It's the result of an extremely rare condition called foetus in foetu. While the condition is well documented, doctors are unsure how it happens.

This latest, bizarre twin-inside-twin story began when Alamjan Nematilaev, a seven-year-old boy from Kazakhstan, complained that he felt something moving inside him. When doctors operated they found what was described as his "twin". It had apparently been growing inside him since birth and had part of a head, some hair and even teeth. His mother said she thought it was something to do with the radiation from the Chernobyl disaster, and at the weekend C4 announced that it would film an autopsy of the dead twin to find out if this was indeed true.

Doctors are unsure what might cause the condition. One theory is that foetus in foetu is simply one of the risks of the embryonic development of twins. Twins arise either because two separate eggs are fertilised by separate sperm or because a single fertilised egg divides into two. The former are known as fraternal twins, the latter identical twins.

"There are various things that can go wrong," says Andrew Calder, a professor of obstetrics and gynaecology at Edinburgh University. Conjoined twins represent the failure of a total separation in identical twins. "The assumption of this foetus in foetu must be that somehow the failure to separate seems to result in one twin being enveloped into the other one," he adds.

Philippe Jeanty, a specialist in birth defects at the Women's Health Alliance in Nashville, suggests a mechanism for this enveloping. One of the earliest places that germ cells - which go on to produce the testicles and the ovaries - develop is the yolk sac attached to the embryo. Rarely, the two yolk sacs of identical twins can end up connecting. If one baby's heart develops before the other's, the connection will make blood circulate from the healthy baby into the yolk sac and backwards into the arteries of the less developed baby. That could stop the second baby's heart developing.

"So now we have a baby that is developing as a parasite of the healthy baby," says Jeanty. As the embryo develops further, the yolk sac is normally drawn back into the foetus. In the case of the foetus in foetu, the healthy baby would draw in what is left of its twin with its yolk sac.

As to why the internal twin was reportedly "moving" inside the seven-year-old boy, Jeanty says that, to some extent, the twin can remain alive as long as it has a blood supply, and it may develop a primitive spinal reflex system. If it gets a lot of blood, it can also develop recognisable features. "Some have limbs and fingers."

This is just one theory though. Lyndon Hill, director of ultrasound at the Magee Women's hospital in Pittsburgh, says some cases of foetus in foetu could just be a teratoma - a type of tumour containing cells that can form virtually any tissue in the body. When these are removed, they are usually found to contain some fat, skin or teeth.

What experts do agree on is that there is no evidence that foetus in foetu or teratomas are caused by radiation damage. "To say it's Chernobyl-related, you'd have to find 10 cases in a one-mile area or something," says Hill. "A much higher prevalence than one single case."