PKU (Phenylketonuria) in your baby

Babies and birth defects

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.

In the United States, about 1 in 10,000 to 15,000 babies is born with PKU each year. The illness happens in all ethnic groups. But it’s more common in white, American Indian or Alaska Native people than in Black, Ashkenazi Jewish or Japanese people.

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.

Your baby has to inherit a gene change for PKU from both parents to have PKU. If they inherit the gene from just one parent, they have the gene change for PKU but they don’t have PKU. When this happens, your baby is called a PKU carrier.

How do you know if your baby has PKU?

All babies are screened for PKU soon after they’re born. Finding PKU early helps doctors treat it so babies can grow up healthy.  

Before your baby leaves the hospital, their health care provider takes a few drops of blood from their heel. The blood is collected on a special paper and sent to a lab for testing. If newborn screening results aren’t normal, it simply means your baby needs more testing. Your baby’s provider can recommend another kind of test, called a diagnostic test. This test can check to see if your baby has PKU or if there is another cause for abnormal test results.

If your baby is tested before they are a full day old, it’s possible for the test to miss PKU. Some experts recommend that if your baby was tested within the first 24 hours of life, they should be tested again at 1 to 2 weeks of age.

What problems can PKU cause?

Babies who have PKU seem normal for the first few months of life. But without treatment, they begin to show signs and symptoms of the illness at about 6 months of age. These include:

  • Jerky movements in the arms and legs
  • Lighter skin and eyes (Babies who have PKU can’t properly make melanin, the pigment in the body that’s responsible for skin and hair color.)
  • Musty body smell
  • Seizures
  • Skin rashes
  • Small head size
  • Taking longer than expected to sit, crawl or walk
  • Losing interest in surroundings
  • Delays in mental and social skills
  • Intellectual disabilities
  • Behavior problems, such as being hyperactive

If your baby has PKU, what kind of treatment do they need?

If your baby has PKU, they may need testing as often as once a week or more often for the first year of life to check their phenylalanine levels. After that, they may have testing once or twice a month throughout childhood.

Your baby needs to follow a special meal plan that is low in phenylalanine. It’s best to start this meal plan as soon as possible, ideally within the first 7 to 10 days of life.

At first, your baby gets a special protein formula that has reduced phenylalanine. Protein is important to help your baby grow and develop. The amount of phenylalanine in the formula is controlled to meet your baby’s individual needs. Your baby also can have some breast milk/chest milk. Breast milk has phenylalanine in it, so talk to your baby’s provider to find out how much your baby can have.

When your baby is ready to eat solid foods, they can eat vegetables, fruits, some grains (like low-protein cereals, breads and pasta) and other low-phenylalanine foods. If your baby has PKU, they should not eat:

  • Milk, cheese, ice cream and other dairy products
  • Eggs
  • Meat and poultry
  • Fish
  • Nuts, seeds, oats, rye, barley, gelatin
  • Beans
  • Food or drinks that contain aspartame. This is an artificial sweetener that has a lot of phenylalanine in it. It’s sold as NutraSweet® and Equal®.

PKU meal plans are different for each baby and can vary over time depending on how much phenylalanine your baby can tolerate. Health care providers at a medical center or clinic that has a special program to treat PKU can help you create a meal plan for your baby. Ask your baby’s health care provider for information on a medical center or clinic that treats PKU.

Your child will follows the PKU meal plan through their whole life. If they eventually get pregnant, they follow their meal plan throughout pregnancy. Most pregnant and parenting people who have PKU can have healthy pregnancies and healthy babies.

The medicine Kuvan® (sapropterin dihydrochloride) can help some people who have PKU. The medicine is more likely to work in people with mild or special forms of PKU. Children who take Kuvan® must follow a special meal plan, but it may not be as strict as one for those not taking the drug. They still need regular blood tests to check phenylalanine levels.

More information

See also: Genetic counselingNewborn screening

Last reviewed: May, 2021