polycephaly

Polycephaly is a congenital condition in which an organism is born with two or more heads. The term comes from Greek poly- “many” and kephale “head.” It is extremely rare in humans and has been described in a variety of animal species. In humans, polycephaly most often results from conjoined twinning, where two embryos only partially separate during early development.

In humans, two common forms are dicephaly, in which two heads rest on a single body with separate brains, and diprosopus, in which there is duplication of facial features on a single skull. Other related conditions include parapagus and craniopagus variants, where the heads may share much of the torso or skull structures.

Polycephaly arises very early in embryogenesis. It is generally attributed to incomplete division of a single fertilized egg (monozygotic twinning) or, less commonly, to the fusion of two embryos. The exact mechanisms are not fully understood and genetic or environmental influences may play a role.

Prenatal ultrasound can detect polycephaly before birth in many cases. After birth, clinicians assess shared organs and vascular connections to gauge viability. Prognosis varies widely and many affected babies have limited life expectancy due to complications. Some individuals survive into adulthood with medical and surgical management.

Treatment is individualized. If separation of anatomically distinct heads with minimal shared organs is possible, surgical separation may be attempted, though it carries substantial risk and is not always feasible. In many cases, care focuses on supportive medical management and quality of life.

Polycephaly has been documented in several animal species, including mammals, birds, and reptiles. Notable human cases include the dicephalic parapagus twins Abby and Brittany Hensel, who lived for many years with two heads sharing one torso.