When One Twin Grows and the Other Does Not: What Families Can Now Be Told

Four 2025 studies of identical twins who share a placenta — what the blood-flow scans really predict, how the bigger twin fares, and why the trial everyone wanted cannot be run

When identical twins share a single placenta, one baby sometimes grows far more slowly than the other. Four studies published during 2025 examined how well doctors can predict what will happen, what becomes of the twin who grows normally, and whether the procedure that might help can ever be properly tested. Together they change what families in this situation can honestly be told.

A shared placenta that was never divided evenly

Most identical twins share one placenta — the organ that delivers oxygen and nutrients from the mother's bloodstream to her babies. Sharing sounds equitable, but the division is rarely equal. If one twin's share of the placenta is small enough, that twin grows poorly. Doctors call this selective fetal growth restriction: selective because it affects one twin and not the other. It occurs in roughly one in seven to one in ten of these pregnancies.

What makes it more than a matter of size is that the two babies' circulations are physically joined. Blood vessels run across the shared placenta and connect one twin's bloodstream to the other's. That connection is what makes the smaller twin's difficulties everyone's difficulties. If the smaller twin dies in the womb, blood can drain rapidly out of the surviving twin through those shared vessels, which can cause serious brain injury or the loss of both babies.

This leaves families and doctors facing a genuinely hard choice, and it is worth naming plainly. Waiting means the smaller twin has more time to grow, but also means accepting a risk that it may die suddenly and take its brother or sister with it. Delivering early protects against that, but a baby born at 30 weeks — however well grown — faces the breathing difficulties, feeding difficulties and long hospital stay that come with prematurity. There is no option that carries no risk.

How doctors have been predicting the future, and why it was never quite reliable

Since 2007, obstetricians have sorted these pregnancies into three groups by looking at the blood flow in the smaller twin's umbilical cord using Doppler ultrasound, a scan that shows the direction and speed of blood movement [1]. In Type I, blood keeps flowing forward between heartbeats — the reassuring pattern. In Type II, that forward flow is persistently absent or even reversed — the pattern long regarded as most dangerous. In Type III, the flow comes and goes, present at some moments and absent at others.

This system has guided care worldwide for almost twenty years, and it did real good: it gave clinicians a shared language and a reason to scan some pregnancies more often than others. A 2017 review pooling many studies confirmed that Type II carried the highest risk of the smaller twin dying before birth [2]. But two uncomfortable facts sat alongside it. First, the pattern was known to change over the course of a pregnancy, meaning a family might be given a prognosis in February that no longer applied in April [3]. Second, doctors could not agree on what to actually do about the more severe types. A survey of 113 specialists published in 2024 found near-unanimity on watching Type I closely and considerable disagreement about everything else [4]. Families in different hospitals were being offered genuinely different care for the same condition.

The label moves, so the prediction has to move with it

The first two of the 2025 studies confirm that the blood-flow category is not a fixed diagnosis. The larger of the two, called the ALIGN Study, gathered 363 pregnancies from five specialist centres in the Netherlands, Belgium, Sweden, Spain and Canada, treated between 2016 and 2022 [5]. In 59% of them — nearly three in five — the blood-flow pattern changed during the pregnancy. Encouragingly, in about one in five pregnancies that started with the intermittent Type III pattern, the flow later returned to normal.

The second study followed 103 pregnancies over twelve years at a single unit in Turin, Italy, where the same team of doctors applied the same protocol throughout [6]. There the pattern changed in about one in three pregnancies. Around a quarter of pregnancies initially labelled Type I — the reassuring group — had to be reclassified later.

The practical lesson is straightforward and genuinely useful. Any prediction given to a family should be based on the most recent scan, not the first one. A reassuring Type I result early in pregnancy is good news for that week; it is not a promise, and it is not a reason to stop scanning. Equally, a worrying Type III result early on is not a verdict: for roughly one family in five, the flow improves.

What actually happens to the babies

When prognosis is based on the most recent scan, the ALIGN numbers are clear. Among pregnancies managed by watchful waiting, the smaller twin was born alive in 98% of cases where the blood flow had returned to normal, 83% where the flow was intermittent, and 70% where it was persistently absent or reversed [5].

The Turin results add something the survival figures alone do not show. Deaths were concentrated in the Type II group — 7.7% of babies, compared with 1.7% in Type I and 2.9% in Type III — and both stillbirths in the whole twelve-year series occurred in Type II pregnancies [6]. But the babies who needed the most intensive-care support were the Type III babies. All of them were admitted to neonatal intensive care; 85% developed respiratory distress syndrome, the breathing difficulty of premature lungs, compared with 45% of Type I babies; and they stayed in intensive care an average of 32 days against 19. The reason is timing: 82% of Type III pregnancies were delivered before 32 weeks. One further finding matters for families: nearly three in ten Type III babies had a birth difference, and more than 85% of those involved the heart — usually a hole between the pumping chambers, which is often treatable and sometimes closes by itself. This confirms an earlier report of more heart differences in these pregnancies [6], [7], and it is a good reason for a detailed scan of the baby's heart at around 20 weeks.

The bigger twin is not simply the lucky one

It is natural to focus on the smaller baby. A French study of 57 normally grown twins born from these pregnancies, followed to their second birthday at a hospital in Marseille, suggests that is a mistake [8]. Compared with babies from uncomplicated shared-placenta twin pregnancies, these children were born earlier — 33 weeks on average against 36 — and needed far more support: 63% went to intensive care against 24%, and 68% needed help with breathing against 26%.

Two findings from this study are worth families knowing about together, because one is worrying and the other is reassuring. The worrying one is that when the researchers compared each normally grown twin with its own smaller sibling, it was the bigger twin who more often had changes on brain MRI scans — 56% against 17%. The bigger twin is the one on the receiving end of the shared circulation, and it is not a bystander. The reassuring one is what happened next. At two years of age, a standard developmental questionnaire covering communication, movement, problem-solving and social skills found no meaningful difference between these children and children from uncomplicated twin pregnancies, and no meaningful difference between the bigger and smaller twin within a pair. Early scan findings did not translate into measurable developmental differences at two.

That reassurance should be held carefully rather than tightly. The study followed fewer than half its children to age two, spanned a decade of changing practice, and used a screening questionnaire rather than a full developmental assessment. Two years is early. It is real evidence, and it is not the last word.

The operation that might help, and the trial that will not happen

For the most severe cases, doctors can offer a procedure that stops the smaller twin's circulation, protecting the bigger twin from the sudden blood loss that would follow if the smaller twin died unexpectedly. It is called selective reduction, and it is as difficult a decision as it sounds.

ALIGN compared what happened to the bigger twin after this procedure with what happened after watchful waiting [5]. After selective reduction, 75% of bigger twins were born alive at 32 weeks or later, against 46% with watchful waiting — an advantage of roughly a third that held up after the researchers adjusted for how severe each pregnancy was to begin with. Babies were born much later on average, at 36 weeks rather than 31 weeks, and serious complications fell from 16% to 5%. But — and this matters enormously in a conversation with parents — the chance of the bigger twin being born alive at all was not significantly different. The procedure did not make survival more likely. It made being born later, and healthier, more likely. Those are different things, and they deserve to be described separately.

A separate 2025 analysis modelled the costs and benefits of the different procedures and concluded that one technique, radiofrequency ablation, looked most economical — while stating explicitly that its own findings were not reliable enough to guide anyone's care [9].

The obvious answer to all this uncertainty would be a randomised trial, in which families agree to have the treatment decided by chance so the two approaches can be compared fairly. The fourth study, from the UK-based FERN project, asked whether such a trial could be done [10]. Twenty-two stakeholders — specialist doctors, neonatal doctors, statisticians and parents who had lived through this — voted on whether it would be acceptable and workable. Not one scenario reached the 50% threshold the researchers had set in advance. Parents and clinicians interviewed separately had said the same thing [11]: asking a family to leave a decision of this weight to chance is not something most people can accept.

What researchers are working on next

That verdict is not a dead end, and it should not be read as one. It redirects the effort. Instead of a randomised trial, researchers are building large international studies that follow many pregnancies from many hospitals and use statistical methods to compare approaches fairly — which is exactly what ALIGN did. A prospective study called CONTRAST is currently recruiting across six specialist centres with the specific aim of predicting, at the moment of diagnosis, which pregnancies will run into trouble.

For families facing this diagnosis now, four things from this year's research are worth carrying into the next appointment. Ask which blood-flow pattern applies today, because it may have changed. Ask for a detailed scan of both babies' hearts. Ask what the plan is for the bigger twin, including whether a brain scan is planned after birth. And if selective reduction is discussed, ask separately about the chance of each baby surviving and the likely week of birth, because the evidence speaks clearly to the second and much less clearly to the first.

References

  1. Gratacós E, Lewi L, Muñoz B, et al. A classification system for selective intrauterine growth restriction in monochorionic pregnancies according to umbilical artery Doppler flow in the smaller twin. Ultrasound Obstet Gynecol. 2007;30:28–34. doi:10.1002/uog.4046 ↩
  2. Buca D, Pagani G, Rizzo G, et al. Outcome of monochorionic twin pregnancy with selective intrauterine growth restriction according to umbilical artery Doppler flow pattern of smaller twin: systematic review and meta-analysis. Ultrasound Obstet Gynecol. 2017;50(5):559–568. doi:10.1002/uog.17362 ↩
  3. Rustico MA, Consonni D, Lanna M, et al. Selective intrauterine growth restriction in monochorionic twins: changing patterns in umbilical artery Doppler flow and outcomes. Ultrasound Obstet Gynecol. 2017;49:387–393. doi:10.1002/uog.15933 ↩
  4. Prasad S, Khalil A, Kirkham JJ, et al. Diagnosis and management of selective fetal growth restriction in monochorionic twin pregnancies: a cross-sectional international survey. BJOG. 2024;131(12):1684–1693. doi:10.1111/1471-0528.17891 ↩
  5. Noll ATR, van Hoogstraten A, Nulens K, et al; ALIGN Study Group. Outcome of monochorionic diamniotic twin pregnancy with selective fetal growth restriction and continuous or intermittent absent or reversed end-diastolic umbilical artery flow: international multicenter cohort study. Ultrasound Obstet Gynecol. 2025;66(1):41–50. doi:10.1002/uog.29241 ↩
  6. Roero S, Arduino S, Ingala A, et al. Management of monochorionic twin pregnancies complicated by selective fetal growth restriction: retrospective single-center 12-year experience. Diagnostics (Basel). 2025;15(20):2653. doi:10.3390/diagnostics15202653 ↩
  7. Faiola S, Casati D, Stellio LN, et al. Congenital heart defects in monochorionic twin pregnancy complicated by selective fetal growth restriction. Ultrasound Obstet Gynecol. 2023;61:504–510. doi:10.1002/uog.26098 ↩
  8. Jarry MA, Topalian N, Cosnard L, D'Ercole C, Chau C, Tosello B. Neonatal and two-year prognosis of eutrophic newborns from monochorionic diamniotic twin pregnancies complicated by selective intrauterine growth restriction. Children (Basel). 2025;12(5):615. doi:10.3390/children12050615 ↩
  9. Morgan JC, Rios J, Kahl T, et al. Management of monochorionic diamniotic twin gestation affected by Type-II selective fetal growth restriction: cost-effectiveness analysis. Ultrasound Obstet Gynecol. 2025;65(1):39–46. doi:10.1002/uog.29135 ↩
  10. Khalil A, Prasad S, Kirkham JJ, Jackson R, Woolfall K; the FERN study group. Feasibility and acceptability of randomized controlled trial of intervention vs expectant management for early-onset selective fetal growth restriction in monochorionic twin pregnancy. Ultrasound Obstet Gynecol. 2025;65(6):671–675. doi:10.1002/uog.29175 ↩
  11. Mitchell TK, Popa M, Ashcroft RE, et al. Balancing key stakeholder priorities and ethical principles to design a trial comparing intervention or expectant management for early-onset selective fetal growth restriction in monochorionic twin pregnancy: FERN qualitative study. BMJ Open. 2024;14(8):e080488. doi:10.1136/bmjopen-2023-080488 ↩