Maternal PKU

Re-printed from: Information Sheet, PKU Handbook, Australasian Society for Inborn Errors of Metabolism, 1996

Making sure your baby will be healthy is very important for you and your partner. There are some things you should know about PKU and pregnancy, whether you plan to have a child now or in the future.

PKU has no effect on your chance of becoming pregnant. If you have PKU your pregnancies will require careful planning and a controlled phenylalanine diet. High levels of phenylalanine and low tyrosine levels will damage the unborn baby. A controlled diet, with blood phenylalanine and tyrosine levels in the safe range, can prevent this damage. The diet needs to start before conception, and continue throughout the pregnancy. It is likely you will need to be on the diet for a year or so for each pregnancy.

Until you are ready to start a family, you should use a safe form of contraception. Discuss this with your doctor, or go to a family panning clinic.

Starting the diet for maternal PKU before you become pregnant is very important to protect your baby. When you decide you wish to become pregnant, you will need to make sure your blood levels are in the safe range before stopping contraception. This means that your diet will probably need to be stricter than usual. Keep in touch with your PKU clinic team. Make an appointment to discuss the diet when you are ready to start a family.

Staying on the PKU diet, when you are an adult, is easier than having to go back to it. Although similar, the diet for maternal PKU is different from the one you had as a child or maybe as an adult. It must provide enough nutrients, for both you to keep healthy, and to meet the needs of your growing baby.

What are the risks?

If you ever become pregnant unexpectedly, when you are off your PKU diet, there is a risk your baby will be harmed. It is important to contact the PKU clinic team and do a blood phenylalanine test immediately.

During pregnancy your blood has to supply the baby with enough food and oxygen to grow normally. If your blood phenylalanine level is high, it can be harmful to the baby, even though the infant does NOT have PKU.

What type of damage can occur?

If you have high phenylalanine levels, permanent damage can occur to the unborn baby even before you miss your first period. This can affect the baby's brain, learning ability, growth and heart development and can sometimes cause a miscarriage. This is why it is so important to have blood levels in the safe range before becoming pregnant.

Poor control of your blood levels during pregnancy can cause slow growth of the baby and his/her brain, which can prevent normal development. A baby's blood level of phenylalanine is always higher than its mother's. This makes it really important for you to keep your blood phenylalanine level in the safe range.

How can you reduce the risks to your baby?

You will need to be on a low phenylalanine diet and supplement with the blood phenylalanine and tyrosine levels in the maternal PKU 'safe' range before stopping contraception. This can greatly reduce the risks to your baby. You will need to continue the recommended diet throughout the pregnancy.

The blood levels need to be checked at least every week. The diet is adjusted to keep your levels in the 'safe' range.

Is a low protein or vegetarian diet ok?

No. If you have relaxed your diet, it may not be adequate to protect and nourish your baby. You should return to a low phenylalanine diet and supplement. You will need protein, vitamin and mineral supplements as part of the diet. Frequent blood tests will tell you if the diet is going well.

How long is the maternal PKU diet needed?

Throughout your pregnancy the blood phenylalanine and tyrosine must be in the safe range. There are no exceptions for parties or special occasions.

Most women will need to be on the strict diet for about a year for each pregnancy. This can be longer if you don't conceive quickly or you take time initially to get your levels down to the 'safe' range.

After the birth of your baby, he/she is no longer at risk from your blood levels. A diet suitable for your own health can be resumed.

What should I do if considering pregnancy?

You and your partner should make an appointment, with your PKU clinic, to discuss things and find out more about the diet you will need.