Thirteen-month-old Milagros Cerron was in prime condition for the surgery, which was to last four to six hours, said Dr. Luis Rubio, leader of the team of 11 surgeons who were to perform the operation.

"Today is the big day awaited by the entire world, by her parents, by her and by us. We have prepared an entire hospital for her," Rubio said in an interview with The Associated Press, adding that he was "tremendously optimistic" the surgery would be successful.

Rare Congenital Defect

Milagros, whose name means "miracles" in Spanish, was born with a rare congenital defect known as sirenomelia, or "mermaid syndrome," which occurs in one out of every 70,000 births.

There are only three known cases of children with the affliction alive in the world today, according to Rubio.

Tuesday night's operation was the first of three complicated surgeries to separate her legs, which are seamlessly fused all the way to her heels. The 11 doctors who will perform the operation include plastic surgeons, pediatricians and heart specialists.

Rubio said the medical team was operating at night because the doctors wanted to perform surgery when the Solidarity Hospital, a public facility that serves 1,500 people daily, would be at its quietest.

The objective is to separate the child's legs from the heels up to the knees. The medical team will examine the knee ligaments to prepare for the next operation, which Rubio said would take place in several months.

Intellectual Development Has Been Remarkable

Probably the greatest challenge Tuesday was how to divide a major artery that crosses from one leg to the other, he said. Preliminary studies indicated it would not be necessary to perform a bypass of the artery, but the team was ready to do so if required, he added.

Milagros weighs 14.75 pounds and measures 25 inches long, about the size and weight of a normal 6-month-old.

Rubio said Milagros had suffered frequent urinary infections because her urinary tract, anus and genitals end in the same opening, almost like a "sewer."

But her intellectual development has been remarkable, he said, as Milagros smiled, trying to grab a laser light Rubio used to show how her legs were connected by the same sheath of skin.

"She has extraordinary psycho-mental development. She communicates well with her surroundings. She babbles words that correspond to her age," he said, picking up Milagros and holding her over his head as she giggled.

'A Little Pained'

In the last three months, doctors have inserted silicone bags filled with saline solution to stretch the skin so it will be able to cover her legs once they are cut apart.

Milagros has a deformed left kidney and a very small right one located very low in her body.

Rubio said Milagros would need up to 15 years of corrective surgeries to reconstruct and repair her sexual, digestive and other internal organs.

"I'm a bit excited and also a little pained because of what my daughter is going through," Milagros' father, Ricardo Cerron, told The Associated Press on Tuesday, only hours before the operation. "But I have faith in the doctors. I have faith in God. I am putting everything in God's hands."