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A teenage boy who went to the doctors with mystery stomach pains was shocked to discover they were were caused by the parasitic foetus of his unborn twin.

Narendra Kumar, 18, sought medical help in his home state of Uttar Pradesh in northern India after suffering chronic pain and vomiting with no explanation.

Medics found a three-stone mass of bone, hair and teeth in his stomach, which turned out to be his twin's malformed foetus.

(Image: Sanjay Pandey)

The parasite had formed an umbilical cord-like structure to the teenager in the early stages of pregnancy, allowing it to live and feed off him.

Following the incredible discovery, Narendra, who had been forced to stop going to school due to his illness, was diagnosed with the rare condition 'foetus in fetu'.

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There are only 200 cases of the condition worldwide on record.

Dr Rajeev Singh was the surgeon who uncovered the cause of the teenager's stomach pains while operating on him at Swaroop Narayan Hospital in Uttar Pradesh.

(Image: Sanjay Pandey) (Image: Sanjay Pandey)

He told MailOnline: "The boy's stomach grew, but his plight went undiscovered for years because neither his parents were of his medical condition nor the doctors could diagnose the condition at an early stage.

"Technically, the foetus was alive and was growing due to metabolic activity in his body."

Although Narendra had previously sought out medical advice, Dr Singh was the first expert to undertake a thorough investigation, allowing him to discover the parasitic twin.

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He performed an ultrasound and CT scan on the teenager - and was astonished by the findings.

Doctors subsequently carried out a three-hour operation to remove the 20cm-wide mass - which featured a staggering 6.5 feet of hair and yellow 'amniotic-like fluid' - from Narendra's stomach.

(Image: Sanjay Pandey)

Now, the teenager is no longer in pain, and is expected to be able to return to school.

His father Prem Chandra, a farmer, told MailOnline: "Now that it has been removed, I am relieved. He can now go back to school and lead a healthy life."

In cases of foetus in fetu, the organs of the foetus have a working blood supply from the healthy twin.

However, the mass has no functional brain, heart, lungs or gastrointestinal tract.

It has no hope of life outside of its twin's body, and can often threaten the life of the carrier.