Australian neurosurgeons have begun removing the bone on the back part of the heads of conjoined twin sisters Trishna and Krishna.
The extremely complicated operation to separate the twins who are joined at the head, began as scheduled at the Royal Childrens' Hospital in Melbourne at 8.30am (AEDT).
After several hours of positioning tubes to ensure there was no pressure on the eyes, surgeons made their first cut about 10am.
Plastic surgeon Tony Holmes said everything was so far going to plan.
"At the moment most of the major (skin) flaps are already up and looking particularly healthy and the bone is exposed... for virtually half the exposure we need," he said.
"We're ready now for the neurosurgeons who are starting up right this minute to start removing the bone on the back half of the head, which is the dangerous bit in that that's where the brain is still joined and there are a couple of blood vessels."
50 per cent chance of brain damage
He said the chance of the operation being a success remains the same at just 25 per cent.
There is a 50 per cent chance they will suffer brain damage and a 25 per cent chance one of the sisters will die.
Mr Holmes said he estimated the twins, aged two, would be separated between 5pm and 6pm on Monday.
"The unknown... is what actually happens when you separate finally the cerebral circulations, because that is a change in hemodynamics (blood movement) so the pressures will be different in each twin," he said.
"It's over those few early minutes when the pressures equilibrate in the brain, that 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."
Mr Holmes said Moira Kelly, who rescued the Bangladeshi orphans, was understandably distressed as she left the girls in the hands of surgeons.
Medical staff 'nervous' over 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."
Mr Holmes said medical staff at the hospital were nervous about the operation.
"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.
"We have had a few ups and downs with these children because of medical problems."
The twins went to sleep in hospital on Sunday and were kept sedated overnight in intensive care. They had an arteriogram, an x-ray showing the remaining blood vessels between the two brains.
Mr Holmes said the remaining blood vessels were minimal, which meant neurosurgeon Virginia Maixner's work slowly separating the twins' vessels had been successful.
Ms Maixner is leading the 16-hour operation with a team of up to 16 surgeons, doctors and nurses.