Doctors successfully separated two-year-old twin girls from Costa Rica conjoined at the chest and abdomen.
The 9-hour surgery performed Monday on Yurelia and Fiorella Rocha-Arias went "much better than anticipated," but their odds of survival are only 50 percent, according to their lead surgeon.
"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," Dr. Gary Hartman said in a statement.
The girls' parents, Maria and José Luis Rocha-Arias, have requested privacy and declined to comment.
The procedure was considered complicated in part because the twins shared a blood supply.
A surgeon is scheduled later this week to try to correct a serious heart defect in Yurelia, who was born with a double outlet right ventricle and other congenital defects.
At their age, the girls may be stronger and able to recover more quickly than younger conjoined twins. But their muscle and skeletons had more time to fuse, complicating the separation, their doctors said.
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.
Researchers theorize that conjoined twins 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 is no definitive environmental or genetic cause.
About five separation surgeries are performed each year in the United States.