Formerly conjoined twins have "excellent" chances of survival after a grueling separation surgery, and one of the toddlers is even breathing on her own, doctors said Friday. Two-year-olds Yurelia and Fiorella Rocha-Arias of San Jose, Costa Rica, were conjoined at the chest and abdomen and shared an oversize liver until Monday, when doctors in Palo Alto separated them during a 9-hour surgery. In a critical milestone, Fiorella - who has always been slightly larger and stronger than Yurelia - was taken off her ventilator Wednesday and has been breathing on her own for two days.

"We are very happy with the outcome so far," said lead surgeon Dr. Gary Hartman of Lucile Packard Children's Hospital. "These are two very strong little girls."

Although Yurelia is still on a ventilator and heavily sedated, the twins' cardiovascular intensive care physician said chances for survival are higher than the 50-50 odds doctors gave them earlier this week.

"Current chances for survival are excellent," said Dr. Gail Wright, who would not provide a specific odds for survival. "They have multiple steps ahead of them but right now things are looking good."

On Wednesday, cardiothoracic surgeon Dr. Frank Hanley performed a six-hour surgery on Yurelia to correct a life-threatening congenital anomaly known as double outlet right ventricle. Kids with the condition have trouble getting enough oxygen to their tissues, and their hearts work abnormally hard.

Another complication for Yurelia: Her heart had become improperly positioned in the chest due to the way her skeleton had fused with her sister's.

"Although it will never resemble a normal heart, it should still function properly," Hanley said.

During the surgery, Hanley created a tunnel from the left ventricle through the right ventricle to the aorta to correct the flow of blood in Yurelia's body. The tunnel passed through and sealed a hole between the sides of Yurelia's heart that had allowed mixing of oxygen-rich and oxygen-poor blood.

Hanley also widened an area in Yurelia's heart near the pulmonary valve that had been constricting the flow of blood to her lungs. Because Yurelia and Fiorella's hearts were connected, it was impossible to correct Yurelia's defect until after the separation.

After heart surgery, Hartman and pediatric plastic surgeon Dr. H. Peter Lorenz reconstructed Yurelia's chest wall.

The girls' parents, Maria and Jose Luis Rocha-Arias, have requested privacy and declined to comment.

The girls arrived in San Francisco on July 25 and received weekly injections of sterile saltwater into balloons placed under their skin. The procedure stretched their skin to compensate for the holes surgeons cut into their abdomens.

Packard doctors are donating their time to treat Yurelia and Fiorella, who have nine older siblings. Mending Kids International, a faith-based nonprofit based in Santa Clarita that helps sick children, arranged transportation and housing.

The hospital is donating medical and clinical care. The Nicaraguan embassy helped pay for the twins' care in Costa Rica.

Earlier this fall, Hartman estimated the cost of the surgery to be $1 million to $2 million. Cedars-Sinai Medical Center in Los Angeles rejected the case, and Packard's ethical board approved it only by a narrow margin.

Researchers estimate the incidence of conjoined twins to be between 1 in 30,000 to 1 in 200,000 worldwide. Most do not survive pregnancy, and most born alive die within 24 hours.

Conjoined twins may result when a single fertilized egg does not totally split during the process of forming identical twins. Another theory holds that two fertilized eggs fuse early in development. There's no definitive environmental or genetic cause.

About five separation surgeries are performed annually in the United States.

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. In early August, doctors in Cleveland called off the long-planned surgery on 3-year-old girls joined at the head, saying the risk to both twins was too great.