A foetal therapy team of specialists in high-risk pregnancies, neonatology and paediatrics was assembled early on at St Mary's Hospital, Manchester, to co-ordinate every aspect of delivery and plan for any eventuality.
The delivery, by Caesarean section on Thursday night, was described as a "relatively straight-forward" operation, although a larger incision than usual was necessary because both twins had to be removed together.
The twins, who weighed 12lb 9oz between them, were described as doing well. Both cried within an hour of being born, although one required help with breathing for a short time. Emergency surgery, if preliminary tests revealed problems, was not ruled out, but the hospital was "genuinely ... optimistic about their survival chances".
The greatest concern of Michael Maresh, the consultant obstetrician in charge of delivery, and Alan Dickson, the paediatric surgeon who will now co-ordinate their future care, was to discover the extent of the girls' conjoined state, and what this meant for their well-being outside the womb. Only then can the prospects for the successful separation of twins be accurately assessed.
Antenatal ultrasound scans had revealed that the babies were joined from their breast bone to navel and shared a liver but "until they were born we could not be absolutely certain," Mr Maresh said. Tests were still being carried out yesterday to see if the girls were sharing a bowel or other abdominal organs.
If the liver is indeed the only major organ the girls share then it will have to be split, but separation should be relatively straight-forward as liver cells are remarkably resilient and regenerate easily.
The fact that each has her own heart, limbs and spinal cord gives them a far greater chance than most such twins, medically known as "conjoined", who suffer more complex fusions.
Siamese twins, so called after the first recorded pair, Chang and Eng who were born in Thailand (formerly Siam) in1811, are essentially identical twins who have failed to separate completely from a single fertilised egg. An estimated one in 100,000 births is affected.
The spectrum of abnormality ranges from two normal healthy individuals connected only by excess skin or superficial tissue, to a person who has an extra body part, such as a leg, as the only evidence of the other twin of conjoinedness.
Until their death in 1993 at the age of 43, Yvonne and Yvette McCarther, from Los Angeles, were the world's longest living Siamese twins. They were joined at the head and shared the same circulation so separation was not an option. They spent their early lives touring as part of a freak show but later went on to develop as gospel singers and in their thirties trained as children's nurses.
The best known recent case is that of Irish twins, Eilish and Katie Holton, born in 1988 and whose early lives up until their separation aged three were movingly documented by Yorkshire television.
The girls were connected at the chest, abdomen and pelvis but had separate hearts, lungs and blood vessels. They shared two legs and had arms on either side, with two fused in the middle of their back. They had two fused livers but separate stomachs and spines. Katie died four days after separation from heart failure which, if they had remained joined, could have threatened Eilish's life too.
The 15-hour operation by a 25-strong team at Great Ormond Street Hospital, London, was, at the time, the most complex separation ever attempted.
If, as seems likely, Nicole and Chloe can be separated with minimal risk to either child, Lewis Spitz and his team at Great Ormond Street are the most likely choice.Reuse content