Twin Reversed Arterial Perfusion Sequence

What TRAP sequence is

Twins Trust describes twin reversed arterial perfusion sequence as a very rare condition affecting monochorionic twin or triplet pregnancies, where the babies share one placenta. One baby develops as expected. The other does not form properly but stays connected to the placenta through blood vessels. The non-developing baby may grow in size but will not look like a fully formed baby and cannot survive independently.

The name describes the mechanism. Blood flows through the shared vessels in the reverse of the normal direction, so the developing baby's heart is pumping blood for both. Twins Trust says that over time this extra work strains the developing baby's heart and, if untreated, may lead to heart failure or loss.

Who it can affect

Twins Trust is precise: TRAP only affects pregnancies where at least two babies share a single placenta, and pregnancies where each baby has their own placenta, dichorionic or trichorionic, are not at risk. It gives the frequency as about one in one hundred monochorionic pregnancies.

For scale, Tommy's puts the distribution of twin pairs at roughly 70 to 75 in 100 dichorionic diamniotic, 20 to 30 in 100 monochorionic diamniotic, and 1 or 2 pairs in 100 monochorionic monoamniotic. So TRAP applies to a minority of an already smaller group. It is genuinely rare, which is part of what makes a diagnosis so isolating.

How it is found

Twins Trust says TRAP is usually picked up on specialist ultrasound scans rather than through symptoms you notice yourself, and that this is why attending every scan you are offered matters.

The scans are already in your schedule. NICE NG137 says women with a monochorionic twin or triplet pregnancy should be offered ultrasound monitoring every 14 days from 16 weeks until birth, with simultaneous monitoring at every assessment for feto-fetal transfusion syndrome, fetal growth restriction and advanced-stage twin anaemia polycythaemia sequence. TRAP is generally identified during that programme.

Why the well baby is the one at risk

This is the part that is hardest to take in. The baby who is in danger is the one who is developing normally, because their heart is doing the work of two circulations. All the monitoring and all the treatment is directed at protecting that baby. The other baby cannot be saved and was never going to be.

Twins Trust says that if TRAP is found later in pregnancy and the developing baby's heart looks healthy, doctors may recommend close monitoring with frequent scans rather than immediate treatment, and that when TRAP is diagnosed earlier, treatment is more often needed.

Where your care moves to

NICE NG137 names twin reverse arterial perfusion sequence explicitly in its list of indications for seeking a consultant opinion from a tertiary level fetal medicine centre, alongside feto-fetal transfusion syndrome, discordant fetal death, suspected twin anaemia polycythaemia sequence, conjoined twins and pregnancies with a shared amnion.

NG137 defines a tertiary level fetal medicine centre as a specialist regional or supra-regional centre with the multidisciplinary team, expertise and infrastructure to assess and manage complicated twin and triplet pregnancies, including complex fetal interventions such as fetoscopic laser ablation and techniques including cord occlusion. Twins Trust says the same in plainer terms: if you are diagnosed you should be referred to a specialist fetal medicine unit experienced in complications of twin and triplet pregnancies.

What treatment involves

Twins Trust says the most common procedure is laser therapy, which blocks the umbilical cord of the non-developing baby and stops blood flowing between the two, reducing the strain on the developing baby's heart and usually allowing them to grow more safely for the rest of the pregnancy.

It describes the procedure as done under local or spinal anaesthetic, so you are awake but your abdomen is numb. A thin needle and hollow tube are passed through the abdomen into the fluid around the non-developing baby, and a tiny telescope and laser fibre are used to seal the blood vessels in the umbilical cord. Afterwards there are regular follow-up scans to check growth and heart function.

Your team will set out the risks and benefits for your specific situation. Timing, the health of the developing baby's heart and how far along the pregnancy is all feed into whether treatment is recommended at all.

Symptoms to report between scans

Twins Trust says most people with TRAP notice no clear symptoms, but that in more advanced cases polyhydramnios, too much amniotic fluid, can build up and cause symptoms similar to twin-to-twin transfusion syndrome. It lists reasons to contact your maternity team urgently: sudden weight gain or your bump becoming much bigger within a day or so, feeling breathless at rest or having palpitations, your abdomen feeling very tight or uncomfortable or looking stretched and shiny, and increased thirst, early contractions or new back or leg pain.

RCOG's multiple pregnancy information gives a shorter version for any monochorionic pregnancy: contact your healthcare professional immediately if you have sudden abdominal pain or swelling, sudden breathlessness, or a change in the babies' movements.

Birth, and afterwards

Twins Trust says your maternity and fetal medicine teams will discuss the safest way to give birth based on your baby's health and your situation, that vaginal birth may be possible if your baby is well and there are no other concerns, and that a caesarean may be recommended if there are worries. It also notes TRAP can increase the risk of preterm labour, and that an earlier birth may be recommended.

It explains what happens at delivery: after the developing baby is born you will also deliver the non-developing baby and any remaining tissue, which often shrinks in size after laser treatment. Ask your team to go through this beforehand, and to go over it as many times as you need.

The part that is not medical

Twins Trust puts it as well as anyone: hearing that one of your babies will not survive is devastating, and it is normal to feel shock, grief, anger or guilt. None of it is your fault. You are carrying a pregnancy and grieving at the same time, and both deserve support. Ask your unit what bereavement support is available, because it exists and is rarely offered without being asked for.