Five-month-old conjoined twins Abygail and Madysen Fitterer, of Bismarck, ND, will undergo separation surgery at the Mayo Clinic on Wednesday, the clinic announced Tuesday. The girls are joined at the chest and share their front chest wall. In October, doctors implanted tissue expanders to stretch the girls' skin enough to close their incisions from the separation surgery. "Abygail and Madysen have been growing in strength and size and character for weeks now," according to a Mayo video news release. The surgical team will be led by Dr. Christopher Moir, who also led the teams that separated conjoined twin girls from Fargo, N.D., in May and twin boys from Mankato in November. "Dr. Moir and the team believe there's a high likelihood of success - that's why they're going forward with it - but in any operation there's certainly risk," Mayo spokesman Lee Aase said. The girls are the daughters of Stacy and Suzy Fitterer, who have not been granting interviews but said in the video distributed by Mayo that they were grateful for the moral support they've received from all over. The girls' preparation for surgery has involved daily therapy and playing with their 2-year-old brother, Nicholas. "They've got to work on range of motion and all that, so that when they are separated they'll be able to develop quicker and get back up to speed with kids their own age," said their father, Stacy Fitterer. "And I think they're overachievers, so they're going to do it." The video showed the girls sitting in a special carrier that holds them upright, and a special chair designed by Mayo engineers to allow them to sit up and get their feet on the ground so they can develop their leg muscles. "They've never been able to lay on their backs but Abby, if you watch her pivot her hips, I don't think it'll be too long before she'll be rolling," Suzy Fitterer said. Abygail and Madysen will be the second set of conjoined twins from North Dakota to undergo separation surgery at Mayo's Eugenio Litta Children's Hospital. Abbigail and Isabelle Carlsen, of Fargo, have been doing well since they were separated in May. The Carlsen twins, now 13 months old, were born attached at the diaphragm, pancreas and liver, and shared a common bile duct and part of an intestine. Aase said the main issue in the Fitterer twins' surgery will be separating and reconstructing their chest walls - though their livers are joined and they share some intestine - whereas the biggest challenge for the surgeons with the Carlsens was their common bile duct. "It's complicated in a different way," Aase said. The surgical team planned to hold a news conference Wednesday evening following the Fitterer twins' operation, which Aase said will be Mayo's fifth separation of conjoined twins. Besides the two sets last year, Mayo performed two other separations around 10 years ago, he said. According to Mayo experts, conjoined twins may develop in as many as one in 50,000 pregnancies, but they account for only one in about 250,000 live births. Stacy Fitterer said he and his wife wanted to thank everyone back home for all their prayers and good wishes. "Believe me, we know they're sending it," he said. "We know we're getting it," Suzy Fitterer chimed in.