Pregnancy & PKU - Q&A

The following information provides some answers to commonly asked questions by pregnant women with PKU or women with PKU planning pregnancy.

Nutricia would like to thank Lyndsey Tomlinson, Sarah Howe and Louise Robertson, Metabolic Dietitians at University Hospitals Birmingham for compiling this Q & A.

Q: Why do I need to follow a strict diet during pregnancy?

A: It is important to keep to a strict low phenylalanine (phe) diet during pregnancy. High phe levels whilst pregnant can interfere with the normal development of the unborn baby as the phe is able to cross the placenta. The damage that can occur is permanent, leading to problems such as damage to the brain (leading to learning difficulties), a small head, congenital heart defects and a low birth weight.

Q: Do I need to plan my pregnancy?

A: Yes, it is always advised if you are thinking of having a baby to talk to your dietitian, so that they can help support you in bringing your phe levels down prior to conceiving. This means that from the moment you become pregnant, you know your levels are in a safe range to keep your baby protected from the potential harm higher phe levels can cause. The risks are much less lower to the baby if you conceive on a strict diet. Your dietitian will advise you how long you need to have your diet established and your levels in range before discontinuing contraception and trying to conceive.

Q: Do men need to follow a special diet if planning a baby?

A: No, this is not required of males with or without PKU. This is because it is the high blood phe levels of the mother during the entire pregnancy that causes the potential harm to the baby. However, lots of support is needed during the pre-conception and pregnancy diet, so the father learning about the diet as well is very helpful.

Q: Q: What is preconception diet?

A: Preconception diet involves reducing blood phe levels down to a stricter target range (120-360µmol/l). It is advisable to try and maintain phe concentrations in the middle of this range to reduce the risk of phe concentrations being too low or too high. This usually involves taking 60-80g of protein equivalent from protein substitute each day and to decrease the number of exchanges, this could be as low as 3 exchanges, until phe concentrations are in the correct range. This varies from person to person depending on how severe their PKU is. During the preconception diet, you will need to send in blood levels once to twice a week.

Q: What happens if I find out I am pregnant and I am not on diet?

A: In this situation we would advise you to telephone your dietitian as soon as you become aware you are pregnant, so that they can advise you of how to bring your blood phe levels down as quickly as possible. If this happens at a weekend, when you are unable to contact them, it is advised that you stop eating all high protein containing foods and start taking 60g /day of your protein substitute; usually 3 drinks or powders a day. It is recommended that you have got your phenylalanine levels within target range ideally before but within the first 8-10 weeks of pregnancy.

Q: Will my diet be stricter than the usual PKU diet I follow?

A: Yes, as you will be expected to achieve a stricter target level of between 120-360µmol/l. You will be on a lower amount of exchanges than normal and will have to ensure they are weighed out strictly. More regular blood phe levels will be needed too.

Q: What support will I receive if I have been off diet for a long time?

A: It can be tricky for ladies to go back onto a strict diet after being on a relaxed diet for a long period of time. Your dietitian will support and guide you with choosing a suitable phe–free protein substitute, how to weigh and choose exchanges, low protein cooking and shopping and they will arrange the prescribing and delivery of your specialist products.

Q: How often will I have to send in blood spots when I am pregnant?

A: Two to three times a week. This allows your dietitian to keep a close eye on your phe concentrations and make any changes as needed.

Q: Will I have to stick to the same amount of exchanges throughout my pregnancy?

A: No, as you progress through the pregnancy, the number of exchanges you will need to keep your phe levels within range will increase. This usually happens from around 16 weeks onwards. This is because the baby is using some of the phe for growth and secondly the baby’s liver will start to metabolise some of the phe to tyrosine. It is important to send in regular bloods to see when your phe levels start to decline. Your dietitian will then advise you to increase your exchanges. Low phe levels (less than 120umol/l) can also be harmful to your baby as it can affect their growth.

Q: Will I need to take any extra nutritional supplements like the general population?

A: As the general population, you will need to take a folic acid tablet (400µg/d) when you stop contraception until the 12th week of pregnancy. All the other requirements for vitamins (including vitamin D) and minerals are in your protein substitute. If you have any concerns about not getting enough vitamins and minerals, please speak to your dietitian who will be able to advise you.

Q: Should I be concerned if I am losing weight on my strict pregnancy diet?

A: Yes. Weight loss during the first trimester can lead to raised phenylalanine concentrations. It is also linked to an increased risk of a small head and low birth weight of the baby. Ensure you are eating regularly and having plenty of low protein foods on prescription to increase your energy intake. Your dietitian will advise you on this and monitor your weight.

Q: Will I need any extra antenatal care?

A: This will vary between metabolic centres. Antenatal care and delivery will be the same as for a non-PKU pregnancy but some obstetric teams may request extra scans as a precaution to monitor the baby’s heart and growth.

Q: Am I limited to what sort of birth I can have if I have PKU?

A: No. From a PKU point of view, this should not affect what sort of birth you can have. Most women choose their antenatal care at their local hospital.

Q: What is the likelihood that my baby will have PKU?

A: To have PKU, your baby will need two recessive PKU genes; one from the Mum and one from the Dad. So even though you have PKU, your baby will only have a chance of getting PKU if the Dad also has a PKU gene and is a carrier. The incidence of this is 1 in 50, making the chance of your baby inheriting PKU 1 in 100, if your partner is a carrier.

Your baby will be tested for PKU on day 5 of life, alongside other metabolic conditions. A blood spot card is used in a similar way to how you test your blood phe levels. This may be done earlier if your post-natal team are aware of your PKU.

Q: Will I find out if my baby has PKU?

A: Yes, you would be informed very quickly if your baby has PKU. Your dietitian may be able to help you find out this information.

Q: Do I have to continue with a low protein diet once my baby is born if I want to breastfeed?

A: No. Once the baby is born, as long as he/she doesn’t have PKU, you can breastfeed whilst being on a relaxed diet or “off diet”. This is because the baby is able to breakdown any extra phe it will get from your milk.

If your baby was to have PKU, you would still be able to give them a certain amount of breast milk (as exchanges) alongside a specialised infant formula for PKU. A paediatric dietitian would be able to advise you.

NOTE: The dietary management for PKU varies for each person so all information presented here is for guidance purposes only. The information provided on this page is in no way intended to replace the care, advice and medical supervision of your healthcare professional. Your own dietitian and/or doctor will advise you on all aspects relating to management of PKU for you and your family. Always consult your healthcare professional before making any changes to your low-protein diet.