Twin Girls Joined at the Chest to Be Separated in 18-Hour Surgery Today

Twin 2-year-old girls were born conjoined at chest, abdomen.

— -- A pair of twin girls conjoined at the chest and abdomen will undergo a lengthy surgery to finally be separated.

In an effort to allow the 2-year-old girls to live independently of one another surgeons and other physicians are performing surgery to be separate the toddlers today. The medical staff who will work on the surgery at Lucile Packard Children's Hospital Stanford, which is part of Stanford University, anticipate that there is a 70 percent chance that both girls survive the arduous operation.

"It's hard to see the numbers and find comfort on the odds. But as you know from the beginning our girls have superseded the doctors expectations of life and will continue to show us their strength," parents Aida and Arturo wrote online earlier this year.

The procedures are expected to take around 18 hours with 50 medical staff attending to the girls, according to Lucile Packard Children's Hospital Stanford.

"This surgery is complex in terms of enabling a good quality of life for the girls after the separation," lead surgeon Dr. Gary Hartman, Division Chief of Pediatric Surgery at Lucile Packard Children's Hospital, said in a statement last week.

The operation is scheduled to start today, but hospital officials declined to give an update on the girls at this time due to the "complex and sensitive nature" of the operation.