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(Newser) - When Xiao Feng was admitted to a hospital in China with an extremely swollen stomach, doctors made a pretty disturbing find: The 2-year-old boy was carrying his own twin inside his stomach.

The undeveloped fetus acted as a parasite, growing until it took up two-thirds of Feng's stomach, Inquisitr reports. Its spine and limbs were fully formed, and it measured 10 inches wide when doctors removed it.

Cases like these are known as cryptodidymus, an extremely rare version of conjoined twins, which form when an egg splits after fertilization but doesn't fully separate.

In cryptodidymus cases, one fetus ends up concealed in the other's body, Medical Daily reports; it can't survive on its own, Metroadds.

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