On Friday, a team of doctors began working to separate the Carlsen twin sisters, who were born five months ago joined at the abdomen and chest. The surgery at the Mayo Clinic in Rochester was expected to take 10 to 12 hours.
The Carlsens have separate hearts but share a pericardium, a membrane around the heart. The girls' livers are fused together and they share a common bile duct. They also are joined at the diaphragm and the pancreas, and they share part of an intestine.

Some final decisions likely will be made in the operating room. The intestine, for example, is a difficult area and doctors will have a complete picture of how it is shared once the surgery begins, Aase said.
A 70-member Mayo Clinic team has cared for the twins since Feb. 24. In early March, doctors implanted inflatable balloons under the girls' skin to stretch it. They periodically filled the balloons with saline so the skin would expand enough to close up the surgical wound.

A team of 30 people, including 18 surgeons from various specialties, were participating in the operation Friday.
One of the girls is expected to need more reconstructive surgery than the other and will be in the operating room about three hours longer. Doctors have not revealed which girl that will be.

Conjoined twins occur once in every 70,000 to 100,000 live births, according to the John Hopkins Children Center. They form when a single fertilized egg fails to divide completely to create two distinct individuals.
Since the mid-1990s, there have been about 250 separations in which one or both twins survived, according to the American Pediatric Surgical Association. The separation of twins conjoined at the abdomen, like the Carlsen girls, has had one of the highest statistical success rates.