Twin-to-twin transfusion syndrome (TTTS): signs, scans and treatment

The RCOG explains that chorionicity refers to whether babies each have their own placenta or share one. Babies who share a placenta (monochorionic) are always identical and have a higher chance of complications. It is checked at the first ultrasound, which the NHS says should be offered at around 11 to 14 weeks to multiple pregnancies. The NHS lists three types of twins: dichorionic diamniotic (separate placentas and sacs), monochorionic diamniotic (one placenta, two sacs) and monochorionic monoamniotic (one placenta and one sac, much rarer).

The NHS says MCDA twins have the highest risk of TTTS, although it can happen in MCMA twins too, and that TTTS affects 10 to 15% of monochorionic twins and can have serious consequences. The RCOG gives about 15 in 100 monochorionic pregnancies. The differences are rounding and different studies, so treat all of these as approximate.

Twins Trust describes blood vessels connecting the babies inside the shared placenta. Usually flow is fairly even. In TTTS blood is redirected from one baby (the donor) to the other (the recipient), so the recipient has extra blood to cope with and often makes more urine, increasing the fluid around them, while the donor may have very little fluid and can become pressed against the wall of the womb. The NHS says it is caused by abnormal connecting blood vessels in the placenta.

Left untreated, Twins Trust says both babies can be at risk: the donor may be smaller and struggle to get enough nutrients and oxygen, and the recipient can develop heart strain. TTTS is graded into stages from mild to more severe to guide decisions about treatment. It can occur at any point but is most often diagnosed before 24 weeks. The RCOG says it can be mild and need no treatment or can be serious, in which case treatment is offered in a hospital with specialist expertise.

Twins Trust says doctors use regular growth and Doppler scans to look for the changes that cause TTTS, and that attending every scan you are offered is among the most important things you can do. The RCOG describes being monitored with frequent scans for signs of TTTS and being under a specialist team including a doctor, a midwife and a sonographer who specialise in multiple pregnancy. By around 16 weeks you should have your first appointment with the specialist team and a detailed care plan; sometimes referral to a fetal medicine unit in a different hospital is recommended.

The RCOG leaflet also says that if you are expecting monochorionic twins you will be offered scans at least every 2 weeks from 16 weeks (for MCMA twins too). The NHS adds that dichorionic twins are usually scanned every 4 weeks, which illustrates why shared-placenta pregnancies are watched more closely. Scan schedules differ between hospitals and nations, so use the schedule your own team gives you.

Twins Trust notes that TTTS can sometimes progress quickly between scans and lists signs to watch for if you are known to be at risk: rapid tummy growth, sometimes within a day; increased discomfort, bloating or shortness of breath; palpitations; increased thirst; early tightenings or contractions; new back or leg pain linked to a build-up of fluid. It stresses that these symptoms also occur in healthy pregnancies, so not to panic, but to get checked.

The RCOG's list is shorter: sudden abdominal pain and/or swelling, sudden breathlessness and any change in the babies' movements. In either case, contact your midwife, maternity triage or specialist team straight away, using the number on your notes, and call 999 if you are very unwell or have severe breathlessness.

Twins Trust says that if TTTS is diagnosed you should be referred to a specialist fetal medicine unit, and that treatment depends on severity, gestation and how each baby is coping. Close monitoring with frequent ultrasound and Doppler scans is often used for mild, stage one TTTS. Amnioreduction removes excess fluid from around the recipient baby using a fine needle; it does not correct the underlying cause but can ease pressure on the womb, give the donor baby more room and lower the risk of very early labour; it is usually used in later pregnancy and in less severe stages and sometimes needs repeating.

Fetoscopic laser ablation is described by Twins Trust as currently the most effective treatment for more severe TTTS, especially stages three and four. A needle and thin tube are passed into the sac around the recipient baby and a fetoscope used to seal with a laser the connecting vessels, so each baby is supplied through their own cord. It is usually done under local or spinal anaesthetic, and extra fluid is often drained at the same time. Twins Trust says that in around 73% of cases at least one baby survives after laser treatment and outcomes are usually better when TTTS is treated early. If it is found late or progresses despite treatment, early delivery may be recommended, with steroid injections offered to help the babies' lungs mature if early delivery is likely.

Twins Trust says a TTTS diagnosis can feel overwhelming and unfair, that it is not your fault, and that you have the right to be involved in decisions about your care. It suggests going to all appointments, eating as well as you can, staying moderately active if comfortable and resting. If you are worried, contact your midwife, doctor or maternity triage straight away. Support, including bereavement support if a baby dies, is available from the charity and from your hospital team. Pathways to fetal medicine centres differ across the UK nations.