*Conjoined twins are already rare, but two heads being born on the same body is one of the most unusual occurrences. An unnamed baby girl in Indonesia was born with the condition, which can cause many difficulties.

From Wikipedia:

Polycephaly is the condition of having more than one head. The term is derived from the Greek stems poly- (Greek: “πολύ”) meaning “multiple” and kephalē- (Greek: “κεφάλη”) meaning “head”. A polycephalic organism may be thought of as one being with a supernumerary body part, or as two or more beings with a shared body.

Two-headed animals (called bicephalic or dicephalic) and three-headed (tricephalic) animals are the only type of multi-headed creatures seen in the real world, and form by the same process as conjoined twins from monozygotic twin embryos.

In humans, there are two different forms of twinning that can lead to two heads being supported by a single torso. In dicephalus parapagus dipus, the two heads are side by side. In craniopagus parasiticus, the two heads are joined directly to each other, but only one head has a functional torso. Survival to adulthood is rare, but does occur in some forms of dicephalus parapagus dipus.

There are many occurrences of multi-headed animals in mythology. In heraldry and vexillology, the double-headed eagle is a common symbol, though no such animal is known to have ever existed.

Two-headed people and animals, though rare, have long been known to exist and documented.

Occurrence in humans

In humans, as in other animals, partial twinning can result in formation of two heads supported by a single torso. Two different ways this can happen are dicephalus parapagus, where there are two heads side by side, and craniopagus parasiticus, where the heads are joined directly.

Dicephalus parapagus dipus

In dicephalus parapagus dipus, the two heads are side by side, on a torso with two legs, with varying levels of twinning of organs and structures within the torso. The shared body may have four arms altogether, or three arms, or two arms only. There are Greek-based medical terms for the variations, e.g. dibrachius means two-armed, tribrachius means three-armed. Both heads may contain a fully formed brain, or one may be anencephalic.[1] If carried to term, dicephalus parapagus twins are usually stillborn, or die soon after birth. Survival to adulthood does however occasionally occur in cases where the twins are born with three to four arms.[2] Chances of survival are improved if two complete hearts are present.[3] Separation surgery is contraindicated, except in cases where one of the twins is clearly dying.[4]

Giacomo and Giovanni Battista Tocci (born between 1875 and 1877), were dicephalus parapagus dipus twins who survived to adulthood. Each had his own pair of arms. They learned to speak several languages, but never learned to walk. Abigail and Brittany Hensel, born in 1990, are another instance of dicephalus parapagus dipus twins who grew up. They were born with two functional arms, plus a vestigial third arm, which was surgically removed. Each twin has her own complete head, heart and spine, and controls one arm and one leg. They developed good motor skills, and completed courses at school and university.[5]

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