The identical twins, born to Mexican parents who came to the US on a tourist visa, were fused at the front, but had separate heads, necks, shoulders, hearts, lungs, arms and legs.

There was an "orderly calm" in the operating room as doctors moved one of the twins to another room, said spokesman Steve Rutledge. "They seem to be doing fine," Rutledge said of the twins.

Doctors planned to work through the night reconstructing the girls' chest walls and pelvis regions and sewing up surgical wounds. The girls remained sedated.

'Going Impeccably'

Surgical director Dr. Henri Ford said earlier in the day that the sisters "looked very healthy and quite good" throughout the operation. "Everything has been going impeccably as one could possibly imagine," he said.

The twins, who were born facing each other, were wheeled into the operating room shortly before 6 a.m. and went under anesthesia. Doctors made their first delicate incision at the breastbone three hours later and then worked their way toward dividing the liver, bladder and genitalia.

The babies' parents waited one floor below the operating room.

"They are very relaxed and very pleased with the progress," Ford said.

The identical twins, born to Mexican parents who came to the US on a tourist visa, were fused at the front, but had separate heads, necks, shoulders, hearts, lungs, arms and legs. Regina was born with one kidney.

The girls shared part of the small intestine and the entire large intestine. Doctors had debated about how to split the large intestine and it was not immediately clear how the organ was eventually divided.

Surgery Was More Complex

Only a few hundred pairs of conjoined twins are born each year worldwide. In the United States, they occur 1 in every 200,000 live births.

Several physicians from the 80-member team working on the twins previously took part in another conjoined twin separation at the hospital in 2003, but this surgery was more complex because more organ systems were involved.

Regina and Renata were born Aug. 2, 2005, at Los Angeles County-USC Medical Center. They were later transferred to Childrens Hospital, where doctors spent months preparing for the separation surgery.

Childrens Hospital declined to disclose the operation's cost, which will be covered by a state health program. The hospital has performed five conjoined twin operations since 1966 including three cases in which both twins survived.