Los Angeles ? One-year-old Guatemalan twins joined at the head were separated in a marathon operation that ended early Tuesday, but one sister was returned to surgery a few hours later because of bleeding on her brain.

Maria Teresa Quiej Alvarez and her sister, Maria de Jesus, were in critical but stable condition Tuesday night at the University of California at Los Angeles Medical Center, but doctors were optimistic about their recovery. They were expected to remain sedated and using breathing tubes for days.

Formerly conjoined twins Maria de Jesus, left, and Maria Teresa Quiej-Alvarez rest in separate beds for the first time in the pediatric intensive care unit at UCLA Medical Center in Los Angeles. Doctors were cautiously optimistic Tuesday night that the girls would make a good recovery.

“I’m absolutely positive they will do OK. I’m absolutely positive if you go and visit them in five years they will be leading a normal life,” said Dr. Jorge Lazareff, the lead neurosurgeon.

The doctors’ sense was that the girls fared well, but it remained to be determined whether they suffered any brain damage, said Dr. John Frazee, another neurosurgeon.

“We just don’t know neurologically. They’re moving, which is a good sign. There’s no way of knowing what the state of affairs is for another week,” Frazee said.

After the 22-hour risky separation surgery, Maria Teresa was wheeled back into the operating room for nearly five more hours because of a buildup of blood on her brain, Lazareff said.

The surgery-related hematoma “was not necessarily an unexpected situation,” said Dr. Michael Karpf, medical director at the medical center.

“This is very complicated surgery, and until we get past several days it will be life-threatening for both of them. We are minute-to-minute, hour-to-hour, day-by-day. We just can’t get ahead of ourselves,” Karpf said earlier Tuesday.

The girls’ parents, Wenceslao Quiej Lopez and Alba Leticia Alvarez, kissed their girls before the surgery.

The girls were born attached at the top of the skull and faced opposite directions. While the two shared bone and blood vessels, they had separate brains. Cases like theirs occur in fewer than one in 2.5 million live births.

The two face still more operations to reconstruct their skulls.

Healing the Children, a nonprofit group in Spokane, Wash., had arranged to bring the sisters from Guatemala for the $1.5 million operation. “The whole country has come together for these kids,” said Naomi Bronstein, a volunteer with the organization.

Surgeons around the world have performed cranial separations only five other times in the past decade, and not all twins have survived.