One pair of conjoined twins is born in approximately every 200,000 live births, but few survive. By some estimates, about half are stillborn, and only 5 percent to 25 percent live past their first day. Those survivors can present a complicated medical and ethical challenge: Which child should the doctors save?

The conjoined boys born in India last week have separate brains but share all other vital organs including their heart, liver and kidneys. Doctors are currently studying which brain controls what limbs before they operate to separate the boys. With only one heart — even with two brains — only one little boy will have a chance to survive.

"In medical literature we have only seen two such cases where one baby has survived when twins had a fused common heart," Dr. Paras Kothari, head of pediatric surgery at Sion Hospital in Mumbai, India, told Central European News.

Many factors go into the decision. Everything from where the twins are conjoined to their gender can influence survival rates, especially from a possible separation.

For twins joined at the upper chest, the most common type of the condition called thoracopagus, separation is almost impossible because the pair shares a heart. Those joined from the breastbone to the waist, a type called omphalopagus, can be separated because they may only share a liver, gastrointestinal tract, and reproductive organs. For those attached at the skull, called craniopagus twins, separation may be possible depending on the actual location of the fusion and whether they share any brain tissue.

Even in the best of cases, though, separation is extremely difficult and complex to pull off, with only 75 percent of surgeries ending up with at least one twin surviving, even today. According to the American Pediatric Surgical Association, only 250 separations worldwide have resulted in at least one twin surviving for an extended period of time. Strange as it might seem, one of the best predictors of survival is simply being a girl, with these twins surviving three times as often as their boy counterparts.

If the Sion Hospital surgical team found that one boy's brain controls all the limbs, they would separate him from his brother, giving him the vital organs. The doctors haven't said what they will do if neither boy's brain is dominant. Even in cases where the medical options are clear cut, the ethics and emotional weight of the decision are fraught for both doctors and parents.

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