Imagine the surprise of doctors in India who discovered a teenage boy’s stomach ache was caused by the malformed fetus of his unborn twin.

Narendra Kumar, 18, was taken to Narayan Swaroop Hospital in Uttar Pradesh, a northern province of India, after suffering from abdominal pain, weight loss, and bouts of vomiting, as reported by Sanjay Pandey for The MailOnline. After a long process of diagnosis, ultrasound and CT scans discovered a 20-centimeter (7.8-inch), 2.5-kilogram (5.5-pound) mass of bone, hair, and teeth in his abdomen.

The mass was reportedly caused by a developmental abnormality called “fetus in fetu.” In this extraordinarily rare condition, one of the fetuses in the early stages of a twin pregnancy envelops the other. The enveloped twin then lives off the nutrients provided by its sibling's blood supply. They’re usually found in the host’s abdomen, although there are reported cases of them being found in skulls and even scrotums, among others.

Speaking to The MailOnline, Dr. Rajeev Singh said, “The boy's stomach grew, but his plight went undiscovered for years because neither his parents were aware of his medical condition nor the doctors could diagnose the condition at an early stage.

“Technically, the fetus was alive and was growing due to metabolic activity in his body.”

He added, “In the three-hour-long surgery, we removed a mass of malformed baby, which had hair, teeth, a poorly developed head, a bony structure of chest and spine, with lots of yellowish amniotic-like fluid in the sac.”

^{Sanjay Pandey}