Conjoined twins , popularly referred to as Siamese twins , [ 1 ] [ 2 ] are twins joined in utero. Most live births are female, with a ratio of Two possible explanations of the cause of conjoined twins have been proposed. The one that is generally accepted is fission , in which the fertilized egg splits partially.
Conjoined twins share a single common chorion , placenta , and amniotic sac in utero , but so do some monozygotic but non-conjoined twins. Chang and Eng Bunker โ were brothers born in Siam now Thailand who traveled widely for many years and were known internationally as the Siamese Twins.
Chang and Eng were joined at the torso by a band of flesh and cartilage, and by their fused livers. In modern times, they could easily have been separated. Partial splitting of the primitive node and streak may result in the formation of conjoined twins. These twins are classified according to the nature and degree of their union.
Occasionally, monozygotic twins are connected only by a common skin bridge or by a common liver bridge. The type of twins formed depends on when and to what extent abnormalities of the node and streak occurred.
Misexpression of genes, such as goosecoid , may also result in conjoined twins. Over- or underexpression of this gene in laboratory animals results in severe malformations of the head region, including duplications, similar to some types of conjoined twins. Conjoined twins are typically classified by the point at which their bodies are joined.
