The foetus, 18 centimetres long and weighing two kilograms (4.4lbs) was found when doctors operated on the boy, Hisham Ragab, a building worker. It has a head, arm, a tongue and fully formed teeth.
The boy, aged 16, had complained of stomach pains an a X-ray showed a swollen sac pressing against his kidneys. Doctors in Egypt reportedly said that examination of the teeth in the foetus's mouth revealed that they were those of a 16-year-old suggesting that the foetus had been growing inside the boy all his life.
Peter Wardle, consultant in reproductive medicine at the University of Bristol, said: "This is quite remarkable, but it is not unique. It is something recognised to happen very rarely."
He said the foetus would have been the boy's identical twin which, instead of developing separately, had grown inside the first twin.
"The single embryo slips to form identical twins, but instead of splitting evenly into two halves, one half becomes surrounded by the other," he said.
The second, internal twin, would then normally die and become mummified. "It simply dies but doesn't disintegrate. It is like a doll just sitting there," Mr Wardle said.
The size of the internal twin suggested that it had survived to 32 or 33 weeks gestation. Mr Wardle said: "That is a remarkable size. It would be absolutely amazing if it had lived for 16 years."
He added that the teeth were formed during pregnancy, but remained erupted until childhood.
A similar condition occurs in women in which when a foetus fails, instead of miscarrying it becomes mummified and remains lodged in the womb. The condition, known as foetus papyraceous, is sometimes discovered in middle- aged women undergoing hysterectomies.
Hospital officials in Egypt said that Hisham Ragab was in good health and he would be released from hospital once he recovers from surgery.Reuse content