Rachipagus conjoined twins: they really do occur!

The myth of rachipagus twins is hereby exploded-the myth, not the rachipagus! In a review of over 1,200 cases of conjoined twins, one classic example of rachipagus was found-two complete infants with dorsal union of the entire head and trunk, with roentgenograms showing clearly visible bony union of the vertebral arches from T6 to L3. After the typical case was discovered, 20 dorsal parasitic twins were reevaluated and reclassified as rachipagus. Two had extensive vertebral fusion and 18 were united in the dorsal midline, all with a meningocele, vertebral anomaly, and/or bony or neural connection. A theory of the embryologic origin of these twins is proposed: two embryonic discs located on diametrically opposing aspects of a single amniotic cavity becoming united in the area of the closing neural folds. This again raises the controversy concerning "fission or fusion" in conjoined twins.