The Nutricia Metabolics team spoke with Lyndsey Tomlinson, Sarah Howe and Louise Robertson, metabolic dietitians, to answer your burning questions about PKU and pregnancy.
Why do I have to follow a strict diet during pregnancy?
It is important to keep to a strict low phenylalanine (Phe) diet during pregnancy. High Phe levels while pregnant can interfere with the normal development of the unborn baby, as Phe can cross the placenta. The damage that can occur is permanent, leading to problems such as learning difficulties , a small head, congenital heart defects and low birth weight.
What is the preconception diet?
A preconception diet involves reducing blood Phe levels to a stricter target range (120–360 µmol/L). It is advisable to maintain Phe concentrations in the middle of this range to reduce the risk of levels being too low or too high. This usually involves taking 60–80 g of protein equivalent from a protein substitute each day and decreasing the number of exchanges — this could be as low as three — 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 or twice a week.
Should I plan my pregnancy?
Yes. It is always advised that if you are thinking of having a baby, you talk to your dietitian so they can help support you in bringing your Phe levels down prior to conceiving. This means that from the moment you become pregnant, 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 lower to the baby if you conceive while following 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.
Do men need to follow a special diet if planning a baby?
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 pregnancy that cause potential harm to the baby. However, lots of support is needed during the preconception and pregnancy diet, so the father learning about the diet as well is very helpful.
What happens if I find out I’m pregnant and not on a diet?
In this situation, we would advise you to telephone your dietitian as soon as you become aware you are pregnant so they can advise you on 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 foods and start taking 60 g per day of your protein substitute — usually three drinks or powders a day. It is recommended that you have your phenylalanine levels within the target range ideally before, but within, the first 8–10 weeks of pregnancy.
Will my diet be stricter than my usual low-protein diet?
Yes. You will be expected to achieve a stricter target level of between 120–360 µmol/L. You will be on a lower number of exchanges than normal and will have to ensure they are weighed out strictly. More regular blood Phe levels will be needed too.
What support will I receive if I have been off diet for a long time?
It can be tricky for women 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 protein substitute, how to weigh and choose exchanges, low-protein cooking and shopping, and will arrange the prescribing and delivery of your specialist products.
How often should I take a blood test if I am pregnant?
During pregnancy, you should have your blood taken two to three times a week so that your dietitian can properly monitor your Phe concentrations and make adjustments if necessary.
Will I have to stick to the same amount of exchanges throughout my pregnancy?
No. As you progress through pregnancy, the number of exchanges you 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 the baby’s liver will start to metabolise some of the Phe to tyrosine. It is important to send in regular blood samples to see when your Phe levels start to decline. Your dietitian will then advise you to increase your exchanges. Low Phe levels (less than 120 µmol/L) can also be harmful to your baby, as they can affect growth.
Will I need to take any extra nutritional supplements like the general population?
As in the general population, you will need to take a folic acid tablet (400 µg/day) from 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.
Should I be concerned if I am losing weight on my strict pregnancy diet?
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 in 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.
Will I need any extra antenatal care?
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.
Am I limited to what sort of birth I can have if I have PKU?
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.
What are the chances of my baby having PKU?
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 postnatal team are aware of your PKU.
Do I have to continue with a low-protein diet once my baby is born if I want to breastfeed?
No. Once the baby is born, as long as he or she doesn’t have PKU, you can breastfeed while being on a relaxed diet or “off diet”. This is because the baby is able to break down any extra Phe it gets from your milk. If your baby were 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.
