Nine hours into surgery, doctors in Utah were cautiously optimistic they would be able to successfully separate four-year-old conjoined twin girls attached from the waist down. Earlier in the day, it was a tearful parting for the Herrin family as the four year-old twins Kendra and Maliyah were wheeled into surgery at Primary Children's Medical Centre in Salt Lake City. There, a team of 30 specialists began performing the gruelling operation that could take as long as 24 hours. Fused at the torso, the girls share a kidney, liver, pelvis, part of the large intestine and a pair of legs. Surgeons plan to separate the liver and intestines and reconstruct their pelvis. Each girl will only be left with one leg. Kendra will get the kidney, while Maliyah will be put on dialysis until she can receive a transplant. "Last night they knew it was tomorrow. We asked them how they felt and they said they felt excited," said Jake Herrin. Doctors advised the family to wait for surgery because the girls' shared a kidney. Now, despite medical concerns, the family decided it was the right time. The girls have been preparing for the surgery since June, when doctors implanted expandable balloons into their mid-section to stretch their skin and muscles. The surgery is the first known separation surgery attempted on twins with a shared kidney. Conjoined twins occur when an embryo fails to completely separate within two weeks of fertilisation. The condition is rare, one in every 50,000 to 100,00 live births, and is most common in female twins. Most conjoined twins survive only a few days.