Doctors at the London’s Great Ormond Street Children’s Hospital have successfully separated twin girls born conjoined at the head.

 Rital and Ritag Gaboura, 11 months old, who were born in Khartoum, Sudan, underwent four complex operations.

The twin sisters were brought to Britain for the procedures by their parents Abdelmajeed Gaboura, 31, and, Enas, 27, who are both doctors.

The girls suffered from the rare condition of craniopagus and the surgery was carried out by a team led by Dr. David Dunaway. In the condition of craniopagus the skulls are fused, but the bodies are separate. These twins can be conjoined at the back of the head, the front of the head, or the side of the head, but not on the face or the base of the skull.

Surgery to separate conjoined twins may range from relatively simple to extremely complex, depending on the point of attachment and the internal organs that are shared. Most cases of separation are extremely risky and life-threatening. In many cases, the surgery results in the death of one or both of the twins, particularly if they are joined at the head.

Last week, 7-month-old twins, connected by their pelvis and lower spine, were separated by doctors at, Le Bonheur Children's Hospital, Memphis, Tenn.

Conjoined twins is a rare phenomenon, the occurrence is estimated to range from 1 in 50,000 births to 1 in 100,000 births.

Two contradicting theories exist to explain the origins of conjoined twins. The older theory is fission, in which the fertilized egg splits partially.

The second and more generally accepted theory is fusion, in which a fertilized egg completely separates, but stem cells (which search for similar cells) find like stem cells on the other twin and fuse the twins together. Conjoined twins share a single common chorion, placenta and amniotic sac, as do some ldentical twins.