Monozygotic twins who are joined in utero. they may be well developed and share only a superficial connection, often in the frontal, transverse or sagittal body plane, or they may share a partial duplication of a body structure. alternatively, there may be a small and incompletely developed twin conjoined to a larger, more fully developed twin.

Conjoined twins, popularly referred to as Siamese twins, are twins joined in utero. It is a very rare phenomenon, estimated to occur in anywhere between one in 50,000 births to one in 200,000 births, with a somewhat higher incidence in southwest Asia and Africa...