Identical twins Regina and Renata Salinas Fierros have faced each other since birth.

On Wednesday, doctors are scheduled to perform delicate surgery to separate the conjoined sisters, who were born 10 months ago connected from the lower abdomen to the pelvis. The marathon operation at Childrens Hospital Los Angeles is expected to last up to a day.

Several physicians from the 80-member team previously took part in another conjoined twin separation at the hospital in 2003. But this surgery is more complex because it involves more organ systems.

Doctors expressed confidence that the surgery will be a success. Separating the sisters would "give them independent lives and hopefully a very promising future," lead surgeon Dr. James Stein said Tuesday.

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. About 10 percent of conjoined cases are similar to Regina and Renata.

The girls are expected to be wheeled into the operating room before daybreak and doctors will give them anesthesia.

Over several hours, surgeons will begin separating the twins — first by dividing their breastbone, liver, intestine, bladders, genitalia and pelvis. Then plastic surgeons will reconstruct the babies' chests, vaginas and pelvises.

Following surgery, the girls will be transferred to the pediatric unit where a team of specialists will care for them during the critical 24 to 48 hours after the operation.

Regina and Renata were born with their faces inches apart on Aug. 2, 2005 at Los Angeles County-USC Medical Center to Mexican parents who came to the U.S. on a tourist visa. The twins were later transferred to Childrens Hospital where doctors spent months preparing for the separation surgery.

Regina is the weaker of the twins and has trouble gaining weight despite her healthy appetite. But doctors said they have seen cases where the feeble conjoined twin improved after separation.

The girls' mother, 23-year-old Sonia, said Tuesday that she felt conflicted heading into the operation.

"We feel nervous and anxious, but at the same time very tranquil," she said in Spanish.