Conjoined twins are identical twins whose bodies fuse in utero and have shared tissue after birth.  They are far rarer than most other forms of multiple births, rarer that triplets, but somewhat more common than quadruplets.  About half of conjoined twins are stillborn.

The mechanism of conjoined twins is still poorly understood.  Identical twins are formed when the developing embryo splits in two during development.  It is believed that conjoined twins form when the two separate eggs join back together somewhat later in development when cells begin to differentiate from stem cells.

In many cases, depending on how closely the two twins are tied together and what tissue they share, they can be separated by surgery.  However, such surgery is dangerous if the twins share common organs, particularly when they are joined at the skull.

Conjoined twins at Wikipedia

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