Twins don’t literally eat each other in the womb, but the reality behind this idea is fascinating. When one twin stops developing early in pregnancy, the surviving twin and the mother’s body absorb the tissue of the lost embryo. This process, called vanishing twin syndrome, is surprisingly common and is the most likely origin of the “twins eating each other” idea. In rarer cases, one twin can divert blood flow from the other or even physically absorb a sibling that remains attached to its body.
Vanishing Twin Syndrome
The most common version of this phenomenon happens in the first trimester. One embryo in a twin pregnancy stops developing, and its tissue is gradually reabsorbed by the pregnant parent’s body and the surviving embryo. The process is completely harmless to both. In most cases, the mother never knows it happened unless an early ultrasound detected two embryos and a later scan showed only one.
This is far more common than most people realize. One frequently cited study found that 71% of twin pregnancies identified before 10 weeks resulted in a single baby at delivery. More recent data from 2023 puts the rate at about 17% in a specific type of twin pregnancy where each embryo has its own placenta. The wide range reflects how many vanishing twins disappear before anyone gets an ultrasound at all.
The cause is almost always a problem with the lost embryo itself, typically a chromosomal abnormality that prevents normal development. The embryo simply stops growing, and the body treats the remaining tissue the way it handles any non-viable cells: breaking it down and reabsorbing it. If the loss happens before eight weeks, the only trace might be a small cyst on the surviving twin’s placenta. If it happens after eight weeks and the tissue isn’t fully reabsorbed, the lost twin can become compressed into a thin, papery remnant (sometimes called fetus papyraceus) that’s found at delivery.
How Reabsorption Can Change the Survivor’s DNA
Here’s where things get strange. In rare cases, when two fertilized eggs fuse together very early in development, or when a surviving twin absorbs cells from a lost sibling, the result is a person carrying two distinct sets of DNA in their body. This is called chimerism, and it means different tissues (blood, skin, organs) can have different genetic profiles.
The most famous case involved Lydia Fairchild, a Washington state woman who in 2002 was told by a DNA test that she was not the biological mother of her own children. It turned out she was a chimera: the cells in her blood had different DNA than the cells in her reproductive organs. Her body contained genetic material from two embryos that had merged early in pregnancy. Most chimeras never discover their condition because there’s no reason to test multiple tissues for DNA. The ones who are identified tend to be found through unusual circumstances like Fairchild’s.
Twin-to-Twin Transfusion Syndrome
A different and more dangerous version of one twin “taking” from another happens in identical twins who share a single placenta. About 10 to 15% of these pregnancies develop twin-to-twin transfusion syndrome, where abnormal blood vessel connections in the shared placenta route blood unevenly between the two fetuses.
The mechanics work like this: an artery from one twin (called the donor) branches into the placenta to pick up oxygen and nutrients. But instead of the corresponding vein carrying that enriched blood back to the same twin, it redirects toward the other twin (the recipient) through an abnormal connection. The recipient gets too much blood. The donor gets too little.
Both twins suffer. The donor twin’s body, starved of adequate blood flow, shunts whatever it has to the brain and heart at the expense of the kidneys and other organs. Kidney failure is a real risk. The recipient twin, flooded with extra blood volume, has to pump harder. Its heart can fail from the overload, and fluid can build up throughout its body. Without treatment, the condition can be fatal for one or both twins. This isn’t reabsorption, but it’s the closest thing to one twin genuinely “taking” resources from the other in a way that causes direct harm.
Parasitic Twins
The rarest and most visually striking version of this phenomenon is the parasitic twin, which is an extremely uncommon type of conjoined twinning. A parasitic twin occurs when conjoined twins begin to develop, but one stops growing while remaining physically attached to the other. The result is a mostly developed baby with a partially formed sibling attached to its body, sometimes as small limbs, tissue masses, or even a partial torso.
There are two main theories for how this happens. The fission theory says a single fertilized egg begins to split into identical twins but doesn’t fully separate, creating conjoined twins. When one of those twins stops developing, it becomes parasitic. The fusion theory proposes the opposite: two separate fertilized eggs grow too close together and fuse, with one eventually ceasing development while staying physically connected. A third theory suggests vascular problems in the uterus may play a role. Parasitic twins are almost always removed surgically after birth.
Risks to the Surviving Twin
When a twin is lost early and fully reabsorbed, the surviving twin faces essentially no added risk. But when the loss happens later in pregnancy, the picture changes. Single fetal death occurs in up to 6.2% of all twin pregnancies, and the consequences for the surviving twin can include premature delivery, growth restriction, and in some cases, neurological injury.
The danger comes from sudden shifts in blood flow between the twins at the time of death, particularly in twins sharing a placenta. When one twin dies, blood pressure changes can cause rapid fluid shifts that temporarily deprive the surviving twin’s brain of adequate blood flow. This can lead to white matter injuries, damage to areas supplied by major brain arteries, or hemorrhagic lesions. The surviving twin is also at risk from whatever underlying condition caused the sibling’s death in the first place, since they share the same uterine environment and often the same blood supply.
These serious outcomes are the exception, not the rule. The vast majority of vanishing twin cases resolve without complications, particularly when the loss occurs in the first trimester before the twins have developed significant vascular connections. Many people who were born as singletons may have started as twins without anyone ever knowing.