Key takeaways
Selective growth restriction happens when one baby is much smaller and not growing as expected
It’s usually caused by one baby receiving a smaller share of the placenta’s blood and nutrients
Regular ultrasound and Doppler scans help your team monitor both babies and guide safe timing of birth
Treatment ranges from close monitoring to early delivery or specialist procedures, depending on severity and gestation
Understanding selective growth restriction in twins and triplets
In any pregnancy, a baby can measure smaller than expected for their gestational age. In a pregnancy with twins, triplets or more, doctors are also watching how evenly your babies are growing.
If one baby is much smaller than the other baby or babies and isn’t growing as expected, this may be called selective intrauterine growth restriction (sIUGR) or selective fetal growth restriction (sFGR). These terms describe a pattern where one baby’s growth is restricted while the other baby or babies grow normally.
Most often, this happens because the placenta isn’t sharing blood and nutrients evenly. One baby may have a smaller “share” of the placenta, so they simply don’t get as much support for normal growth as their sibling. This can feel frightening, but you’re not alone and you haven’t caused this.
How selective growth restriction is found and how common it is
Selective growth restriction is usually picked up during routine ultrasound scans. Your sonographer will measure each baby and plot their growth on a chart.
Doctors may diagnose growth restriction when there is more than a 10% difference between the estimated weights of the babies, or when one baby is dropping away from their usual growth line on the chart.
You may also be offered Doppler scans. These measure blood flow in each umbilical cord and check the amount of amniotic fluid around each baby. In sIUGR, the smaller baby may have reduced blood flow and less fluid around them.
Some babies affected by Twin-to-Twin Transfusion Syndrome (TTTS) (LINK - Inf16) also have sIUGR, but they are different conditions. TTTS is mainly about an unequal flow of blood between the babies. sIUGR is more about how the placenta is shared, its size and how well it works for each baby.
Growth restriction can happen in any twin or triplet pregnancy, whether your babies share a placenta or each have their own. It affects around 10–15% of twin or triplet pregnancies.
Most parents don’t notice symptoms themselves. Sometimes, if the placenta isn’t working well, there can be signs of pre-eclampsia in the mother, such as high blood pressure or protein in the urine. This is why it’s so important to attend every appointment and have your blood pressure and urine checked regularly.
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Possible complications and treatment choices for sIUGR
Selective growth restriction can affect the smaller baby’s growth and development, both before and after birth. In more severe cases, it can also affect the larger baby, especially if they share a placenta.
You should be referred for specialist review, usually with a fetal medicine team. You’ll likely be offered frequent ultrasound scans to:
- Monitor the weight difference between the babies
- Check cord blood flow with Dopplers
- Measure amniotic fluid around each baby
In many pregnancies, blood flow remains reassuring. In these situations, extra monitoring may be all that’s needed and you may continue the pregnancy for as long as it’s safe.
If blood flow patterns become worrying, your team will talk you through possible treatments. After around twenty four weeks, the most common option is to deliver your babies early, often by caesarean section, so they can be cared for in a neonatal unit.
When severe growth restriction happens very early in pregnancy, the decisions can be much harder. Sometimes doctors may advise continuing the pregnancy, even if the smaller baby is unlikely to survive, to give the other baby the best chance of being born at a safer gestation. These are heartbreaking conversations, and you should always be given time, information and emotional support.
Management options in shared-placenta pregnancies
If your twins share a single placenta (a monochorionic pregnancy), there are three main approaches you might hear about. The best option depends on how unwell the smaller baby is, the blood flow patterns and how many weeks pregnant you are.
Expectant (conservative) management
If the restriction isn’t too severe and blood flow stays reassuring, your team may recommend very close monitoring. This can mean scans every one to two weeks or, in some cases, even daily checks and a hospital stay. The risk is that if the smaller twin dies in the womb, this can affect the larger baby, including a risk of brain injury or death.
Selective fetial reduction
If the growth restriction is very severe and it’s too early to safely deliver both babies, doctors may discuss a procedure that stops blood flow in the smaller baby’s umbilical cord. This leads to the smaller baby’s death but can protect the larger twin, giving them a better chance of reaching a safer gestation. The terminology can feel harsh, and it’s completely understandable to find these discussions overwhelming.
Laser ablation
Another option in some cases is laser ablation. A laser is used to block the blood vessels that connect the babies on the surface of the placenta, separating their circulations. This can protect the larger twin if the smaller twin dies, but it may increase the risk that the smaller twin won’t survive.
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Looking after yourself and finding support
If you’ve been told that one of your babies is smaller, it’s normal to feel shocked, anxious or overwhelmed. Try to use your appointments to ask questions, understand your options and talk through what matters most to you and your family. You’re allowed to ask for things to be explained more than once.
You can’t change how the placenta formed, but there are positive steps you can take. Attending all your appointments, eating as healthily as you can, staying moderately active and getting enough rest are all important for you and your babies.
You can use the pregnancy tracker (antenatal care checklist - Link) to keep on top of the appointments and tests you should be offered throughout your pregnancy. If you’re struggling emotionally, ask your midwife, doctor or specialist nurse about extra support.
Above all, remember that you don’t have to face this on your own. Twins Trust and your healthcare team are here to help you navigate the information, understand your choices and feel as supported and empowered as possible through each stage of your pregnancy.
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