A young baby has died hours after landmark surgery in the Dominican Republic to remove a live second head.
Rebeca was thought to be the first to survive with this condition
Doctors said the baby's blood refused to clot, leading to her death.
Earlier surgeons had said baby Rebeca Martinez was doing well after a complex 11-hour operation carried out by a team of 18 people on Friday.
Born with the head of an undeveloped conjoined twin fused to the top of her skull, it was regarded as highly unusual for her to survive beyond birth.
In the delicate operation, surgeons had to cut off undeveloped tissue, clip the veins and arteries and close Rebeca's skull using a bone graft from another part of her body.
"She was too little to resist the surgery," the baby's mother Maria Gisela Hiciano told the Associated Press news agency from the hospital in Santo Domingo where surgery took place.
The team was led by Jorge Lazareff, who successfully separated Guatemalan conjoined twins in 2002.
A spokesman for the charity Cure International - which is meeting the estimated $100,000 cost of surgery - earlier said that as the surgeons came out of the theatre they had unanimously said the operation was a "great success".
But one of the lead brain surgeons, Benjamin Rivera, told AP the infant had lost a lot of blood in the operation and had many transfusions, but that her blood would not clot.
"In that case, you can't do anything. This is the worst
complication that can happen in this kind of surgery," he said.
Rebeca, who was born in mid-December, was only the eighth documented case in the world of a condition known as parasitic twins. The other seven all died before birth.
Her second head had a partially-developed brain, ears, eyes and lips, and if it had continued to grow it would have prevented Rebeca's brain from developing.
The features on the second head reportedly moved when Rebeca was being fed.
Dr Lazareff, the director of Paediatric Neurosurgery at UCLA's Mattel Children's Hospital, said the head on top had been growing faster than the lower one.