When One Twin Slowly Gives Blood to the Other: Better News for Families Facing a Rare Twin Condition

Four studies from 2025 and 2026 on twin anemia–polycythemia sequence — what they found about survival, brain development, and what happens if one twin dies before birth

Identical twins who share one placenta sometimes also share blood vessels inside it, and in a rare condition called twin anemia–polycythemia sequence a few of those vessels let red blood cells drift slowly from one twin to the other over many weeks. One baby ends up short of red cells and the other ends up with too many. Four studies published in 2025 and 2026 have followed hundreds of these pregnancies and their children, and the picture that emerges is more reassuring than doctors expected — with the main remaining risk coming from being born too early rather than from the blood-sharing itself.

If you have been given this diagnosis, or told that your twins are being watched for it, this article explains what the condition is, what the newest research found, and what it means for the weeks ahead.

A problem that did not have a name until 2007

Doctors have long known that identical twins sharing a placenta can run into trouble because their blood vessels are joined. The best-known version is twin-to-twin transfusion syndrome, in which a large amount of blood moves from one twin to the other quite quickly. It is dramatic and easy to spot: one twin is surrounded by far too much amniotic fluid and the other by almost none.

For years, twins with a quieter version of the same problem went unrecognised. Their fluid volumes looked normal, so nothing seemed wrong — yet at birth, one baby was strikingly pale and the other deep red. In 2007 a Dutch team described two such twin pairs and argued that this was a condition in its own right ^[1]. By 2010 it had a name — twin anemia–polycythemia sequence, usually shortened to TAPS — along with a way to diagnose it before birth and a five-stage system to describe how advanced it is ^[2]. The diagnosis uses an ultrasound measurement of how fast blood moves through an artery in each baby's brain: blood flows faster when it is thin and anemic, and slower when it is thick with extra red cells. In 2019 that method was refined to compare the two babies against each other rather than against fixed thresholds, which picked up more cases ^[3].

TAPS arises on its own in roughly three to five of every hundred identical twin pregnancies with a shared placenta. It can also appear after laser surgery for twin-to-twin transfusion syndrome, when a few tiny vessel connections survive the treatment. A large European trial published in 2014 tested a more thorough laser technique — treating the whole dividing line across the placenta rather than only the visible connections — at five specialist hospitals. Among 274 women, it reduced TAPS from 16% to 3%, which was a real advance but not a cure ^[4]. Studies that followed, including one covering 370 pregnancies across 17 fetal-treatment centres, compared watching and waiting, blood transfusion to the baby before birth, laser surgery and early delivery — and found no single approach that was clearly best ^[5]. Families and clinicians were left in an uncomfortable position: everyone agreed on how to look for TAPS, but nobody agreed on what to do when it was found. A 2025 expert consensus statement from a North American network of fetal-treatment centres described exactly that gap ^[6].

What the four new studies looked at

The first, from a large hospital in Israel, reviewed twelve years of records covering 32 affected pregnancies and 57 babies, asking which features at diagnosis predicted a difficult pregnancy ^[7]. The second, led from the Netherlands, used an international register of TAPS pregnancies from ten fetal-treatment centres in eight countries to examine the 39 pregnancies in which one twin died before birth ^[8] — a frightening situation in shared-placenta twins, because blood can rush from the survivor into the twin who has died. The third, led from Italy, gathered every published report of that same situation and combined the results statistically ^[9]. The fourth, from the Dutch national centre for complicated twin pregnancies in Leiden, examined brain scans and long-term development in 256 babies born after TAPS over more than twenty years ^[10].

Finding it early is a warning sign

The Israeli team found that the earlier in pregnancy TAPS showed up, the harder the pregnancy went. Pregnancies in which one or both twins did not survive had been diagnosed at around 21 weeks, compared with about 27 weeks when both twins survived. The same pattern held for babies who had changes on brain scans before birth: those pregnancies were diagnosed at about 22 weeks rather than 26 ^[7].

This does not mean that finding the condition early is harmful. It means that when the condition is severe enough to show itself early, it tends to behave more aggressively. Overall in that group, 86% of babies survived: both twins survived in 78% of pregnancies, one twin survived in 16%, and neither survived in 6% ^[7].

If one twin dies, the other usually does far better than feared

This is the finding most likely to change a difficult conversation. In the international register, the twin who died was almost always the anemic one — 90% of cases — and this happened at around 23 weeks on average. Among the 38 surviving twins, none developed new anemia, none needed an emergency blood transfusion before birth, and brain scans done during pregnancy in 13 of them showed nothing abnormal. The survivors stayed in the womb for another nine weeks on average and were born at around 34 weeks. Not one of the 37 survivors who had a brain scan on their first day of life had severe brain injury. About one in five had a serious newborn complication, most often breathing difficulty from being born early ^[8].

The Italian-led analysis of all the published evidence reached the same conclusion, though with less precision because the numbers are small: the surviving twin died in about 5% of cases, and none died after birth. Signs of brain injury appeared on scans after birth in around 9% of survivors and on scans before birth in around 15% ^[9]. For comparison, in shared-placenta twins generally — without TAPS — when one twin dies before birth, the other dies in roughly 40% of cases and has abnormal brain scans after birth in over 40% ^[11].

Researchers think the explanation lies in the plumbing. In TAPS the shared vessels are unusually few and unusually narrow — less than a millimetre across — so when one twin dies, there is simply no wide channel for blood to rush through. The very feature that makes TAPS develop so slowly appears to protect the survivor at the worst moment ^[8], ^[9].

Two practical points follow. There is no need to rush delivery after one twin dies, because the harm, if it happens at all, happens at the moment of the death rather than in the weeks afterwards. And an emergency transfusion to the surviving baby is not usually needed if the brain-artery blood-flow measurement is normal ^[8].

Brain development: prematurity matters more than the blood-sharing

The Leiden study looked at 256 babies. About 17% had some form of brain injury, and 7% had severe injury — but more than nine out of ten of these injuries happened after birth, not before, and the most common type was bleeding into the fluid spaces of the brain, which is a well-known complication of being born early. The single strongest risk factor was gestational age at birth: each week earlier a baby was born raised the odds of brain injury by about 31%. Whether a baby had been the anemic or the over-full twin made no significant difference, and neither did whether the TAPS had happened on its own or after laser surgery ^[10].

Development was assessed in 166 of these children at around two and a half years. Just under three-quarters were developing normally, 18% had mild difficulties and 8% had moderate to severe difficulties. Children who had been the anemic twin had roughly twice the rate of any difficulty. Strikingly, in 10 of the 14 children with the most significant difficulties, the brain scans did not explain what was seen — and the most common problem in that group was deafness in both ears, which affected four children, all of them anemic twins. Researchers suspect the long months of anemia before birth, together with poor growth, low protein levels and low iron stores, contribute in ways that scans cannot yet show ^[10]. The same team has separately shown that the anemic twin often has low iron after birth while the over-full twin can have too much ^[12].

What this means for your family

Three things are worth taking from this research. If TAPS is found early in pregnancy, expect closer monitoring — not because the monitoring caused a problem, but because early appearance signals a more active condition. If the worst happens and one twin dies, the outlook for the other is much better than the general figures for shared-placenta twins would suggest, and there is usually no reason to deliver immediately. And because most brain injury in these babies happens after birth and is linked to prematurity, every week of pregnancy that can safely be gained is genuinely valuable.

Practically, ask for a brain ultrasound in the first three days after birth for any baby born after TAPS, whatever their gestational age, since that is when most injury shows up. Ask about a hearing test, particularly for the twin who was anemic. And ask to be enrolled in developmental follow-up, which in the Dutch centre continues to school age ^[10]. Your team will also expect some familiar newborn issues: roughly half of anemic twins need a blood transfusion, about a quarter of over-full twins need some of their thick blood replaced with fluid, and jaundice needing light therapy is common in both ^[10].

What researchers are working on next

The biggest open question is whether laser surgery helps in TAPS that arises on its own. A randomised trial is running now, coordinated from the Leiden centre, comparing laser surgery with standard care; its main measure is how long pregnancies last ^[10]. A second gap is long-term development after one twin dies — remarkably, not a single published study has yet followed those children into childhood ^[9]. A third is better brain imaging: because ordinary ultrasound cannot see subtle changes in how the brain matures and wires itself, more advanced scanning may eventually explain the difficulties that current scans miss ^[10].

None of this makes a TAPS diagnosis easy to hear. But the direction of the evidence over the last two years has been consistently toward better outcomes than were once assumed, and toward a clearer sense of where the real risks lie.

References

1. Lopriore E, Middeldorp JM, Oepkes D, Kanhai HH, Walther FJ, Vandenbussche FPHA. Twin anemia–polycythemia sequence in two monochorionic twin pairs without oligo-polyhydramnios sequence. Placenta. 2007;28(1):47–51. doi:10.1016/j.placenta.2006.01.010 ↩
2. Slaghekke F, Kist WJ, Oepkes D, et al. Twin anemia-polycythemia sequence: diagnostic criteria, classification, perinatal management and outcome. Fetal Diagnosis and Therapy. 2010;27(4):181–190. doi:10.1159/000304512 ↩
3. Tollenaar LSA, Lopriore E, Middeldorp JM, Haak MC, Klumper FJ, Oepkes D, Slaghekke F. Improved prediction of twin anemia–polycythemia sequence by delta middle cerebral artery peak systolic velocity: new antenatal classification system. Ultrasound in Obstetrics & Gynecology. 2019;53(6):788–793. doi:10.1002/uog.20096 ↩
4. Slaghekke F, Lopriore E, Lewi L, et al. Fetoscopic laser coagulation of the vascular equator versus selective coagulation for twin-to-twin transfusion syndrome: an open-label randomised controlled trial. The Lancet. 2014;383(9935):2144–2151. doi:10.1016/S0140-6736(13)62419-8 ↩
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6. Brock CO, Baschat AA, Moldenhauer JS, Ryan G, Johnson A. Surveillance of monochorionic twins for detection of twin-twin transfusion syndrome and twin anemia polycythemia sequence: a North American Fetal Therapy Network consensus statement. American Journal of Obstetrics and Gynecology. 2025;233(5):353–362. doi:10.1016/j.ajog.2025.07.001 ↩
7. Zloto K, Rosenthal N, Cohen S, Avnet H, Weisz B, Yinon Y. Predictors of Adverse Perinatal Outcome in Twin Anemia Polycythemia Sequence: Evidence From a Single Center Cohort Study. Prenatal Diagnosis. 2025;45(13):1737–1745. doi:10.1002/pd.70003 ↩
8. van de Sande MJA, Lopriore E, Lanna M, et al. Single fetal demise in twin anemia–polycythemia sequence: perinatal outcome of surviving cotwin. Ultrasound in Obstetrics & Gynecology. 2025;66(1):51–55. doi:10.1002/uog.29242 ↩
9. Piergianni M, Khalil A, Rizzo G, et al. Perinatal outcomes following single intrauterine death in monochorionic twin pregnancies complicated by twin anemia polycythemia sequence: Systematic review and meta-analysis. Acta Obstetricia et Gynecologica Scandinavica. 2026;105(6):1017–1025. doi:10.1111/aogs.70189 ↩
10. Rondagh M, van der Spoel JW, Lopriore E, et al. Brain injury in twin anemia–polycythemia sequence: prevalence, severity and long-term neurodevelopmental outcome. Ultrasound in Obstetrics & Gynecology. 2026;67(4):470–481. doi:10.1002/uog.70209 ↩
11. Mackie FL, Rigby A, Morris RK, Kilby MD. Prognosis of the co-twin following spontaneous single intrauterine fetal death in twin pregnancies: a systematic review and meta-analysis. BJOG. 2019;126(5):569–578. doi:10.1111/1471-0528.15530 ↩
12. Rondagh M, van de Sande MJA, Tollenaar LSA, et al. Twin anemia polycythemia sequence and the risk of iron deficiency in donors and iron overload in recipients. Journal of Perinatology. 2026;46(5):850–852. doi:10.1038/s41372-025-02386-y ↩