What are conjoined twins?
Conjoined twins develop in the womb from a single egg and grow attached together. The area of the join can vary and some conjoined twins may share body parts and organs.
Delivering two babies attached together can be a challenge for doctors, as well as planning for separating the babies at a later stage so they can live independent lives if possible.
Conjoined twins are rare. One of the earliest documented cases was that of Mary and Eliza Chulkhurst, who were born in 1100 in Biddenden in Kent, and were known as the Biddenden Maids. They lived until they were 34.
Successful separation of a set of conjoined twins was first reported in 1689 in Basel, Switzerland.
Survival of both, or one twin, often depends on how the babies are joined together, and whether they have enough separate organs to survive alone. Being joined at the chest is reported to have the greatest chance of survival from separation surgery.
As well as the medical issues facing surgeons, there are moral and ethical decisions for them and the babies' parents.
When single organs are shared, the twin who receives the organ system can live, while the other twin cannot.
A similar problem arises when limbs are shared.
When a brain is shared, these twins are usually inseparable.