Most conjoined twins are separated before their first birthday but the Herrins have been forced to wait for their operation because of the risks associated with separating their shared kidney in addition to their shared liver, pelvis, legs and part of the large intestine.

Some doctors counselled against separation because of the length of time they had been joined and because their conjunction is not life-threatening. But the Utah twins have told their parents and psychologists that they want to lead separate lives.

Yesterday morning the fair-haired, blue-eyed twins were wheeled into the operating theatre in Primary Children’s Medical Centre, Salt Lake City, to begin the marathon surgery, which is expected to take between 14 and 30 hours.

Each girl will keep one leg, while Kendra retains the shared kidney. Maliyah will be put on dialysis until she is strong enough to receive a transplant kidney donated by her mother, Erin. Surgeons will divide the girls’ liver, giving them each a piece that will grow into an entire organ, separate their intestines and construct a pelvis each from the divided bone structure.

Preparations for surgery began in June, when doctors implanted expanding balloons into the girls’ torso to stretch the skin and muscles that will be used to close their abdomens after surgery. The process of filling the balloons with saline solution each week has been more painful than expected. During the treatment, the girls have slept on a 3,000lb oval hospital bed filled with sand to reduce the pressure on their tender skin. To help them to understand what is happening to them, Kendra and Maliyah have been given a set of conjoined stuffed dolls to play with and mimic their experiences. When they were given injections to fill their abdominal balloons, the dolls had them too.

Two weeks ago Kendra performed separation surgery on the dolls as her sister watched. “They gave the babies medicine and said they were so brave,” their mother wrote on the family’s website. “It is incredible how much they understand.”

Psychologists have worked with the girls to ensure that they are ready for such a dramatic change. Several experts have disputed the wisdom of surgery. One in every 50,000 to 100,000 births results in conjoined twins. Only about 20 per cent survive to become candidates for separation, and most surgery occurs when the twins are six to twelve months old.

“The reason for that is partly psychological, partly mechanical,” Rebecka Meyers, the hospital’s chief of pediatric surgery said. “If Maliyah had had a kidney, these girls would have been separated long ago.”

In their last web posting before the surgery, the Herrins said that they were shedding many tears. “Not because we are scared, but because we are mourning their last day conjoined.”

Explore World News

• Iraq News

• US & Americas News

• Europe News

• Middle East News

• Asia News

• Africa News

• Ireland News

Times Recommends

• Congo's rangers risk their lives to guard gorillas
• Pearl farming in China losing its lustre
• Axl Rose mystery as album arrives at last