio). This test checks amniotic fluid from the amniotic sac around your baby for birth defects and genetic conditions. You can get this test at 15 to 20 weeks of pregnancy.

Talk to your provider or genetic counselor if you’re thinking of having either of these tests.

How is maternal PKU treated in pregnancy?

The good news is that most pregnant women who have PKU can have healthy babies if they follow their PKU meal plan. This is a special meal plan that is low in phenylalanine.

PKU meal plans are different for each person and depend on how much phenylalanine your body can take. Health care providers at a medical center or clinic that has a special program to treat PKU can help you create a PKU meal plan. These providers can monitor your pregnancy and help you change your meal plan to help keep your blood phenylalanine at the right level. Ask your health care provider for information on a medical center or clinic that treats PKU.

Your health care provider may order ultrasound tests during your pregnancy to check on your baby’s growth.

Be sure to follow your PKU meal plan at least 3 months before getting pregnant and throughout pregnancy. If you just found out you’re pregnant, go back to your PKU meal plan right away. You can get weekly blood tests during pregnancy to make sure your phenylalanine levels aren’t too high.

See also: PKU (Phenylketonuria), Genetic counseling, Your family health history, Newborn screening

Last reviewed: January, 2013