What life is like now for formerly conjoined twin sisters after separation surgery Eva and Erika Sandoval have spent most of their young lives in hospitals.

 -- An encore presentation of this report will air on "Nightline" on Thursday, Nov. 24 at 12:35 a.m. ET

Like many toddlers, Eva and Erika Sandoval spent their third birthday party playing with friends and family, laughing and eating cake -- a Princess Sofia cake from the Disney TV series, “Sophia the First,” for Eva and a Woody from “Toy Story” cake for Erika.

The twin girls each had their own party outfits -- Eva was dressed as Princess Sophia, Erika was dressed as Woody.

But for their parents, Aida and Art Sandoval, this was more than just their daughters’ birthday. It was a miracle.

This was the first birthday the twins, who were born conjoined and who had spent most of their young lives in a hospital, had celebrated as separate individuals. Doctors weren’t certain they would make it this far.

When then-44-year-old Aida Sandoval found out she was pregnant three years ago with twins, she and her husband were surprised but “so excited.” But as they started to plan to add two more to their family of five, the couple received devastating news. Her doctor referred her to a specialist who told her that the twins were conjoined and may not survive.

“[The doctor] asked me to call Art,” Aida said. “And I couldn’t. Like, how do I tell him? … So the doctor had to call him.”

Conjoined twins are a rare phenomenon, and their chances of survival are even rarer. About half are stillborn and only 35 percent survive beyond their first day, according to the University of Maryland Medical Center.

The Sandovals, who live outside of Sacramento, reached out to Dr. Gary Hartman at Lucile Packard Stanford Children’s Hospital in Palo Alto, California, an expert in the world of conjoined twins and who had six successful separation surgeries under his belt. When Dr. Hartman reviewed the Sandovals’ case, he said he was honest with the couple about the twins’ chances of survival.

“What we told them was [what] we thought [which was] we didn’t know that they could be separated,” he said. “We said, ‘You would need to assume … that they would never walk’ … We weren’t real optimistic about quality of life.”

Doctors gave the Sandovals the option of terminating the pregnancy.

"We talked about it, we talked about it," Art said. "It was like - let’s give them a chance....You know? It’s- if it was meant to be, it's meant to be."

At 33 weeks, Aida gave birth to Erika Rose and Eva Victoria. The girls were joined from the sternum all the way down to the pelvis and they shared a third leg.

The first time seeing them was “very emotional,” Aida Sandoval said.

“They have tubes, they had the little covers over their eyes,” she said. “You can’t carry them, they’re very fragile … you feel helpless because you question yourself, ‘Are we doing the right thing?’ But then you talk to them, you say, ‘You are strong, and you’re going to get through this.’”

The twins spent the first few months of their lives in the neonatal intensive care unit. They weren’t deemed strong enough to go home until they were 7 months old.

When the Sandovals were allowed to finally take them home, Aida said she dressed them in little gowns. Over time, they reached milestones together -- their first words, learning to stand and developing their own personalities, with Eva as the talkative one and Erika as the observer.

But by the time they reached 2 years old, Eva had grown stronger than her sister, and then their health started to decline, so the decision was made to attempt to separate them.

In trying to explain the surgery to the girls, Aida said, “I would always role play, and [say], ‘Some magic is going to happen, and Dr. Hartman is going to be your magician.’”

The surgery was very risky. The American Pediatric Surgical Association said that at that point, only 250 separation surgeries have been successfully performed in the world, and doctors told the Sandovals that there was a 30 percent chance one of the twins would die.

Art Sandoval said they weighed the odds, but in the end they knew their girls were fighters, and he said they believed, “They will pull through this.”

On the morning of the surgery, Dec. 7, 2016, the medical team, led by Hartman, started with a prayer asking for unity, strength and guidance. Hartman’s plan was to separate the organs in the girls’ chests first and then move down the abdomen and finish with the pelvis.

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After five-and-a-half hours, the girls were separated successfully, but the surgery was far from over. The medical team realized they didn’t have enough skin to close the girls’ incisions so they had to turn to their once shared third leg.

“They had told us earlier that they may have been able to use that third leg and keep it and give it to Erika, but when it came down to it, there wasn’t enough tissue to cover up Erika. So they had to use the tissue of the leg,” Art said.

“That was really hard,” Aida added. “It’s just like a punch in the gut.”

After 13 hours of surgery, Eva was wheeled into recovery. Her sister Erika followed two hours later. Their parents were on pins and needles until they saw the girls.

“I was excited just to know that they were alive still,” Aida said. “Just to know -- see them breathing.”

In the weeks that followed, Erika, the once smaller twin, thrived, making tremendous progress in physical therapy, but recovery was a bigger challenge for Eva.

Through all of this, Aida was mostly parenting solo as Art, who had to keep working full-time to cover the medical bills, drove the three hours from their home in Sacramento to the hospital in Palo Alto every weekend to visit her and the twins.

Finally, three months after their separation surgery, the twins were allowed to go home. Under the care of their local hospital, Eva and Erika continued to go to physical therapy and were fitted for wheelchairs. At home, the family settled into a new routine, with Aida being able to walk outside with a stroller built for two for the first time.

The best part of this whole journey, Aida said, was “finally bringing them home and being a family.”

“We’ll still say, ‘Dr. Hartman and his team performed magic,’” she continued. “Eva will see her scar and she goes, ‘My sister was right here.’ And I said, ‘Yes, your sister was right there.’”

The girls made amazing progress, surpassing all expectations, but they still have more challenges ahead. They have been on feeding tubes since birth and are just now getting used to eating solid foods.

But they are embracing their independence as typical toddlers, full of curiosity and mischief.

“The worst part is over,” Aida said. “We’re here. We’re done … when you think about it or when you talk about it, those emotions do come back. But right now, it’s just looking forward, looking to the future … and they are vivacious, they are just -- they’re spunky little girls.”