DALLAS -- A team of 18 doctors began a complicated separation surgery Saturday in an attempt to give 2-year-old conjoined twins from Egypt a chance at independent lives.

The risks are high: One or both of the boys may die, and even if they survive, some brain damage is possible.

"Ahmed and Mohamed Ibrahim have begun the first stage of their surgery that will physically separate them and -- we all hope -- will give them the opportunity to grow and develop like other brothers," said Dr. Jim Thomas, chief of critical care medicine at Children's Medical Center Dallas.

By midafternoon, doctors entered a critical stage as neurosurgeons prepared to separate an intricate web of shared blood vessels, Thomas said.

Thomas relayed a message from Dr. Dale Swift, one of five pediatric neurosurgeons: "Everything is going fine. There have been no problems."

Doctors spent more than a year planning the surgery to separate the boys joined at the crown of the head. The operation was expected to take a team of 50 to 60 medical personnel anywhere from 18 to 90 hours to finish.

Relatives of the boys in the tiny village of el-Homr, near the southern Egyptian city of Qus, prayed for their safe return.

"It is all in God's hands now," an uncle of the twins, Nasser Mohammed Ibrahim, said in a telephone interview. "He is the one who can save their lives."

Thomas said much of Saturday morning was spent positioning the boys in a specially made bed that lets doctors swivel their bodies for easy access to the front and back of their heads.

Craniofacial surgeons planned to open the boys' scalp and remove skin expanders inserted about five months ago. The extra skin and tissue created by the expanders will cover the head wounds.

While each boy has his own brain, they share an extensive attachment of blood vessels, which neurosurgeons must separate.

Left hemisphere at risk

On the right side of each boy's brain, the blood flows in a normal fashion -- into the brain and back to the boy. On the left side, though, the blood flows from one boy into the other.

"During the operation, the left hemisphere is going to be at risk," Swift said earlier this week.

He said the hope is that the blood will drain into other deep veins. The worst possible situation, Swift said, is that the blood can't get out and the hemisphere becomes swollen and damaged.

"We think the result will be somewhere in the middle," he said.

Tissue from the boys' thighs will be used to cover the brain and bone fragments exposed by the surgery.

"The moment they're separated may seem like a great moment, but it's not," Swift said. "We can't stop doing anything. There's no relaxing right then and there because you have to get this all closed up."

After the surgery, the boys would go to an intensive care unit, where they will remain in a drug-induced coma for three to five days, Swift said.

The boys were born June 2, 2001, by Caesarean section to Sabah Abu el-Wafa and her husband, Ibrahim Mohammed Ibrahim.

Dallas-based World Craniofacial Foundation, a nonprofit group that helps children with deformities of the head and face, arranged to bring the boys to Dallas in June 2002 for an evaluation.

A team of specialists determined that the boys could be separated, though the risks include brain damage and death. The boys' father told doctors he felt it was worth it to give them a chance at a normal life.

Thomas acknowledged the risks but said ethics boards at two hospitals had reviewed the case.

"Nobody goes into this lightly," said Swift, the pediatric neurosurgeon. "At this point, we think it's the right thing to do."

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On the Net:

World Craniofacial Foundation: http://www.worldcf.org

Children's Medical Center: http://www.childrens.com