Surgeons at the Mayo Clinic in Rochester, Minn., Friday began to separate Abbigail and Isabelle Carlsen, five-month-old cojoined twins from Fargo, N.D.

The daylong surgery was expected to involve three dozen doctors, nurses and other specialists, the Minneapolis Star Tribune said.

The twins, who have been hospitalized in Rochester since February awaiting surgery, are connected at the chest and abdomen. They share a liver and parts of their digestive systems. Surgeons have said the separation will be tricky because of the intricate connections.

The Mayo Clinic team has spent months studying the girls' anatomy in detail in preparation for the surgery. "It's fair to say that we know these girls inside and out," said Dr. Christopher Moir, the lead surgeon, at a meeting of the team Thursday.

Copyright 2006 by United Press International