It took surgeons less than three hours to separate conjoined twins Abygail and Madysen Fitterer at the Mayo Clinic on Wednesday.

The 5-month-old girls from Bismarck, N.D., spent their first night apart recovering in intensive care following what their lead surgeon called "a flawless operation."We're happy to report that they're no longer conjoined," Dr. Christopher Moir said at a news conference following the surgery.

There were some "tense moments," Moir said, especially involving the girls' hearts, which overlapped into each other's chests. But "everything went exactly as we had hoped and exactly as we had planned."

The girls were born Aug. 8 in Minneapolis. They had been at Mayo's Eugenio Litta Children's Hospital for 99 days going through preparations for surgery, said Moir, who led a surgical team of some 40 people.

The girls, who shared a chest wall and some internal organs, were the third set of conjoined twins separated at the Mayo Clinic in less than a year.

Afterward, Stacy Fitterer said his daughters looked beautiful. His wife, Suzy, holding Nicholas, 2, added: "I don't know how to thank people for giving not one but two children that chance at a normal life. I don't know how we'll ever thank you."

Surgeons began the operation at 9:50 a.m., and were able to transfer the girls to separate operating-room tables for the first time by 12:30 p.m. Two teams of reconstructive surgeons spent another 3 ½ hours rebuilding the girls' chest walls, using synthetic patches of plastic to cover part of their hearts.

Stacy Fitterer said the family waited anxiously for updates throughout the day. "Everything was positive," he said. "We just gave a little cheer after each one."

Moir said that additional surgery may be necessary as the girls grow up. Moir, a pediatric surgeon, also led the team that separated Abbigail and Isabelle Carlsen of Fargo, N.D., last May, and who performed emergency surgery in November when Valerie James of Mankato gave birth to conjoined twin sons.

When asked how the surgeries compared in complexity, Moir said: "These are phenomenal surgeries." Each one, he said, "is off the scale."

The Fitterer twins are expected to remain in intensive care for a few weeks, Moir said. He said he hoped they would go home soon, after they've had a chance to recover and gain weight.

Stacy Fitterer said he appreciated the outpouring of support from both friends and strangers. "I look at our two girls and all the other children that we've seen in the hospital," he said. "Because ours are conjoined, everyone makes that such a special thing because that's so rare. But I've seen so many other children that need the prayers and well wishes just as much as our girls did."

He said the families at the children's hospital formed a support system all their own. "We always finish off by saying 'I hope we never see you again, except maybe at the Mall of America.' "

Maura Lerner • 612-673-7384 • mlerner@startribune.com