A marathon and risky operation to separate Bangladeshi twins joined at the head, which they have only a one in four chance of both surviving, was going well today, surgeons said. The 16-strong team was on "tenterhooks" over the delicate bid to separate Trishna and Krishna, aged two, which will take about 16 hours, plastic surgeon Tony Holmes told reporters at Melbourne's Royal Children's Hospital.

"It is a stressful time for any group of surgeons with this sort of case. They only come along really once in a lifetime and I think everybody has been on tenterhooks," he said. Doctors began at 10:00 am (2300 GMT Sunday) by cutting through the bone which joins the girls via the top of the head and were working to separate the connected section of brain and blood vessels. "The unknown... is what actually happens when you separate finally the cerebral circulations, because that is a change in haemodynamics (blood movement) so the pressures will be different in each twin," Holmes said.

"It's over those few early minutes when the pressures equilibrate in the brain, they're the things that we're worried about. "But the children are prepared as well as could possibly be and we're cautiously optimistic that everything is going particularly well." By 9:00 pm surgeons had almost finished separating the twins' brains but the girls were still joined by a bone bridge between their skulls. "It's just going absolutely according to plan at the moment," Ian McKenzie, the hospital's director of anaesthesia and pain management, told reporters. "We really couldn't be happier. I suppose we'd be slightly happier if it was a little bit quicker, just like you, but we're actually right on the time frame we were expecting anyway," the doctor said. "So, actually, it's great. We're prepared for it, and it's going exactly according to plan."

The whole procedure is expected to finish in the early hours of Tuesday. The girls, who were placed in a Dhaka orphanage at birth, were close to death when they arrived in Australia two years ago but both are now thriving after undergoing a series of preparatory operations. The Children First Foundation (CFF) flew the girls to Australia because of poor separation survival rates in their native Bangladesh, where only two children have survived four operations in recent years.

Holmes said the children's legal guardian, Moira Kelly, looked "relatively distressed" as Trishna and Krishna were wheeled into the operation. "When the children went into the operating theatre... Moira was there giving them, you know, a farewell kiss and good luck," he said. "She was relatively distressed as one would be if it was your child. The kids were fine, OK, they looked as healthy and happy as anything but they were sedated." Separating conjoined twins is a notoriously difficult procedure, with attempts in Britain and Bangladesh both failing over the past year, although Saudi doctors successfully divided a pair of Egyptian brothers in February.

-The Independent