Surgeons in Northern California began a nine-hour operation on Tuesday to separate a pair of twin 2-year-old girls conjoined at the chest and abdomen.

Doctors at Stanford University's Lucile Packard Children's Hospital were expected to complete the procedure on Angelica and Angelina Sabuco by about 3:30 p.m. Pacific time, hospital spokeswoman Erin Digitale said.

The girls, classified by doctors as thoraco-omphalopagus twins, are joined at the chest and abdomen and have fused livers, diaphragms, breast bones and abdominal walls. They have separate hearts, brains, kidneys, stomachs and intestines.

It's a large surgical field which required extensive preparation. Doctors were making sure the girls had all the IV lines they would need, Digitale said.

The sisters, who were born in the Philippines and now live in San Jose, California, can count to 10 and love stories and music, according to the hospital, which said that lead surgeon Dr. Gary Hartman has separated six sets of conjoined twins over his career.

In the United States, about six such surgeries take place every year. But 75 percent of conjoined twins do not survive pregnancy, according to the hospital.