Selective Fetal Growth Restriction in Twins

A different measurement from the singleton version

In a singleton pregnancy, growth concerns are usually raised by a baby falling off a centile line or by a fundal height measurement that does not match the dates. Neither applies here. NICE says plainly: do not use abdominal palpation or symphysis–fundal height measurements to monitor for fetal growth restriction in a dichorionic twin or trichorionic triplet pregnancy, and repeats the same instruction for monochorionic pregnancies. The measuring tape is not a twin tool.

Instead, twin growth is assessed by comparing the babies with each other and with the centile chart at the same time. This page is about how that differs from growth restriction in a single pregnancy — it is not a general account of fetal growth restriction, which behaves differently and is investigated differently when there is only one baby.

How discordance is calculated

NICE asks teams to calculate and document estimated fetal weight discordance using a specific formula: the estimated weight of the larger baby minus the estimated weight of the smaller baby, divided by the estimated weight of the larger baby, multiplied by 100. For trichorionic triplets the same calculation is done twice, comparing the largest baby with the smallest and with the middle baby.

That percentage is written in your notes at every growth scan. It is the number your team is watching, and it is the number worth asking for.

The thresholds that change your care

NICE sets two, and they apply to dichorionic and monochorionic pregnancies alike.

• 20% discordance, and/or any baby below the 10th centile for gestational age: increase diagnostic monitoring in the second and third trimesters to at least weekly, and include Doppler assessment of the umbilical artery flow for each baby.
• 25% discordance and a baby below the 10th centile: refer to a tertiary level fetal medicine centre, because NICE describes this combination as "a clinically important indicator of selective fetal growth restriction".

Note the difference between the two. The first threshold is met by either finding. The second requires both together. NICE also lists fetal weight discordance of 25% or more with a baby below the 10th centile among the specific indications for a consultant opinion from a tertiary fetal medicine centre.

Twins Trust describes a lower trigger for specialist review in practice, saying doctors may diagnose growth restriction when there is more than a 10% difference between estimated weights, or when one baby drops away from their usual growth line. Different services set their referral bars differently; NICE sets the floor, not the ceiling.

When monitoring starts, and how often

The schedules differ by chorionicity. For dichorionic twins and trichorionic triplets, NICE says growth discordance monitoring is offered at each ultrasound scan from 24 weeks, with intervals not exceeding 28 days for twins and 14 days for trichorionic triplets. For monochorionic twins and triplets, monitoring starts at 16 weeks and the interval should not exceed 14 days.

Both use two or more biometric parameters plus amniotic fluid assessment, with fluid measured as the deepest vertical pocket on either side of the membrane.

Why a shared placenta changes everything

Twins Trust explains the usual cause: the placenta is not sharing blood and nutrients evenly, so one baby has a smaller share and simply does not get as much support for growth. It says this can happen in any twin or triplet pregnancy, whether the babies share a placenta or each have their own, and puts the frequency at around 10 to 15% of twin or triplet pregnancies.

What differs in a monochorionic pregnancy is the consequence. Because the circulations are connected, Twins Trust says that if the smaller twin dies in the womb this can affect the larger baby, including a risk of brain injury or death. That risk does not exist when each baby has their own placenta, and it is the reason the management options diverge so sharply.

RCOG's Green-top Guideline No. 51 lists selective growth restriction among the specific complications associated with inter-twin vascular anastomoses, alongside twin–twin transfusion syndrome and twin anaemia-polycythaemia sequence.

How it is told apart from TTTS

The two are often confused because both involve one baby doing worse than the other. Twins Trust draws the distinction clearly: TTTS is mainly about unequal blood flow between the babies, whereas selective growth restriction is more about how the placenta is shared, its size, and how well it works for each baby. Some pregnancies have both. The fluid measurements are what separate them on a scan — TTTS produces a marked fluid difference between the sacs, while growth restriction on its own may not.

Will you notice anything?

Usually not. Twins Trust says most parents do not notice symptoms themselves. It adds one important exception: if the placenta is not working well, there can be signs of pre-eclampsia in the mother, such as raised blood pressure or protein in the urine — which is why blood pressure and urine checks at every appointment matter, and why they are not just a formality.

What treatment can involve

Twins Trust sets out the range, and it depends heavily on gestation and on whether the placenta is shared.

Watching closely

If blood flow patterns stay reassuring, extra monitoring may be all that is needed, and the pregnancy can continue for as long as it is safe. In monochorionic pregnancies this can mean scans every one to two weeks or, in some cases, daily checks and a hospital stay.

Earlier birth

After around twenty-four weeks, Twins Trust says the most common option when blood flow becomes worrying is to deliver the babies early, often by caesarean, so they can be cared for in a neonatal unit.

Fetal procedures

Where restriction is severe and it is too early to deliver safely, Twins Trust describes two specialist options discussed in monochorionic pregnancies: a procedure that stops blood flow in the smaller baby's cord, which ends that baby's life but can protect the larger twin, and laser ablation to separate the two circulations on the placenta. Both are described honestly as difficult conversations, and Twins Trust says you should always be given time, information and emotional support.

Where the research is going

Twins Trust notes it is supporting the FERN study, which is examining how early selective growth restriction is currently managed in monochorionic twins in the UK and whether a future trial comparing treatment options would be feasible. That is a fair signal of how much genuine uncertainty remains in this area, and a reasonable thing to raise with a fetal medicine team.

The practical takeaway

Ask for the discordance percentage at each growth scan and write it down. A single number in isolation says little; the trend across scans is what your team is acting on, and it is the clearest way to follow what is happening to your own pregnancy.