Surgeons in California on Tuesday began separation surgery for 2-year-old twins Angelina and Angelica Sabuco joined at the chest.

The separation surgery will be done at Stanford University's Lucile Packard Children's Hospital. This type of surgery is made about six times in a year in the United States.

In preparation for their separation surgery, the conjoined twins had tissue expanders inserted in four places under their skin. Those expanders prompt growth of extra skin that will be used to repair their separation, the hospital said in a statement.

The twin girls were born in the Philippines in August 2009, joined at the chest and abdomen. Their livers, diaphragms, breast bones, chest and abdominal wall muscles are fused. They have separated hearts, brains, kidneys, stomachs and intestines.

Most conjoined twins do not survive pregnancy. Because they occur so rarely, it is difficult to determine an exact frequency. The occurrence of conjoined twins is estimated to range from 1 in 50,000 births to 1 in 100,000 births worldwide, and the overall survival rate is approximately 25 percent, according to Lucile Packard Children's Hospital.

Angelica and Angelina Sabuco at 3-months-old.

Angelica and Angelina Sabuco at their 2nd Birthday.