This phenomenon of unknown cause arises during the process of monozygotic (identical) twinning. The two bodies formed from the one egg do not separate completely, or there is a partial doubling of the body. Many different forms are present. In one group, one of the twins is rudimentary and is attached to its co-twin as a parasitic twin. In other forms, a relatively normal body has two heads (dicephalus). The most common type consists of twins joined in the thoracic area (thoracopagus). In association with the abnormal area of fusion, malformations are regularly seen, e.g. heart malformations in thoracopagus twins, or anencephalus in dicephalus. Malformations are also common in areas other than the site of fusion. If the twins survive delivery, surgical separation is sometimes contemplated; its success depends on the degree of fusion and the presence of life-threatening malformations. One separation of American conjoined infant twins was attempted in 1993 with the intentional sacrifice of one for the other, when it was clear that both could not survive.
This is an exceptional rare malformation with a rate of 1 birth per 100,000. Japan has a slightly higher rate. There is an excess of females.