Surgeons started separating twin girls joined in the chest and abdomen at Stanford University's Lucile Packard Children's Hospital in California on Tuesday morning.

The delicate operation on two-year-olds Angelina and Angelica Sabuco, of San Jose is scheduled to last until mid-afternoon, said hospital spokeswoman Reena Mukamal, according to the Associated Press.

Leading the surgery is Dr. Gary Hartman, who had performed five similar operations in the U.S. He expects the operation to go smoothly.

The separation of the Sabucos will entail cutting their skin and muscles and then separating their diaphragms and livers. The critical part of the operation is the severing of the liver, which may lead to blood loss.

The operation was done as doctors found that the Sabucos, who face each other, will worsen the deformities of their muscles and bones over time and the death of one will cause the death of the other twin within hours.

The twins' mother, Ginady, is with the girls, whose condition is called thoraco-omphalopagus. Their livers, diaphragms, breastbones, chest and abdominal wall muscles are fused. But they have separate hearts, brains, kidneys, stomachs and intestines.

The girls, who were born in the Philippines but now live in San Jose, will stay in the hospital for three weeks after the operation to recover.