A Brazillian mother has given birth to full-functioning conjoined twins that have two autonomous brains, separate spines but just one heart.
Experts believe that separation would be fatal for the twins, who have been named Jesus and Emanuel, due to the single set of internal organs.
Specialists are keeping a close eye on the duo in case they develop any health issues in the future.
Dr Neila Dahas who is treating the brothers said separating them is not an option at this stage.
"What we know statistically is that the children who undergo surgery and survive are the children who have less organs in common," she said.
"What we've got to think about at this moment is to maintain the children in good condition and see how they will develop."
The twins were born by Caesarean section weighing it at 9.9lbs on Monday in a hospital in Para, Brazil.
Hospital director Claudioner Assis de Vasconcelos said their mother gave birth after complaining of abdominal pains.
Mr de Vasconcelos said: "Despite all the problems we have as a small interior hospital we managed to save both mother and baby, which was our aim."
One famous case of conjoined twins is the widely documented case of Abigail and Brittany Hensel who were born with a similar condition to Jesus and Emanuel in the U.S. in 1990.
Patrik O'Brien, spokesman for the UK's Royal College of Obstetrician and Gynaecologists, said: "A lot of work is needed, in terms of scans and tests, before doctors will know if they can separate them or not, and just how organs and blood vessels are shared and linked.
"It takes quite a while before they can decide how feasible it is."