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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.

Conjoined twins are exceedingly rare and occur between every 1 in 30,000 to 1 in 200,000 live births, according to the hospital. To take on the difficult surgery to separate Erika and Eva, the medical team created a 3D model of the girls' shared abdomen. As the surgery progresses, their MRI, CT scans and the 3D model will be used to help guide the surgeons.

\"You can think of their anatomy as two people above the rib cage, merging almost into one below the bellybutton,\" Dr. Peter Lorenz, a professor of plastic and reconstructive surgery at Stanford University Medical Center who will lead the reconstructive phase of the twins’ procedure, said in a statement.

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.

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