Sept. 20, 2011 --

One-year-old conjoined twins -- Rital and Ritag Gaboura from Sudan -- were recently surgically separated and are recovering steadily and appear to be in good condition. The twins, who were joined at the head, underwent four operations, the most recent of which happened last month. Despite the success story, the twins faced long odds when surgeons first tackled their case, given the complexity of their condition.

Conjoined twins are identical, or monozygotic, twins that fail to separate in the womb and are instead fused together. They can be fused virtually anywhere along the body: by the head, as is the case with the Gaboura sisters, by the chest, abdomen, torso or back. Conjoined twins needn't be symmetrical, as is the case with parasitic twins, in which the smaller individual relies on the larger one for survival. Most conjoined twins are thoraco-omphalopagus twins, where the individuals are fused along the chest to the stomach, maintaining two separate bodies. Instances of twins sharing a single body, both externally, and even internally, is called "fetus in fetu" -- in which case one body lives inside the other.

Conjoined twins represent one in every 200,000 live births. Only a fraction of these twins make it past their first day, which is why the condition appears more rare than it really is. One in 10 million live conjoined twin births survive through infancy. Females are more likely to survive than males by a ratio of three-to-one.

For conjoined twins that make it through infancy, surgical separation is sometimes an option, but it's a potentially risky proposition, depending on how the twins are fused. Craniopagus twins are connected by the head, but have two separate bodies and faces. This condition occurs in less than 5 percent of all conjoined twin births. The condition is also one of the more surgically complex, given the potential damage to the brain of each twin during the one or more operations necessary to separate them. In this photo, twins from Iran, Ladan and Laleh Bijani, pose for photos ahead of surgical separation. They had the same condition as the Gabouras from Sudan. Unfortunately, they didn't have the same outcome and both twins died following the operation.

Of the many ways conjoined twins can be fused together, one, interconnecting vital organs such as the heart or liver, makes separation without forfeiting the lives of one of the twins nearly impossible. Thoracopagus twins, for example, always share a heart and make up to 20 percent of conjoined twin cases. This raises ethical questions, since doctors and parents may have to choose which life to save should separation be necessary, as often one of these vital organs is not enough to keep both twins alive. These conjoined twins were born to a woman named Khalida at Nishtar hospital in Multan, Pakistan, in 2007. They share a heart and a liver.

When conjoined twins appear as two heads on a single body, as is the case with these conjoined twins born in Indonesia in 2006, they often share many vital organs, several of which may be doubled up within the same body. These conjoined individuals are known as dicephalic twins. When twins share a single body, separation is impossible.

Although quality of life is the reason most consider when deciding to surgically separate conjoined twins, in many cases the operations are necessary, since the twins' lives are at risk if they remain conjoined. A single heart isn't designed to pump blood to the muscles and organs of two full bodies. When separation is an option, however, parents are most often inclined to elect it, particularly in cases such as these conjoined twins born in Beijing in 2007, two-thirds of a set of triplets.

Although the Gabouras are expected to make a full recovery from their procedure, they will continue to need further surgeries as they age to improve their skulls' shape. Until then, for as long as they're here, Rital and Ritag are just two ordinary baby girls who have already overcome extraordinary odds together.