All about conjoined twins

Conjoined twins are formed from a single fertilized egg which only undergoes incomplete separation at the embryological development stage. They are genetically identical, always of the same sex and share the same amniotic cavity and placenta.
Usually, twins develop either when a woman releases two eggs (fraternal twins) instead of the usual one or she produces only one egg but that divides after fertilization
(Identical or paternal twins).
Eng and Chang Bunker born in Thailand (then called Siam, thus the term Siamese twins) in 1811 were a set of famous conjoined twins.
They were joined at the lower chest by narrow band of flesh, which connected their livers. They were put on show at circuses and later lived in the United States where they married and had several children. They died at the age of 63.

Some types of conjoined twins
Conjoined twins are usually classified by the point at which they are joined.

Thoracopagus: this is the most common form of conjoined twins, connected on the upper front part of the torso. As they share a heart it is almost impossible to separate them.

Omphalopagus: connected from the breast bone to the waist, this is another common form. These conjoined twins share a liver and gastrointestinal or genitourinary functions, but only rarely a heart.

Pygopagus: the twins are joined at the rump.

Craniopagus: the twins are joined at the cranium (the top of the head or skull). This is a rare form and difficult to separate though such surgical separations have been performed successfully.

Cephalopagus: joined on the upper half of the body, with two faces on opposite sides of a conjoined head, sharing one heart. This is extremely rare.

Parasitic twins: also called asymmetrical conjoined twins or heteropagus twins, they are extremely rare. There is and incomplete component (parasite) that is normally smaller and dependent on the host (autosite).

Foetus in fetu: an imperfect foetus is contained completely within the body of its sibling.