A conjoined twin has died in hospital after an operation to separate her from her sister. The twins were jointed at the chest and shared a liver but had separated hearts. Hope Williams died after 12 hours of surgery at Great Ormond Street Hospital in London.

The twin’s mother is 18-year-old Laura Williams, who is Britain’s youngest-ever mother to give birth to conjoined twins. The birth went well and doctors were hopeful both babies would survive the surgery because they had separate hearts. Doctors said the technical separation worked well, but the baby died because her lungs failed her. Her sister also needed support for her breathing, but was gradually improving, Professor Pierro, head of the surgical team which operated on the twins, said.

Siamese, or conjoined twins, occur when the single egg from which identical twins develop fails to divide properly after conception. Conjoined twining is a fairly rare occurrence; about 1 in 200 sets of identical twins are born conjoined. Since the late twentieth century, many medical advances have resulted in the successful surgical separation of conjoined twins.

Conjoined twining usually happens in women aged between 25 and 40 and the survival rate is somewhere between five and 25 per cent depending on where the children are joined. Those who share an abdomen, usually share the liver, bile ducts from the liver, and parts of the intestines.

More than 40 staffs were involved in the separation of the Williams twins at the London hospital.