Parasitic Evil Twins: The Bizarre And Dangerous Fetus In Fetu Syndrome

Parasitic twins are often the subject of cheesy horror flicks or macabre urban legends. But these over-the-top stories about twins have their roots in a very real medical phenomenon: fetus in fetu syndrome, which experts at the National Center for Biotechnology Information, or NCBI, have described as "a rare condition that has been defined as the presence of one twin in the body of the other."

This medical oddity happens in the womb. Two babies will begin to grow until eventually, a dominant one will begin to "absorb" the other into its body. Usually, fetus in fetu syndrome is diagnosed only after the surviving twin is born. But in some rare - and grisly - cases, the remains of the absorbed twin aren't discovered until much later.

So, what do these parasitic twins look like? Sometimes the encapsulated twin is astonishingly developed and lifelike, and sometimes it resembles a medical illustration gone wrong. Whatever form it takes, this mysterious medical condition is as fascinating as it is tragic.

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  • The Condition Was First Discovered In The 18th Century

    The Condition Was First Discovered In The 18th Century
    Photo: Nisreen M Khalifa et al / Wikimedia Commons / CC BY 2.0

    The first case of fetus in fetu was described in the late 18th century by Dr. Johann Friedrich Meckel, a German anatomist. Even in this age of advanced technology, however, the condition remains few and far between.

    As of 2019, there have been fewer than 200 cases recorded worldwide.

  • The Condition Is Incredibly Rare

    Fetus in fetu is probably an inevitable horror movie fear for would-be parents, but there's almost surely no reason to worry. It's one of the world's rarest conditions, estimated to occur in only one in 500,000 births.

    Even if the syndrome does come to pass, it's usually caught quickly and corrected immediately upon the surviving baby's birth.

  • The Fetuses Can Settle Almost Anywhere In Their Twins' Bodies

    The Fetuses Can Settle Almost Anywhere In Their Twins' Bodies
    Photo: Nisreen M Khalifa et al / Wikimedia Commons / CC BY 2.0

    Though most hidden fetuses settle in the abdominal cavity, they are by no means limited to that area. Parasitic twins have also been found in GI tracts, and in the form of a bulge in the coccyx (tailbone). Fetuses may also manifest as multiple bulges.

    In short, it's theoretically possible for a fetus to shift almost anywhere within its twin's body.

  • The Fetuses Can Be Vampiric

    The Fetuses Can Be Vampiric
    Photo: Johann Schenk / Wikimedia Commons / Public Domain

    There's a reason they're called "parasitic" twins. Consider the case of 36-year-old Sanju Bhagat, whose absorbed sibling attached itself directly to his blood supply. Doctors removed the twin, and while it was more developed than expected, it did not survive.