In New York Monday, doctors performed the first of at least three operations to separate 18-month-old twins, who are joined at the tops of their heads.

Carl and Clarence Aguirre underwent the first round of surgery at Montefiore Children's Hospital in the Bronx. The procedure entails inserting inflatable balloons under their scalps to gradually stretch their skin so there will be enough to cover their heads once they are separated.

Conjoined twins occur in roughly one of every 200,000 live births, but twins joined at the head are a rarity, occurring in one in 10 million live births.

Dr. Walter Molofsky is a pediatric neurologist at New York's Beth Israel Hospital. He says there are a number of serious challenges for operations of this type.

"The first problem is the skull, and they're connected," he said. "You have basically two bony structures that are attached to each other. A plan has to be put in place. So, as part of the procedure you have to separate the bones, and you're going to have to cover those bones with a skin flap. The next problem is arterial supply. The brain gets its blood from vessels that leave the heart and go up into the head. You have to make sure preliminarily, when they're separated, there's sufficient blood supply to each brain."

The Aguirre twins share a single major vein that drains blood from the brain to the heart.

Dr. Molofsky says the next challenge will be to ensure adequate blood flow out of the brain and back to the heart, and then evaluating the post-operative conditions of the twins' brains.

"What about the brain itself? Are they really two separate [brains] that are wrapped together, or are there any parts of the brain that are actually fused, such that when you separate the twins, one twin might have more of an intact brain than the other," he said. "Part of the problem is, during that process of separation, [something] may create a complication. That is, even though the brains could be relatively intact, if blood supply isn't maintained, or if the drainage isn't completely intact, you may end up having damage occur."

The conjoined twins arrived in New York City from the Philippines on September 10, and have been gaining weight and receiving physical therapy to prepare them for the highly risky procedure. The entire series of operations is expected to take from two to three months to complete.

Their surgery comes one week after two-year-old Egyptian boys were separated in a lengthy operation in Dallas, Texas.