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Surgeons at Lucile Packard Children's Hospital in Palo Alto separated two conjoined twin girls from Costa Rica Tuesday who now have a 50 percent chance of survival, the hospital announced.

Two-year-old Yurelia and Fiorella Rocha-Arias were born joined at the abdomen and chest. They shared a liver and the right atriums of their heart were connected. Their separation began at 6:30 a.m. Monday and was completed nine hours later, according to the hospital.

"The separation is the culmination of months of planning by several multidisciplinary teams. It was a very risky and complicated surgery, and the outcome is still unknown. The girls face many more hurdles in their path to recovery," lead surgeon Dr. Gary Hartman said in a statement.

"The separation went much better than anticipated. Dozens of people worked together seamlessly, and the girls' cardiac function actually improved when they were separated."

Yurelia is scheduled to undergo a second surgery later this week to correct a serious heart defect.

The hospital has set up a Web site with more information about the surgery and updates about the twins' condition at twins.lpch.org .

— Bay City News Service