In a rare but true story, a baby was born pregnant with his twin brother. British newspaper The Independent reports that doctors found a male baby with legs, one arm and a brain lodged behind the stomach of his brother who was born in Mumbra, India. The unidentified newborn is healthy after undergoing an operation to have his twin removed. The mother, 19 years old, first learned of her condition during a visit to the doctor’s office earlier this month. The radiologist noticed the mass inside the baby boy’s fetal sac.

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The condition is known as fetus in fetu, a very rare situation which occurs when a fetus, that’s not completely formed, grows inside the body of the other baby. In this case, the parasitic twin (as is deemed by the condition) had no skull.

As the paper reports, the mass can go unnoticed until it causes health problems later on. New Scientist reported on a 10-month-old baby in Indonesia whose belly grew to the size of a melon. His parents were worried the growth was due to a tumor but doctors found a parasitic fetus in the boy’s abdomen following a scan.

English woman Jenny Kavanagh had a mass inside her ovary for more than 40 years, according to The Independent. Severe period pains led doctors to find a mass which had a face, eye, tooth and black hair.

New Scientist explains that the first case was documented around the year 1800 and that there are less than 200 cases of fetus in fetu around the world. The parasitic fetus can be found in areas including the chest, skull and scrotum, the magazine writes.

Dr. Mark Umstad of Melbourne, Australia, tells the publication that fetus in fetu occurs when twin embryos come from the same egg and share a placenta. After four weeks post-inception, embryos fold to form their bodies. Umstad believes that since twins are situated close together in the womb, one can fold into the other while developing. As Umstad told New Scientist, the location of the parasitic fetus depends on where it was stuck as it was absorbed into its twin. However, some health professionals believe there is no real explanation for why this occurs.

"Weird things happen early, early in the pregnancy that we just don't understand," Dr. Draion Burch, an obstetrician and gynecologist in Pittsburgh, told LiveScience. "This is one of those medical mysteries."

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Like in Kavanagh’s case, years can go by without any signs or symptoms. However, the surviving baby can be malnourished as the twin who was absorbed relies on its sibling for nutrients.

New Scientist poses that fertility treatments could potentially increase the prevalence of this condition as twins are associated with reproductive treatments.

“However, since fetus in fetu only occurs in about 1 in 500,000 births currently, we might be talking about one extra case a year globally,” he told the magazine.

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