Twin-to-Twin Transfusion Syndrome (TTTS)

What TTTS actually is

NICE defines feto-fetal transfusion syndrome as what happens "when blood moves from one baby to another. The baby that loses the blood is called the donor and the baby receiving the blood is called the recipient." It is, in NICE's words, "a complication of monochorionic multiple pregnancies arising from shared placental circulation", and it is called twin-to-twin transfusion syndrome when there are two babies.

RCOG's Green-top Guideline No. 51 explains the underlying anatomy: the particular challenges of monochorionic pregnancies arise from vascular placental anastomoses that "are almost universal and connect the umbilical circulations of both twins". Those connections are normal. TTTS is what happens when the flow across them stops being balanced.

Who can get it, and who cannot

This is the single most useful thing to know. TTTS requires a shared placenta. If your twins are dichorionic — a placenta each — they cannot develop TTTS. The NHS says the risk is higher for monochorionic diamniotic twins but that it can happen in monochorionic monoamniotic twins too. Twins Trust notes it can also affect triplet pregnancies where babies share a placenta.

How common it is

The NHS states that "TTTS affects 10 to 15% of monochorionic twins and can have serious consequences." Twins Trust gives the same range, around 10% to 15% of monochorionic twin pregnancies, and adds that although it can happen at any point, it is most often diagnosed before twenty-four weeks. Tommy's expresses it as around 15 in 100 monochorionic pregnancies.

Set against RCOG's figure that roughly 30% of UK twin pregnancies are monochorionic, TTTS is uncommon across twin pregnancies as a whole — but for the group at risk, it is common enough that a whole surveillance programme is built around it.

What the fortnightly scans are looking for

NICE recommends offering diagnostic monitoring for feto-fetal transfusion syndrome to all monochorionic twin and triplet pregnancies, "with ultrasound every 14 days from 16 weeks until birth". The measurement is amniotic fluid: NICE asks for ultrasound assessment with the amniotic membrane visible in the image, measuring the deepest vertical pocket of fluid on either side of that membrane.

The logic follows the physiology. Twins Trust explains that the recipient baby copes with extra blood by producing more urine, which becomes amniotic fluid, so the fluid around them increases. The donor baby may have very little fluid and can end up "stuck" against the wall of the womb.

NICE also asks for simultaneous monitoring. At every ultrasound assessment in a monochorionic pregnancy, teams should look for feto-fetal transfusion syndrome, fetal growth restriction and advanced-stage twin anaemia polycythaemia sequence together, explaining that the relative likelihood of each changes with gestation but that all three can occur at any gestational age.

The numbers that trigger action

NICE sets out explicit thresholds, and they are worth knowing because they explain what your team is reacting to.

• Increase monitoring to at least weekly, with umbilical artery Doppler for each baby, if there is a difference in deepest vertical pocket depth of 4 cm or more between the sacs.
• Refer to a tertiary level fetal medicine centre if one baby's sac has a deepest vertical pocket of less than 2 cm and another baby's sac measures over 8 cm before 20 weeks and 0 days, or over 10 cm from 20 weeks and 0 days.
• Refer back to your named specialist obstetrician for further assessment if one sac is in the normal range and another measures either less than 2 cm or 8 cm or more.

NICE also lists feto-fetal transfusion syndrome among the conditions for which a consultant opinion from a tertiary level fetal medicine centre should be sought.

Symptoms worth reporting between scans

TTTS is usually found on ultrasound rather than through anything you feel, and Twins Trust says so directly. But it also warns that TTTS can progress quickly between scans, and lists changes to report straight away if you are known to be at risk: rapid tummy growth or suddenly feeling much larger than expected, sometimes within a single day, and palpitations where your heart feels like it is racing or pounding.

Twins Trust adds the reassurance that these can happen in healthy pregnancies too, so the instruction is to get checked, not to panic. Contacting maternity triage is the right response to a bump that changes shape overnight.

What happens if it is diagnosed

Twins Trust explains that once TTTS is confirmed it is graded into stages, from mild to more severe, and that this staging guides treatment. Care moves to a specialist fetal medicine unit.

Close monitoring

For mild, stage one disease, Twins Trust says close monitoring is often used, with frequent ultrasound and Doppler scans tracking growth, fluid and heart function.

Amnioreduction

Excess fluid can be drained from around the recipient baby through a fine needle. Twins Trust is clear that this does not correct the underlying cause but can ease pressure on the womb and reduce the risk of very early labour, and that it sometimes needs repeating.

Fetoscopic laser

Twins Trust describes fetoscopic laser ablation as currently the most effective treatment for more severe TTTS, especially stages three and four. A fetoscope is passed into the sac and a laser seals the connecting vessels on the placenta so each baby is supplied only through their own cord. On Twins Trust's figures, at least one baby survives in around 73% of cases after laser treatment, with better outcomes when TTTS is found and treated early.

Early birth

If TTTS is diagnosed late or progresses despite treatment, delivery may be recommended so the babies can be cared for in a neonatal unit. Twins Trust says steroid injections to mature the babies' lungs should be offered if early delivery is likely.

How this differs from complications in a singleton pregnancy

It is worth being explicit, because TTTS is often lumped in with general pregnancy complications and it does not belong there. It has no equivalent in a singleton pregnancy. It is not caused by anything you did, it cannot be prevented by diet, rest or aspirin, and it is not detected by blood pressure, urine testing or fundal height. The only thing that finds it is the fortnightly scan — which is why NICE builds the entire monochorionic schedule around it, and why attending every appointment matters more in this group than almost anywhere else in maternity care.