Doctors at Lucile Packard Children's Hospital said they would attempt to separate 2-year-old twin girls who are conjoined at the chest and abdomen.

Surgery on Yurelia and Fiorella Rocha-Arias of San Jose, Costa Rica, is expected to take place in late November, after their skin has been stretched to cover the large gap where they have been connected.

The survival rate for separation surgery for twins joined primarily at the chest — known as thoraco-omphalopagus twins — is about 50 percent, doctors said Thursday. But rates vary widely, depending in part on the extent of heart defects.

"We hope to send home two girls who are healthy and happy," lead surgeon Gary Hartman said. "I can envision these girls, a few years from now, flipping through a photo album and calling mama and saying, 'Look mama! This is a picture of us when we were connected.'"

Since arriving in San Francisco on July 25, the girls have been receiving weekly injections of sterile saltwater into balloons placed beneath their skin. This procedure should stretch their skin to compensate for the hole that surgeons will cut into their abdomens.

The girls are connected at the right atria of their hearts, the chamber that receives blood from the rest of the body, and they share some blood and a single liver.

At their age, Yurelia and Fiorella may be stronger and quicker to recover than younger twins; before coming to California, they were only hospitalized a few times for colds and respiratory infections.

But, doctors warn, their muscle and skeletons had more time to fuse than younger twins who undergo surgery, possibly complicating the procedure.

Because of their face-to-face positioning, the girls are unable to walk, and their backs are becoming crooked. Although they reach for things together and play peacefully, caregivers say the toddlers are increasingly willful and want to do things by themselves.

Cardiothoracic surgeon Dr. Frank Hanley said it was impossible to say how long the twins would live if the surgery did not take place.

"It's an unfathomable question," he said. "As they mature, one is probably going to grow faster than the other, and the discrepancies will grow. They can't even walk together now, just stand up. It's more about quality of life."

If the surgery goes well, the girls face more operations to correct heart and lung problems, and psychological care. Costa Rican media have followed the girls, and it's unclear how they would adjust to relative normalcy.

"That celebrity status is going to fade, and we're concerned there may be psychological issues," Hanley said. "These girls really view themselves as a unit, and for lack of better word there may be separation issues."

About five separation surgeries are performed annually in the United States, according to Packard data. On Aug. 29, doctors at Children's Hospital of Philadelphia separated 1-year-olds Matthew and Andrew Goodman, who shared a liver, pancreas and other organs.