The conjoined twins Faith and Hope Williams were undergoing emergency separation surgery at Great Ormond Street Hospital for Children in London last night.

Doctors took the decision to to carry out the surgery after the girls’ condition worsened on Monday night.

Their mother, Laura, became the world’s youngest mother of conjoined twins at just 18. The babies, delivered by Caesarean section last week after 35 weeks' gestation, had a combined weight of 10lb 8oz.

Agostino Pierro, Nuffield Professor of Paediatric Surgery at Great Ormond Street, said at the weekend that the sisters’ hearts had significant congenital abnormalities that could require surgery.

“The current concern is that the two hearts and the joined circulation raise a risk that the children might suddenly deteriorate and need emergency separation surgery," he said.

Mr Pierro said that the large blood vessels run from one child to the other, and are joined at the liver and intestine.

Doctors wanted to leave separation surgery until the children were older and stronger but feared it would be too risky to delay. They were due to make a decision yesterday on carrying out the surgery later this week but decided on Monday night that an emergency operation was vital.

Mrs Williams and her husband, Aled, 28, from Shrewsbury, had refused to have a termination after doctors at their local hospital said they would probably not live beyond two weeks.

After their birth Mrs Williams said: “We knew conjoined twins rarely make it through the first 24 hours, and we could see that one girl was a bit smaller than the other so we called the little one Hope and the bigger one Faith.”

A spokeswoman for Great Ormond Street Hospital said: “The separation of conjoined twins Faith and Hope Williams has been brought forward and is currently taking place, after some concerns developed last night.

“Obviously, the final decision about separation surgery was made by the family.

“Laura and Aled Williams would like to thank all the hospitals involved in their care for everything they have done for the family so far, in particular, the surgeons and other staff who are currently in surgery conducting the separation.”

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