Read this medical news today:

Baby born with 'foot in brain':


Doctors operated on three-day old Sam Esquibel after finding what looked like a microscopic tumour on an MRI scan.
But while removing the growth, they also found a nearly perfect foot and the partial formation of another foot, a hand and a thigh.


The entire story is here:
BBC NEWS | Health | Baby born with 'foot in brain'

Yes very unusual. From my limited coverage of this in med school I'd go with the "foetus in foetu" hypothesis.

I recall observing an operation that involved a teratoma as mentioned in the article and it contained hair and 4 teeth. Located on middle third of the thigh. We knew it would be unusual before the surgery though as the teeth showed up on x ray, although the hair was a bit of a surprise!

Stephen King wrote a book called 'The Dark Half'. This was about a character who turned out to have had a twin brother who was partially absorbed early on in the womb. The explanation in the book was that this phenomenon of one twin 'absorbing' another is actually fairly common. We rarely know as it happens at the point at which both 'twins' are just a bunch of cells.

Obviouly Stephen King writes fiction, but after reading that book I also read that it was based on a hypothesis that explained the appearance of noses and limbs growing, fairly randomly, out of the bodies of otherwise healthy individuals.

cmarkod wrote:

Yes very unusual. From my limited coverage of this in med school I'd go with the "foetus in foetu" hypothesis.

I believe that process of absorbing a twin at a very early stage of develpment is also responsible for chimerism. This was once thought to be exceedingly rare condition easily recognized by a pattern of alternating skin coloration, but in recent years has shown up in a number of individuals due to DNA testing. Apparently some parts of these people's bodies would test with one DNA pattern, and others with an alternate, but closely-related one. In the first two cases I heard of, two women were believed not to be the mothers of their children, though the fathers tested out for having fathered them. Imagine how strange that must have felt before it was explained.