Dicephalic parapagus (/daɪˈsɛfəlɪk/) is a rare form of partial twinning with two heads side by side on one torso.[1] Infants conjoined this way are sometimes called "two-headed babies" in popular media.[2] The condition is also called parapagus dicephalus.[1]
If carried to term, most dicephalic twins are stillborn, or die soon after birth.[3] A small number are known to have survived to adulthood.[3]
The extent to which limbs and organs are duplicated varies from case to case. One head may be only partially developed (anencephalic),[4] or both may be complete. In some cases, two complete hearts are present as well, which improves their chances of survival.[5] The total number of arms may be two, three or four.[3]
Their prospects are best if no attempt is made to separate them, except in cases in which one twin is clearly dying.[6]