Dicephalous

Dicephalous, also called dicephaly, is a rare congenital condition defined by two heads attached to a single body. It typically results from abnormal embryonic development, most commonly due to incomplete splitting of a single fertilized egg (a form of monozygotic twinning) or from fusion events early in development that produce two heads on one trunk. The condition is discussed within the broader context of twinning anomalies and varies in presentation.

In humans, the most frequently described form features two heads sharing a single torso, with two necks. Each head usually contains a brain and sensory organs, but the extent to which internal organs are shared or duplicated varies. Some cases involve partial sharing of the spinal column or other structures, and the availability of organs such as the heart or digestive system can differ between individuals. The prognosis is highly dependent on the extent of neural and organ sharing; many affected fetuses do not survive to term, and among those born alive, survival beyond infancy is uncommon and typically requires intensive medical care. Surgical separation is rarely feasible, especially when the brains are involved or major organs are shared.

Diagnosis is typically made or clarified through prenatal ultrasound, with further imaging such as MRI or CT after birth to delineate anatomy. The condition is exceedingly rare in humans but has been documented in animals as well, contributing to research on embryonic development and twinning. Related topics include conjoined twins and polycephaly.