Twin Anaemia Polycythaemia Sequence (TAPS)

What TAPS is

Twin anaemia polycythaemia sequence, usually shortened to TAPS, is a complication of pregnancies where babies share a placenta. Twins Trust explains the mechanism: tiny blood vessels connect the babies across the shared placenta, and in TAPS these allow blood to move slowly from one baby to the other. Over time, one baby becomes anaemic — too few red blood cells to carry oxygen well — while the other becomes polycythaemic, with too many. Both babies' hearts end up working harder than they should.

RCOG's Green-top Guideline No. 51 lists twin anaemia-polycythaemia sequence among the specific complications associated with the inter-twin vascular connections that are almost universal in monochorionic placentas, alongside twin–twin transfusion syndrome and selective growth restriction.

Who can develop it

Only pregnancies where at least two babies share a single placenta. Twins Trust is explicit: if each baby has their own placenta — dichorionic twins or trichorionic triplets — they are not at risk of TAPS. It can arise on its own, or after laser treatment for twin-to-twin transfusion syndrome.

How common it is

Twins Trust puts TAPS at around three to five percent of monochorionic pregnancies. That makes it substantially less common than TTTS, which the NHS puts at 10 to 15% of monochorionic twins, but it is not vanishingly rare within the group at risk.

Why it is easy to miss

This is the defining feature of TAPS and the reason it needs a separate screening test. Twins Trust explains that unlike TTTS, TAPS usually does not cause big changes in the amount of amniotic fluid around each baby. The fluid on either side of the membrane can look near-identical. Every measurement that finds TTTS — deepest vertical pocket on each side, the 2 cm and 8 cm and 10 cm thresholds NICE sets — can be entirely normal in a pregnancy with TAPS.

NICE addresses this by asking teams to look for several things at once. It says to offer simultaneous monitoring for feto-fetal transfusion syndrome, fetal growth restriction and advanced-stage TAPS at every ultrasound assessment in a monochorionic pregnancy, and to explain that "the relative likelihood of each complication changes with advancing gestation but that they can all occur at any gestational age".

The test that does find it

The measurement is middle cerebral artery peak systolic velocity, usually written as MCA-PSV. It is a Doppler measurement of how fast blood is moving through an artery in the baby's brain. Twins Trust explains the principle in plain terms: if a baby is anaemic the blood is thinner and flows faster, and if a baby is polycythaemic the blood is thicker and flows more slowly. One baby with fast flow and one with slow flow is the pattern that raises TAPS.

Who NICE says should be screened weekly

NICE does not recommend weekly MCA-PSV for every monochorionic pregnancy. It targets two specific groups, and says to offer weekly ultrasound monitoring for TAPS from 16 weeks of pregnancy using MCA-PSV to women whose pregnancies are complicated by:

• feto-fetal transfusion syndrome that has been treated by fetoscopic laser therapy, or
• selective fetal growth restriction, which NICE defines here as estimated weight discordance of 25% or more together with a baby below the 10th centile for gestational age.

Separately, NICE says that in any monochorionic pregnancy showing cardiovascular compromise such as fetal hydrops or cardiomegaly, or unexplained isolated polyhydramnios, or an abnormal umbilical artery, teams should perform MCA-PSV measurements to help detect advanced-stage TAPS and "seek management advice immediately from a tertiary level fetal medicine specialist".

NICE also lists suspected TAPS among the specific indications for a consultant opinion from a tertiary level fetal medicine centre.

What you will and will not feel

Twins Trust says most of the time you will not feel any clear symptoms yourself, and that TAPS is usually picked up on scans rather than through changes you notice. That is the honest answer, and it is why attendance at every scan carries more weight here than a symptom list would. If something does feel different, Twins Trust's advice is still to contact your midwife, doctor or maternity triage rather than wait for the next appointment.

Staging and what treatment can involve

Twins Trust says that once diagnosed, TAPS is graded into five stages, from mild to very severe, and that this guides treatment. Because it is rare, referral to a specialist fetal medicine unit with experience of twin and triplet complications is expected. Scans then run at least every two weeks, usually combining growth measurements, Doppler and amniotic fluid assessment so the team can tell TAPS apart from TTTS.

Expectant management

If TAPS is mild, Twins Trust says the team may suggest close monitoring without intervention.

Fetoscopic laser

The connecting vessels are found and sealed so that each baby is supplied only through their own part of the placenta. Twins Trust describes it as done under local anaesthetic or an epidural, using a needle and thin tube replaced by a small telescope and a laser fibre.

Transfusion in the womb

If the anaemic baby is very unwell they may need a blood transfusion given directly into the umbilical cord under ultrasound guidance. If the polycythaemic baby's blood is very thick, Twins Trust describes an exchange transfusion in which some blood is removed and replaced with a similar volume of saline to thin it and ease the strain on the heart.

Planned early birth

Sometimes the safest option is to deliver so the babies can be treated on the neonatal unit. Twins Trust says steroid injections to help the babies' lungs mature should be offered if early delivery is likely, and that the timing and mode of birth should be discussed with you.

Diagnosis after birth

Twins Trust notes that TAPS is sometimes only confirmed after birth, by checking the babies' blood and examining the placenta for the tiny vessel connections. If your babies were monochorionic and one was noticeably paler and one noticeably ruddier at birth, that is a question worth asking the neonatal team, because it changes how both babies are followed up.

Why this is not the same conversation as anaemia in pregnancy

It is worth separating two things that share a word. Maternal anaemia is common in twin pregnancies — NICE asks for an extra full blood count at 20 to 24 weeks precisely because of it — and it is about your iron stores. TAPS is about the babies' blood, caused by connections in a shared placenta, and no amount of iron supplementation prevents or treats it. They are unrelated problems that happen to appear in the same pregnancy.