ROCHESTER, Minn. (AP) - The Mayo Clinic said Wednesday it's preparing to separate another pair of conjoined twins from North Dakota.

Eight-week-old Abygail and Madysen Fitterer, of Bismarck, N.D., on Tuesday underwent the first procedure leading up to their surgery. The daughters of Stacy and Suzy Fitterer had tissue expanders placed in them during a four-hour procedure.

After two weeks, the expanders will slowly be filled with saline solution to give each girl enough skin after separation to close their incisions.

Mayo teams separated Abbigail and Isabelle Carlsen, of Fargo, N.D., on May 12. Their recovery was hailed as a success that resulted from a "team of teams," a concept coined by the lead surgeon, Dr. Christopher Moir. Thirty staff members participated in the Carlsens' daylong surgery, including 18 surgeons.

Moir will also lead the Fitterer girls' surgery, which will be his fifth separation of conjoined twins.

As the Fitterers waited for the birth of their daughters Aug. 8 in Minneapolis, a Mayo news release said, they drew encouragement from the extensive news coverage about the successful separation of the Carlsen twins.

"We're proud of our little girls," Suzy Fitterer said in the statement. "They're strong, and they've made it through each step of this process so far with flying colors."

The mother said they decided to share their story because they want family, friends and others to be updated about their girls' conditions. "And if we can give the same hope and encouragement to others, we want to do that," she said.

Moir said in the same statement that Tuesday's tissue-expansion surgery was "a good dry run" for the separation.

"One of our concerns was how well the girls would tolerate the anesthesia and breathing tubes, especially when they are so young and have the complexity of being conjoined and having shared blood circulation," he said. "Abygail and Madysen came through yesterday's operation and bounced back really well. That was an important step."

The Fitterers said they will not do media interviews at this time.

"We've got one goal for the next several weeks," Stacy Fitterer said, "and that's seeing our girls continue to grow and get stronger so they will be prepared for the day of separation. We want to focus on that, and on our son, Nicholas."

The family, including Nicholas, 2, will stay in Rochester until the separation surgery, which Mayo said "will likely take place before the end of the year."

They expressed gratitude for all the good wishes they've received.

"The outpouring of support and prayers has been overwhelming," Stacy Fitterer said. "This has been a real source of strength for us."