Pregnancy puts you face-to-face with mother nature's stranger aspects. For instance, do twins really eat each other in the womb? This seemingly bizarre idea is actually a fascinating phenomenon.
First, though, it’s helpful to understand what’s really going on when one twin seems to vanish from the womb. “A vanishing twin is when a fetus in a multifetal pregnancy dies in utero in the early first trimester. Over the course of the pregnancy, it typically is resorbed,” Jeffrey A. Kuller, M.D., Professor of Obstetrics and Gynecology in the Division of Maternal-Fetal Medicine at Duke University School of Medicine, tells Romper. “If it happens early, the twin completely disappears,” Errol Norwitz, M.D., Ph.D., Chief Scientific Officer and Chairman of the Department of Obstetrics and Gynecology at Tufts Medical Center, agrees.
Although it is a difficult truth, the loss of one twin during pregnancy is not at all rare. “It is most commonly due to an abnormality in the demised twin, usually a chromosome abnormality. It is common and may occur in 20-30 % of multifetal pregnancies,” says Dr. Kuller.
This vanishing twin phenomenon has likely been going on for some time, but medical professionals only became aware of it recently thanks to advances in medical technology. Vanishing twin syndrome “is a very common phenomenon that was not previously recognized until first trimester ultrasound became a regular part of prenatal care,” says Dr. Kuller. “It’s only because of early detection that we even know about them,” says Dr. Norwitz. And as Dr. Norwitz further explains, this may be the reason behind some false positive test results for Down syndrome early in pregnancy. “On rare occasions, the screening test will come back as high risk for Down syndrome, then the test for the fetus comes back as completely normal. It may be a case of vanishing twin, where the twin had Down syndrome and disappeared, then the twin’s DNA is still in the placenta.”
But does one sibling really eat the other one? Not so much. The vanished embryo gets reabsorbed back into the mother's body, according to Baby Center. It is a miscarriage, but the typical signs — such as bleeding — are often absent, given the reabsorption of cellular material. In general, the pregnancy will progress as normal from there.
As with any miscarriage, though, the exact causes are mysterious and in no way the mother's fault. Chances are, vanishing twin syndrome results from the same genetic anomalies that would cause any miscarriage, as Baby Center explained. Vanishing twin syndrome is a natural aspect of pregnancy that has probably gone on forever, without notice, until modern ultrasounds brought the event to light. There is high drama in those growing little cells.
In extremely rare cases, the deceased embryo's cells are absorbed by the surviving twin. Under these conditions, the remaining fetus becomes a chimera, or a person with two sets of DNA, according to Scientific American. Under this set of conditions, a person could go through life with two different blood types. For example, a woman preparing for a kidney transplant was identified as genetically different from her own biological children, a fact that caused some confusion until her chimerism was discovered, according to the New England Journal of Medicine. She had absorbed a twin's DNA in utero. But the children she delivered were definitely hers.
On a more positive note, experiencing a vanishing twin does not necessarily mean the second pregnancy is in jeopardy. “The outcome for the remaining pregnancy is generally very favorable,” says Dr. Kuller. “Some studies suggest a higher rate of fetal growth restriction in the surviving fetus, but this risk is still likely to be low,” says Dr. Norwitz, who adds that the remaining fetus’ survival may be affected by whether both twins shared a blood supply, but most do not. In other words, if one dies, the other twin is generally not affected very much.
Oh, and if the "eating a twin" idea still sounds familiar, it could be a fact you picked up from Shark Week. Although human embryos are pretty peaceful, some animal species do cannibalize one another in utero. For instance, about a dozen sand tiger sharks begin the gestation journey, then the largest will eat all but one of the siblings in utero, according to Live Science. For humans, though, there's no need to worry about such behavior. Although the vanishing twin syndrome may sound bizarre, it’s likely been a part of the human condition for a long, long, while.
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