Pregnancy puts you face-to-face with mother nature's scarier aspects. For instance, do twins really eat each other in the womb? This idea that has been highlighted by movies and TV shows is actually a serious issue.
Although it is a difficult truth, the loss of one twin during pregnancy is not uncommon. Known as vanishing twin syndrome, it refers to the miscarriage and reabsorption of one sibling in the earliest stages of pregnancy, according to the American Pregnancy Association (APA). For example, a mother might be carrying twins at her first ultrasound but only show one fetus at the second ultrasound, all within the initial weeks of gestation. In the past 70 years, advances in ultrasound technology have brought this phenomenon to light, although it's probably been happening forever. As further explained by the APA, about 20 to 30 percent of multifetal pregnancies will result in vanishing twin syndrome. It's a common, if somewhat bizarre, occurrence.
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 further explained by Baby Center. Vanishing twin syndrome is a natural aspect of pregnancy that has probably gone on forever, without notice, until modern ultrasounds brought light to the event. 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.
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.