One twin can die in the womb while the other survives, and it happens far more often than most people realize. In the first trimester, the loss of one twin occurs in roughly 15 to 35 percent of all twin pregnancies, a phenomenon known as vanishing twin syndrome. Later losses are less common but carry higher stakes for the surviving twin, with risks that depend heavily on whether the twins share a placenta and on how far along the pregnancy has progressed.
Vanishing Twin Syndrome in the First Trimester
The most frequent version of single twin loss happens so early that many parents never know a second embryo existed. Vanishing twin syndrome refers to the first-trimester loss of one twin, where the embryonic tissue is gradually reabsorbed by the mother’s body, the surviving twin, or both. The pregnancy then continues as a singleton. Because early ultrasound has become routine, clinicians now detect initial twin pregnancies that would have gone unnoticed a generation ago, and they sometimes see one gestational sac shrink and disappear between scans.
The reported prevalence of vanishing twin syndrome sits between 15 and 35 percent of twin pregnancies, with several factors raising the odds: older maternal age, the number of embryos transferred during IVF, and the initial number of gestational sacs visible on early ultrasound.1PubMed. The vanishing twin: Diagnosis and implications When the loss occurs early enough, the surviving twin typically develops without further complications. The body handles the reabsorption efficiently, and the pregnancy proceeds as though it were always a singleton.
One practical wrinkle, though, is that a vanishing twin can skew prenatal blood screening results. In pregnancies where one twin was lost, certain maternal blood markers shift. Pregnancy-associated plasma protein A rises by about 20 percent, alpha-fetoprotein increases by roughly 10 percent, and dimeric inhibin A goes up by about 13 percent compared with normal singleton pregnancies.2PubMed. First and second trimester maternal serum markers in pregnancies with a vanishing twin These shifts can trigger false-positive results on routine screening tests for chromosomal abnormalities, leading to unnecessary anxiety or follow-up procedures. If you’ve had an early ultrasound showing twins but later carried a singleton, that information is worth mentioning to your provider before interpreting any screening results.
Why Sharing a Placenta Changes Everything
The single biggest factor determining what happens to the surviving twin is whether the twins share a placenta. Identical twins who share one placenta (monochorionic twins) face dramatically different risks than fraternal twins or identical twins with separate placentas (dichorionic twins). In shared-placenta pregnancies, blood vessels on the placental surface connect the two twins’ circulations. When one twin dies, the surviving twin can lose blood rapidly through these connections, experiencing a sudden drop in blood pressure and oxygen delivery. This hemodynamic crisis is the root cause of most serious complications in the survivor.
The numbers are stark. In a large cohort study, the overall risk of early pregnancy loss before 24 weeks was about 60 per 1,000 fetuses in monochorionic twins versus roughly 7 per 1,000 in dichorionic twins, a ninefold difference.3PubMed. Early fetal loss in monochorionic and dichorionic twin pregnancies: analysis of the Southwest Thames Obstetric Research Collaborative (STORK) multiple pregnancy cohort Research on outcomes after one twin dies has found that the probability of the surviving twin also dying is around 12 percent for monochorionic twins compared with about 4 percent for dichorionic twins. Monochorionic survivors also face nearly a fivefold increased likelihood of neurodevelopmental problems.4PubMed Central. Conservative Management of a Monochorionic Twin Pregnancy with an Intrauterine Fetal Death at 20–21 Weeks and Successful Term Delivery of the Second Twin
With dichorionic twins, the placentas are separate, so there is no shared vascular network. When one dies, the surviving twin is physically insulated from the hemodynamic consequences. The risks are not zero: preterm labor, growth restriction, and preeclampsia can still occur. But the danger of acute brain injury or organ damage from sudden blood loss simply does not apply the way it does when blood vessels are interconnected.
Evidence of this vascular crisis has been documented even in the first trimester. A case report showed hemodynamic changes consistent with blood shifting between twins (feto-fetal hemorrhage) after one died in a monochorionic pregnancy, suggesting that the shared circulation poses risks from very early in gestation.5PubMed. Temporal ultrasound findings in the first trimester of a dual fetal demise occurring in a monochorionic diamniotic twin gestation
When the Loss Happens Matters
Timing shapes outcomes almost as much as placental type. The gestational age at which one twin dies determines the range and severity of risks for the survivor, and the relationship is not a simple linear one.
If the death occurs in the first trimester, the surviving twin usually develops normally, as described with vanishing twin syndrome. But if the loss happens after roughly 17 weeks, the risks climb: preterm labor, growth restriction, preeclampsia, and perinatal death all become more likely. Interestingly, the picture improves somewhat if the loss occurs very late, at 33 weeks or beyond, because the surviving twin is already mature enough to deliver with good chances of a healthy outcome.6PubMed Central. Single Fetal Demise in Twin Pregnancy—A Great Concern but Still a Favorable Outcome
The risk of preterm birth rises sharply when one twin dies. In a large analysis of over 3,000 twin pregnancies, those complicated by a single death delivered at a median of 32 weeks compared with nearly 37 weeks for uncomplicated twins. The risk of delivery before 28 weeks was more than twelve times higher in pregnancies where one twin had died.7PubMed. Influence of chorionicity and gestational age at single fetal loss on risk of preterm birth in twin pregnancy: analysis of STORK multiple pregnancy cohort Later gestational age at the time of the co-twin’s death was associated with even higher preterm-birth risk, which makes intuitive sense: the further along the pregnancy, the more biological disruption the loss causes.
In a recent single-center study of monochorionic pregnancies where one twin died, 69 pregnancies continued. The average delivery age was about 35 weeks, around 62 percent delivered preterm, and about half of the surviving babies required neonatal intensive care.8PubMed Central. Perinatal outcomes of spontaneous single fetal death in monochorionic twin pregnancies: a single-center retrospective study Those numbers sound alarming, but they also show that the majority of survivors pulled through, even in the higher-risk monochorionic category.
Brain Injury in the Surviving Twin
The complication that weighs most heavily on clinicians and parents is the risk of brain damage in the survivor. When one monochorionic twin dies, the sudden drop in blood volume and pressure can deprive the surviving twin’s brain of oxygen and blood flow. The damage can range from small focal lesions, essentially localized spots of injury, to widespread tissue destruction.
A multicenter study using MRI on 42 brain-injured monochorionic survivors cataloged the patterns. Some showed broad damage across the brain’s white matter (periventricular leukomalacia and generalized encephalomalacia), while others had localized injuries, either hemorrhagic or nonhemorrhagic. Twins from pregnancies that had also been complicated by twin-to-twin transfusion syndrome were more than twice as likely to have focal brain lesions.9PubMed. Brain-injured Survivors of Monochorionic Twin Pregnancies Complicated by Single Intrauterine Death: MR Findings in a Multicenter Study Brain injury rates among surviving monochorionic twins after a co-twin death are estimated at between 18 and 35 percent.10PubMed Central. Fetal Growth Restriction in the Survivor Twin Following Spontaneous Demise in Monochorionic Pregnancy: A Case Report Highlighting Aplasia Cutis
Detecting this injury early is critical for planning delivery and counseling parents. Fetal MRI, especially with a technique called diffusion-weighted imaging, is more sensitive than ultrasound for spotting the early signs of cerebral ischemia in these pregnancies.11PubMed Central. Diffusion MRI findings in monochorionic twin pregnancies after intrauterine fetal death There is also a Doppler ultrasound measure, looking at blood flow speed in the brain’s middle cerebral artery, that can help predict which surviving twins are at greatest risk. In one study, survivors whose peak blood flow velocity in this artery was elevated were significantly more likely to show brain injury on MRI: about 69 percent of those with high flow velocities had cerebral injury, versus around 39 percent of those with normal readings.12PubMed. Early imaging predictors of fetal cerebral ischemic injury in monochorionic twin pregnancy complicated by spontaneous single intrauterine death When early MRI showed no restricted diffusion and a second scan was done later, no ischemic changes appeared, suggesting that a clean early scan is genuinely reassuring.
Twin-to-Twin Transfusion Syndrome as a Trigger
Twin-to-twin transfusion syndrome is one of the most common reasons one monochorionic twin dies while the other survives. It develops in roughly 10 to 15 percent of monochorionic pregnancies when unbalanced blood flow through the shared placental vessels causes one twin (the donor) to become anemic and growth-restricted while the other (the recipient) becomes overloaded with blood and fluid.13PubMed Central. Twin to twin transfusion syndrome Without treatment, advanced cases are fatal for both twins in the vast majority of cases.
The primary treatment is fetoscopic laser surgery, where a surgeon uses a thin scope inserted through the mother’s abdomen to seal off the problematic blood-vessel connections on the placental surface. The procedure dramatically improves survival but carries its own risks, including the possibility that one or both twins die during or after the intervention.14PubMed Central. Continuation of Pregnancy After Donor Twin Demise Following Fetoscopic Laser Photocoagulation to Treat Twin-Twin Transfusion Syndrome When one twin dies after laser treatment, the laser seal on the vessels often protects the surviving twin from the acute hemodynamic collapse that would otherwise occur through the shared connections. That is the whole rationale behind the surgery: by closing those channels, the twins’ circulations become functionally separate.
Long-term follow-up of children surviving twin-to-twin transfusion syndrome is encouraging. Survival rates after laser surgery range from the mid-70s to mid-80s percent, and among survivors, cerebral palsy rates are low, around 1 to 2 percent, with cognitive or motor impairments in roughly 2 to 8 percent.15The Indonesian Journal of General Medicine. How Does Fetal Surgery Impact Long-Term Neurological And Developmental Outcomes In Children With Twin-Twin Transfusion Syndrome? An earlier study did find that IQ scores averaged about 8 points lower than a comparison group, but this decrement was concentrated in children born before 33 weeks, suggesting that prematurity, rather than the syndrome itself, was the main driver of the difference.16PubMed. The long term neurologic outcome of children from pregnancies complicated by twin-to-twin transfusion syndrome
Selective Reduction and Cord Occlusion
Sometimes the death of one twin is not spontaneous but deliberate. When one twin has a severe anomaly incompatible with life, or when continuing a shared-placenta pregnancy puts both twins at high risk, doctors may recommend selectively ending one twin’s life to protect the other. In monochorionic pregnancies, the standard injection of potassium chloride used in singleton reductions cannot be used, because the shared circulation would deliver the drug to the healthy twin as well.
Instead, clinicians use techniques that physically block blood flow through the affected twin’s umbilical cord, such as radiofrequency ablation or cord occlusion. In one early series, 11 patients underwent ultrasound-guided cord occlusion before 28 weeks. The average gestational age at the procedure was 24 weeks, and the average delivery happened at around 31 weeks. Eight of the surviving babies were alive and well at follow-up.17PubMed. Selective feticide in monochorionic twin pregnancies by ultrasound-guided umbilical cord occlusion While these procedures carry real risks of preterm labor and, rarely, loss of the healthy twin, they offer a way to protect a survivor when continuing the pregnancy uninterrupted would threaten both.
What Physically Happens to the Dead Twin
What becomes of the twin who dies depends on when the death occurs. In the first trimester, the embryonic tissue is usually reabsorbed entirely. The ultrasound shows a shrinking gestational sac, and within a few weeks, no trace remains.
When the death happens in the second or third trimester, the body cannot simply reabsorb a larger fetus. Instead, the dead twin is gradually compressed by the growing placenta and the surviving twin, becoming flat and parchment-like over time. Clinicians call this “fetus papyraceous.” The compressed remnant remains in the uterus until delivery, when it is typically found attached to the placenta or membranes.18PubMed Central. Fetus Papyraceous in Monochorionic Diamniotic Twins For parents, encountering this at delivery can be deeply distressing, and the experience often comes as a shock if they were not adequately prepared.
Risks to the Mother
Most of the medical attention focuses on the surviving twin, but the mother faces her own set of complications. A retained dead fetus releases tissue substances called thromboplastins into the maternal circulation, which in rare cases can trigger disseminated intravascular coagulation, a dangerous clotting disorder where the blood’s ability to clot and to stay fluid both go haywire simultaneously. Although this is uncommon, it can be fatal for both the mother and the surviving fetus if not caught and managed.19Journal of South Asian Federation of Obstetrics and Gynaecology. Single Intrauterine Fetal Demise in Twin Pregnancies and Pregnancy Outcomes Clinicians typically monitor the mother’s clotting function with regular blood tests after a single fetal death is diagnosed, especially if delivery is being delayed to give the surviving twin more time to mature.
The presence of a uterine blood collection (intrauterine hematoma) early in a twin pregnancy also signals higher risk. Pregnancies with a hematoma had a vanishing twin rate of about 27 percent compared with 19 percent without one, and these pregnancies also saw increased rates of preterm birth and postpartum hemorrhage for the mother.20PubMed Central. Vanishing twin syndrome is associated with first-trimester intrauterine hematoma in twin pregnancies after in vitro fertilization
The Grief That Nobody Validates
Losing one twin while the other survives puts parents in an emotional position that few people around them understand. You are simultaneously grieving a dead child and caring for a living one, often while still pregnant. Well-meaning friends and family tend to respond by encouraging you to focus on the surviving baby, which sounds reasonable on the surface but functionally tells you that your loss does not deserve its own space.
A qualitative study of mothers who experienced this found that healthcare providers sometimes discouraged them from seeing or physically holding their deceased baby after delivery. Mothers who were denied this contact reported lasting guilt and regret. The researchers described the experience as “disenfranchised grief,” a loss that society does not openly recognize or support.21PubMed Central. A womb and a tomb: mothers’ experiences of single fetal loss in a twin pregnancy Another study framed the unique difficulty as managing stigma while trying to grieve authentically: parents felt pressure to perform gratitude for their surviving child, which made it harder to process the death of the other.22PubMed. Pearls of wisdom. Maternal guidance for navigating grief after loss of one twin from a multiple birth pregnancy
If you are going through this, or know someone who is, the research supports what should be obvious: both experiences, the joy and the grief, are real and deserve acknowledgment. Being happy about your surviving child does not erase the loss, and mourning the lost child does not mean you are ungrateful for the one who lived.
Administrative Tangles and Registration Issues
An aspect of twin loss that rarely makes it into medical discussions is the bureaucratic confusion surrounding birth and death registration. When one twin is born alive and the other is stillborn, the paperwork can become surprisingly messy. Errors in coding twin births, including incorrect assignment of whether twins are identical or fraternal, have been documented as a persistent problem in birth registries. These mistakes affect more than statistics: since identical twins who shared a placenta face far greater risks of morbidity and infant death than fraternal twins, getting the classification wrong can mislead future medical decisions and complicate any later efforts to determine whether clinical care was appropriate.23Cambridge University Press (Twin Research). Errors in birth registrations and coding of twins and higher order multiples
Parents navigating this often find themselves dealing with confusing paperwork: a birth certificate for one child and, depending on gestational age and local laws, a death certificate, a certificate of stillbirth, or nothing at all for the other. In many jurisdictions, a fetal death before a certain gestational threshold, commonly around 20 weeks, does not receive any official documentation. For parents who deeply felt that pregnancy as carrying two children, the lack of any legal acknowledgment of the second baby adds another layer to the grief.
How Animal Research Fills in the Gaps
Much of what scientists understand about the biological mechanics of embryo loss in multi-fetal pregnancies comes from animal studies, particularly in mice, where large litter sizes make partial litter loss easy to study. Mouse models have allowed researchers to watch, in real time with high-frequency ultrasound, how an embryo dies and is cleared from the uterus while its siblings continue developing normally.
The resorption process in mice unfolds in stages: the failing embryo first shows growth retardation, then develops a slow heartbeat and fluid accumulation around the heart, then stops developing entirely. Afterward, the embryo is broken down and absorbed by maternal immune cells.24PubMed Central. Early detection and staging of spontaneous embryo resorption by ultrasound biomicroscopy in murine pregnancy A closer look at the cellular level shows that the embryo undergoes programmed cell death first, then the wall of the embryonic vesicle ruptures, and maternal immune cells, predominantly neutrophils, invade and clear the debris through a sterile inflammatory process.25PubMed Central. Spontaneous embryo resorption in the mouse is triggered by embryonic apoptosis followed by rapid removal via maternal sterile purulent inflammation Critically, the neighboring embryos are unaffected throughout this process, their placentas remain intact, and they continue growing on schedule.
The parallel to human vanishing twin syndrome is imperfect, since mouse embryos have separate implantation sites rather than a shared placenta. But the basic principle, that the maternal body can efficiently clear a failed embryo without harming its siblings, is consistent across species and helps explain why early vanishing twins in humans so rarely cause problems for the survivor.