A pair of 29-year-old Iranian twin sisters, joined at the head, said Saturday their fate was in God's hands as they prepared to walk into a marathon operation that could finally separate them — or could leave one or both of them brain dead.

After a lifetime of compromises on everything from when to wake up each day to what career to pursue, Ladan and Laleh Bijani (search) said they preferred to face the dangers of the surgery — which could last up to four days — rather than continue living joined.

"If God wants us to live the rest of our lives as two separate, independent individuals, we will," Ladan said.

Sunday morning, they plan to walk into the operating room at Singapore's Raffles Hospital (search) — rather than be put to sleep beforehand and wheeled in — as a sign of courage.

"We've never been as confident as we are now," Ladan said. "We are prepared by all means to embrace the risks and walk into the operation room."

The operation will mark the first time surgeons have tried to separate adult craniopagus twins (search) — siblings born joined at the head — since the procedure was first successfully performed in 1952. The surgery has so far only been performed on infants, whose brains can more easily recover.

An international team of 28 doctors and about 100 medical assistants will participate in the surgery.

Ladan said they would spend the hours before the operation reading the Quran and performing ritual Muslim ablutions. "We feel closer to God that way," she said.

Ladan spoke just before doctors conducted four hours of last-minute tests on the sisters to study how blood flows through their brains.

The tests revealed a new medical reason for the surgery to proceed, lead neurosurgeon Dr. Keith Goh said. The pressure inside the twins' brains was more than twice what it should be.

The discovery led doctors "to believe this is something quite necessary, not cosmetic or frivolous," Goh told a news conference late Saturday.

"Rest assured, we're all here to help you. Please stay positive," Dr. Benjamin Carson, one of six international experts assisting in the surgery, told the sisters when he met them on the eve of the operation, according to a hospital statement.

The sisters each have a 50-50 chance of survival, said Carson, a Johns Hopkins neurosurgeon from Baltimore. But he said he expected the surgery to be a success.

The surgeons were making their own preparations ahead of the long surgery, he said: getting enough sleep and not drinking too much liquid.

The surgeons' biggest challenge will be dealing with a shared vein that drains blood from the women's brains. German doctors concluded in 1996 that vein made the surgery too dangerous.

One sister will have to have a graft to replace the shared vein, probably a vein taken from a leg, Carson said.

He compared the veins to a city's road network and said the task surgeons faced was to identify traffic jams and create detours. The largest vein was the size of a finger, he said.

Saturday's tests were aimed at finding alternative blood channels and to see if a bypass was necessary, Nair said. The tests, led by French neuroradiologist Dr. Pierre-Louis Lasjaunias, lasted four hours.

The discovery of high pressure in their brains explained why Laleh had suffered chronic headaches and meant medical intervention would have eventually been necessary, Goh told reporters, without elaborating.

The twins will remain seated throughout the operation — a standard practice in brain surgery — which will last at least 48 hours and could take four days.

The Bijani sisters, born in Firouzabad, southern Iran, in 1974, have separate brains that lie next to each other in a joined skull. Their heads are connected but their bodies are otherwise distinct.

The twins have wanted to be separated ever since they first opened their eyes, Ladan told a news conference last month. They told reporters they long for simple things such as seeing each other's face.

They came to Singapore in November after hearing about Goh's success in separating 18-month-old Nepalese infants who were also joined at the head.

Both sisters studied law because Ladan wanted to be a lawyer. But after the surgery, Laleh wants to move to Tehran to be a journalist, while Ladan wants to move back home with her parents and continue her studies to qualify as a lawyer.