In the Indian state of West Bengal, a 4-year-old boy recently checked into a hospital, complaining of bad stomachaches. Doctors suspected he had a tumor in his abdomen. However, ultrasounds and a CT scan revealed a surprising find: inside the young patient’s stomach was a developed fetus, complete with hands, legs, nails, and a partially-formed head.

According to the International Business Times, the boy had a condition called "fetus in fetu"—a baby-within-a-baby. Scientists say fetus in fetu likely occurs when twin fetuses are developing in the womb and one absorbs the other. Parts of the subsumed fetus remain inside the surviving infant's body, and are sometimes discovered by physicians after birth.

In the past two decades, highly publicized cases of fetus in fetu have included a foot and other body parts that were discovered in a newborn's brain, as well as a man whose twin remained in his abdomen until he had it surgically removed in his twenties. However, fetus in fetu is a rare occurrence that’s estimated to occur in only 1 in 500,000 live births. Since the 19th century, fewer than 100 cases have been reported.

Aside from the condition’s shock factor, patients diagnosed with fetus in fetu tend to be perfectly healthy once the growth is taken out—just like the boy in India, who's apparently doing just fine after undergoing surgery.

[h/t Atlas Obscura, International Business Times]