Dicephalic parapagus twins

Dicephalic parapagus (/daɪˈsɛfəlɪk/) is a rare form of partial twinning with two heads side by side on one torso. Infants conjoined this way are sometimes called "two-headed babies" in popular media. The condition is also called parapagus dicephalus.

If carried to term, most dicephalic twins are stillborn, or die soon after birth. A small number are known to have survived to adulthood.

The extent to which limbs and organs are duplicated varies from case to case. One head may be only partially developed (anencephalic), or both may be complete. In some cases, two complete hearts are present as well, which improves their chances of survival. The total number of arms may be two, three or four.

Their prospects are best if no attempt is made to separate them, except in cases in which one twin is clearly dying.