Milagros Cerron, whose name means "miracles" in Spanish, is already lucky to be alive, as children born with sirenomelia, or "mermaid syndrome" usually die within days.
Later this month, a team of doctors will perform the operation at a charity hospital for the poor in the Peruvian capital, Lima.
From the waist up she is a happy, smiling, normal child, but Milagros' legs are fused and her splayed feet resemble the mythical creature's tail.
She was born to a poor family in the mountain city of Huancayo, 125 miles east of Lima. Her father, 24-year-old Ricardo Cerron, admitted he was initially "in total despair" at her appearance, but has since adjusted and is now nervous about the outcome of his daughter's operation.
"I keep thinking about what's going to happen and how the operation's going to be," he said.
Almost none of the children born with this condition survive more than a few days because of defects to their internal organs. The only person who is known to have survived in the long term is a 16-year-old American, Tiffany Yorks, whose legs were separated before she was one year old.
Milagros' abdomen merges into her legs, which are connected by skin down to the feet, which are splayed in a V-shape. She has normal bone structure and independent movement within the two joined legs.
Although most of her internal organs are fine she has just one kidney. None of her major organs will be part of the operation, surgeons said.
The head doctor at the hospital, Luis Rubio, said the operation was expected to take five hours and would be performed by a team that includes trauma surgeons, plastic surgeons, cardiovascular surgeons, neurologists, gynaecologists and a paediatricians.
Her mother, Sara Arauco, 19, said: "My dream is that everything is going to turn out well."
If the operation is a success, the infant will need further surgery in the future to rotate her feet forward and reconstruct her genitalia, according to her doctors.
"We want to dream that she could one day run or ride a bike," said Dr Rubio. "But if we could just give her the ability to be independent, that's enough."