LONDON (AP) — Sudanese twins born with the tops of their heads joined together have been separated in a rare and risky series of operations at a London children’s hospital.

Facing the World, a charity which helps disfigured children, said it had helped fund the four-stage operation on 11-month-old Rital and Ritag Gaboura.

Twins born joined at the head are known as craniopagus twins and they occur in about one in 2.5 million births. Separating them can be dangerous, especially if — as in this case — there’s significant blood flow between their brains.

“It’s extremely high-risk,” said Dr. James Goodrich, who coordinated a similar separation of conjoined twins at New York’s Montefiore Children’s Hospital in 2004.

But the alternative can be just as bad. Because conjoined twins almost never pump the blood across their bodies evenly, the strongest sibling strains his or her heart trying to pick up the slack. Facing the World said that Ritag’s overworked heart was already failing by the time her family arrived in Britain.

The charity said that the separation took place in stages at London’s Great Ormond Street Hospital. Two operations took place in May. Tissue expanders — essentially balloons intended to help stretch the babies’ skin over their newly exposed heads — were inserted in July. The final separation took place on Aug. 15.

“Incidences of surviving twins with this condition is extremely rare,” lead surgeon David Dunaway said in a statement released by the charity. “The task presented innumerable challenges and we were all very aware of our responsibilities to the family and these two little girls.”

The charity released before and after photographs of the twins. The before photo showed the two sprawled out on a bed, with their heads joined just above the hairline to form what appeared to be a single, solid unit. The after photograph showed both side-by-side looking alert and healthy, clutching white stuffed animals.

Although rare, operations to separate twins linked by their heads aren’t unheard of. The U.S. National Library of Medicine records that one of the first successful operations to separate craniopagus twins took place in 1956.

In 2003, surgeons in Dallas separated 2-year-old Egyptian twins joined at the head, and a year later Goodrich was one of a team of doctors which separated Filipino twins in four major surgeries that took place over 10 months.

Even successful operations can leave neurological damage, although that didn’t immediately appear to be the case with Rital and Ritag.

“Within days the twins were back on the general ward interacting and playing as before,” the charity said. Its executive coordinator, Sarah Driver-Jowitt, predicted that the girls’ parents — who haven’t been named — may soon return home “with two healthy, separate girls.”

Goodrich agreed.

“So far the results have been exceptional,” he said. “I think both kids will do quite well.”