conjoined twins, two fetuses developed from the same ovum who are physically united at birth. The defect ranges from a superficial anatomical union of varying extent between equally or nearly equally formed fetuses to one in which only a part of the body is duplicated or in which a small, incompletely developed fetus, or parasite, is attached to a more fully formed one, the autosite. Conjoined twins result when separation of the blastomeres in early embryonic development does not occur until a late cleavage phase and is incomplete, causing the fused condition. Viability depends on the extent of the fusion and the degree of development of the fetuses. Formerly called Siamese twins.
