Peruvian surgeons have performed an initial operation to separate the fused legs of a 9-month-old girl with a rare congenital defect known as sirenomelia (search), or "mermaid syndrome," her doctor said Thursday.

Dr. Luis Rubio said his team inserted three silicone bags on Tuesday into the tight coating of skin around Milagros Cerron's (search) legs.

He estimated the baby would be ready for a complicated surgery to separate her legs in 40 to 45 days — not February 24, as originally planned.

"The child went through nearly three and a half hours of general anesthesia to place these skin expanders," Rubio said, adding that the operation was a good indicator of how Milagros will withstand the more difficult surgery ahead.

"The little girl's vital signs were optimum throughout," he said.

The condition occurs in one out of every 70,000 births and there are only three known cases of children with the affliction alive in the world today.

The deformity is almost always fatal within days of delivery due to serious defects to vital organs. But Milagros — whose name means "miracles" in Spanish — has survived.

Although most of Milagros' organs, including her heart and lungs, are in perfect condition, she was born with serious internal defects, including a deformed left kidney and a very small right one located very low in her body.

Sirenomelia is usually fatal because of complications associated with abnormal kidney and bladder development and function.

Milagros' doctors have managed to stave off kidney and bladder infections, allowing her to continue to gain weight and grow, Rubio said.