Yurelia and Fiorella Rocha-Arias of San Jose, Costa Rica , underwent the nine-hour surgery Monday. Though it went well, the odds of survival for such separated conjoined twins is about 50 percent, Dr. Gary Hartman said.
"There's a lot of relief, but it's not really over," Hartman said in a phone interview with The Associated Press. "The point where we would relax significantly would be when they're off the breathing machines."
Yurelia and Fiorella are in the critical care facility at Lucile Packard Children's Hospital in Palo Alto. Both are heavily sedated and have ventilators and feeding tubes.
Yurelia is receiving intermittent paralyzing agents because she is going back to the operating room later this week, where a cardiothoracic surgeon will try to correct a serious heart defect.
Fiorella does not appear to need follow-up surgeries, and anesthesiologists will lighten her sedation Tuesday or Wednesday, he said.
The girls' parents, Maria and Jose Luis Rocha-Arias, have requested privacy and declined to comment.
The procedure was considered complicated, in part because the twins shared a blood supply.
Doctors had to clamp off valves to see what would happen if neither girl shared blood with the other. Neither girl had problems, "in fact, each of them in some manner improved," Hartman said.
The girls also had appeared to share a liver, but the organ turned out to be two individual livers fused at the widest point and successfully separated by doctors.
Yurelia and Fiorella's parents moved from Nicaragua to Costa Rica, which has better medical facilities, after an ultrasound showed conjoined twins. The Nicaraguan embassy helped pay for the twins' care in Costa Rica. The girls arrived in San Francisco on July 25.
Packard doctors are donating their time, and the hospital is donating medical and clinical care, 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.
Earlier this fall, Hartman estimated the cost of the surgery to be $1 million ( EUR 680,000) to $2 million ( EUR 1.37 million).
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. About five separation surgeries are performed annually in the United States .