Fetuses (Image: Leonardo da Vinci/Wikimedia

In India’s West Bengal, a four-year-old boy was taken to the doctor after complaining of a stomach ache. The doctors who examined him diagnosed him with a tumor that required surgical removal. But when they performed the operation, they extracted something wholly unexpected: a mass that  had hands, legs, nails and a “partially formed head,” the IB Times reports

It was a case of fetus in fetu—a baby within a baby.

In all of medical literature, there are fewer than 100 reported cases of this rare condition. Although scientists aren’t entirely sure how it happens, most likely, two fetuses begin developing in a woman’s womb, but in the process of gestating, one subsumes the other. The forgotten twin, though, doesn’t always disappear entirely; sometimes it remains in attenuated form in the body of the surviving baby.

2000 review of fetus in fetu cases estimated the rate of this condition at 1 in every 500,000 live births. Around 135 million people are born into the world every year, which means fetus in fetu happens hundreds of times annually, in theory.  

The reports that have made it into public view can be quite gruesome: one man carried his attenuated twin with him until his 30s, and when it was removed, “its fingernails were quite long,” ABC News reports. In another case, doctors found a tiny foot in a baby’s brain

Even in these extreme instances, though, once the growth is removed, the people who’ve been carrying around parts of another baby inside them tend to be just fine.

Bonus finds: A moustached kingfishera Cypriot tomb from 2,400 years ago

Every day, we highlight one newly lost or found object, curiosity or wonder. Discover something unusual or amazing? Tell us about it! Send your finds to sarah.laskow@atlasobscura.com.