Do you want to have children with phenylketonuria (PKU) or hyperphenylalanemia (HPA)? Fortunately, you can! For this you have to follow an adapted diet, which you have to follow well both before and during your pregnancy. You can read all about it on this page.
This page explains how to prepare for pregnancy, manage your diet during each stage, and care for yourself and your baby after birth.
PKU and HPA before pregnancy
Low Phe levels before pregnancy
Maintaining low Phe levels before conception is essential. During pregnancy, the baby receives all nutrients through the placenta, including phenylalanine. Because of how the placenta functions, Phe levels in the baby’s blood are naturally higher than in the mother’s. High maternal Phe can harm the baby’s growth and brain development.
Your healthcare team will help you achieve good metabolic control before trying to conceive, usually by returning to a strict low protein diet and using your prescribed protein substitute. They will also advise how long Phe levels need to be stable before pregnancy.
Back on a stricter diet
If you’ve relaxed your diet in recent years, this can take adjustment. Involve your partner, family, and friends, understanding why this matters helps them support you better. Your metabolic care team will guide you every step of the way.
The PKU diet in pregnancy
An adapted PKU diet is important to support your baby’s healthy growth and development. The target phenylalanine range during pregnancy is lower than at other times. Your metabolic dietitian will design a personalised plan to maintain good nutritional status and healthy weight gain.
Careful use of your prescribed protein substitute and accurate tracking of your protein intake are essential. Depending on the products you use, your dietitian may recommend extra folic acid, vitamin D, or DHA supplements.
Common pregnancy symptoms and how to manage them
Fatigue
During pregnancy, your body uses more energy to support the growing placenta and baby. Fatigue is common. Following your low protein diet as prescribed helps maintain energy levels. Tip: Prepare extra portions of low protein meals on days when you have more energy and freeze them for busier times.
Nausea
Nausea, especially in the morning, is common. An empty stomach can increase acidity and make symptoms worse. Eat small, frequent meals and avoid long gaps between snacks. Stay hydrated by drinking small sips of fluid throughout the day. If nausea or vomiting prevents you from eating enough, your Phe levels may rise. Contact your dietitian immediately for advice, they may suggest adding extra energy sources to your meals or drinks.
Tip: Keep a small snack on your bedside table, such as low protein crackers or a piece of fruit, and eat it before getting up.
Changes in taste and smell
Pregnancy hormones can alter your sense of taste and smell, making familiar foods or your protein substitute seem unappealing. Many amino acid preparations come in different flavours and formats, speak to your dietitian about alternatives if needed.
Tip: Try flavour variations in your low protein meals, add herbs or spices for savoury dishes, or cinnamon and sugar for sweet options.
Hydration and caffeine
Aim to drink 1.5–2 litres of fluid per day. Water is best, but other drinks count too. Limit caffeinated drinks such as coffee and black or green tea.
Caffeine-free teas, such as rooibos, chamomile, ginger, or mint, are safe options. Be cautious with ‘diet’ or ‘light’ drinks, as they may contain aspartame (E951), a source of phenylalanine that must be avoided in PKU.
Exercise and movement
Hormonal changes can cause mood swings. If you feel anxious about pregnancy or childbirth, talk to your care team or create a birth plan to share with your midwife or obstetrician. This can provide reassurance and help reduce stress.
Stress and emotional health
With all the extra hormones during pregnancy, it's quite normal to experience mood swings. Your body is working hard to support the development of your child. If you have any worries, it helps to talk about them.
If you're worried about childbirth, creating a birth plan can help. In it, you can outline your wishes for the birth. Discuss this plan with your midwife or gynaecologist, as this can provide peace of mind and reduce tension.
Drink enough
Staying hydrated is important, especially during pregnancy. Aim to drink 1.5 to 2 liters of fluid each day. While water is always the best choice, other beverages can count toward your total intake.
However, be mindful of your consumption of caffeinated drinks like coffee, black tea, and green tea. It's important to limit caffeine for the health of your unborn baby, as the fetus takes longer to break it down. High caffeine consumption can also slow fetal growth. Always discuss your personal caffeine limit with your dietitian.
Caffeine-free teas that can generally be enjoyed without restriction include rooibos, chamomile, ginger, and mint. Remember that the fluid you mix with your amino acid preparation also counts toward your daily goal. For example, if you take your preparation three times a day, that may already account for about 0.5 liters of your fluid intake!
Tip: Be cautious with 'diet' or 'light' drinks. Many of these contain the sweetener aspartame, which is a source of phenylalanine and must be avoided in a PKU diet. On food labels, look for aspartame listed by name, as its E-number (E951), or as part of a blend like Aspartame-acesulfame salt (E962).
Sports
Sports and exercise are beneficial for everyone. If your pregnancy is progressing smoothly and you have no complications, you can generally continue to exercise right up until delivery. Staying active offers advantages for both you and your fetus. For example, regular exercise can improve blood flow to the placenta, support fetal brain development, reduce the risk of common pregnancy ailments, and promote a good night's sleep.
Always listen carefully to your body. Stop immediately if you feel dizzy, nauseous, or experience contractions (a hardening of the abdomen). Even if you're not a seasoned athlete, aiming for at least 30 minutes of daily, low-impact exercise, such as walking or cycling, is beneficial.
Tip: After exercising, take your amino acid preparation within half an hour, together with sufficient fluids. This helps to replenish the fluids and minerals lost through perspiration and supports muscle recovery.
Stress and tension
With all the extra hormones during pregnancy, it's quite normal to experience mood swings. Your body is working hard to support the development of your child. If you have any worries, it helps to talk about them.
If you're worried about childbirth, creating a birth plan can help. In it, you can outline your wishes for the birth. Discuss this plan with your midwife or gynaecologist, as this can provide peace of mind and reduce tension.
PKU or HPA after pregnancy
After pregnancy Breastfeeding with PKU or HPA Breastfeeding is encouraged if possible. If your baby has PKU, they will need a combination of limited breast milk and a special amino acid-based formula.
Your metabolic team will provide tailored guidance. If your baby does not have PKU, you can breastfeed as normal, your breast milk remains excellent quality, even while following a low protein diet.
Tip: Breastfeeding can help lower your Phe levels naturally, as some phenylalanine is used to produce breast milk. Your dietitian may adjust your protein allowance during this time.
Breastfeeding with a metabolic disorder
Breast milk is the most natural food, with a composition precisely tailored to the needs of a child. Therefore, if breastfeeding is possible, it is highly recommended.
It's important to know if your baby also has PKU. An infant with PKU should only drink a limited amount of breast milk and will also receive a special amino acid-based formula. Your metabolic treatment team will provide you with extensive guidance on this topic.
If your baby does not have PKU, you can breastfeed as usual. Your breast milk is of excellent quality, even when you are on a PKU diet.
Tip: Breastfeeding can also have a significant benefit for your own PKU diet. Your body uses Phe from your diet to produce the protein in your breast milk. Because of this, your dietitian may allow you to consume more protein while you are breastfeeding.
Postpartum PKU management
After childbirth, it remains important to follow your PKU diet and keep Phe levels stable. High Phe after birth can lead to mood changes and concentration difficulties, whereas maintaining good control can help you adapt to new routines and enjoy parenthood. The diet is usually less strict than during pregnancy, making it easier to manage.
Newborn Screening (Heel-Prick Test) In many countries, newborns are screened for metabolic disorders between day 4 and day 7 after birth.
Discuss with your metabolic team when your baby will be tested to confirm whether they also have PKU or another metabolic condition. Returning to your pre-pregnancy weight Weight loss should be gradual and supervised by your dietitian. Losing weight too quickly can increase Phe levels.
Regular light activity and spreading protein intake evenly throughout the day help maintain metabolic balance. Returning to work Balancing work and parenthood takes planning.
Prepare low protein meals and protein substitutes in advance. If breastfeeding, arrange a private space at work for expressing milk. Maintaining good metabolic control can help reduce fatigue and improve focus as you adjust to life with your new baby.
Conclusion Having a baby when you have PKU or HPA is entirely possible with the right preparation, diet, and medical support. Managing your Phe levels before, during, and after pregnancy is crucial for your baby’s healthy development. With the guidance of your metabolic team, and the support of your partner, family, and friends, you can enjoy a healthy pregnancy and positive postpartum experience.
