Twins who survived one of the rarest separation surgeries in the world are now thriving toddlers "They're both super happy little girls."

A set of twin girls who captured the world's attention after surviving an 11-hour head separation surgery are now healthy 2-year-olds.

"We had no expectations," mom Heather Delaney of Mooresville, North Carolina, told "Good Morning America." "The fact they are doing as well as they are is amazing to us. It's really cool to watch them grow and change and turn into these little people."

Erin and Abby Delaney were both born 2 pounds, 1 ounce on July 24, 2016. When Heather Delaney was 11 weeks pregnant, doctors discovered the twins were joined at the head. Heather and Riley Delaney consulted with Children's Hospital of Philadelphia (CHOP) for separation surgery of their children.

Being conjoined by the head is the rarest form and occurs about six times in 10 million births, according to CHOP. And what's even more rare, the Delaney twins were totally fused, with their connection extending deep into brain tissue. Doctors were particularly concerned that they shared a superior sagittal sinus, which is the large vessel that carries blood from the brain to the heart.

In June 2017, the girls underwent surgery performed by a 30-person team.

Erin and Abby received physical, occupational and speech therapy over the course of several months in the hospital. Erin was discharged from the hospital after 435 days. She and her parents stayed at Philadelphia Ronald McDonald House until Abby was discharged one month later. The family returned home to North Carolina before Thanksgiving 2017.

"They're both super happy little girls," Delaney said. "The fact they are doing as well as they are is amazing to us."

Dr. Jesse Taylor, now chief of the division of plastic and reconstructive surgery at CHOP, had co-led Erin and Abby's surgeries with Dr. Gregory Heuer.

Taylor told "GMA" it was his first craniopagus twin surgery.

"No one had ever done a separation the way we were doing the separation and in many ways it was totally novel," Taylor said.

"They're really exceeding our expectations and doing quite well," he added of the twins. "They're technically a little bit delayed, about 6 months behind, but all-in-all they're doing incredibly."

Delaney said Erin is crawling and Abby rolls and sits up. Like many toddlers, both are showing off their spunky personalities and exploring the world around them.

"Erin, she likes to steal the remotes off the coffee table," Delaney said. "They're just starting to interact with each other so it's funny catching Abby make this face [telling Erin], 'Don't take my toy.' She's a sassy little girl, Abby."

Delaney said her family's goal is to spread hope to parents of children with medical challenges.

"Our girls are the example of impossible being possible," she added. "I call them our miracle babies."