Just 18 weeks into her pregnancy, Karen Lefave was given bittersweet news.

The Sault Ste. Marie, Ont., mother was pregnant with identical twin girls, but one of the babies appeared to have a serious problem with her heart. A few months later, doctors determined that the unborn infant had a rare congenital heart defect.

For the child to have a shot at survival, she’d require two rounds of open-heart surgery shortly after birth.

To make matters worse, the twins shared the same placenta and the same amniotic sac, which put them at risk of deadly umbilical cord entanglements at any time.

Doctors told Karen and her husband, Greg Lefave, that the odds were about 50/50 that the girls would survive.

“That was in and of itself hard to deal with, as you can imagine,” Karen told CTV’s Canada AM on Friday morning.

About three months after the heart defect was discovered, Georgia and Charlize were born premature at 31 weeks in May 2014. That’s when Charlize was whisked away to the Hospital for Sick Children in Toronto while her sister went to Mount Sinai Hospital’s neonatal intensive care unit.

The early birth significantly worsened Charlize’s shot at survival, as doctors were forced to wait two months for her to grow to 5 ½ lbs., just large enough to handle the life-saving surgery.

Charlize’s health was monitored by Dr. Fraser Golding, who said there were “frequent ups and downs” and that plans for the infant’s care were prone to abrupt change.

“I am not certain if I have had to say the words 'cautiously optimistic' to a single family so many times before,” Dr. Golding said in a 2014 press release.

But the complex surgery went well, and five months after Charlize was born, she went home for the first time in October, 2014. In early 2015, the little girl was successfully operated on again.

The family credits the happy ending with the fact that they had plenty of medical information from the outset.

“We were really fortunate to have the type of twins that we had that resulted in a lot of follow-up, we had a lot of time to prepare for it,” Greg said.

Charlize’s father added that doctors are amazed by the little girl’s progress.

“I think that’s the coolest part of the journey for me so far is just seeing the shock and awe every time they see her. They see her growing, they see her walking and they’re just in shock,” he said.

The family credits SickKids with saving their daughter’s life.

“We wouldn’t have our family -- Georgia wouldn’t have her sister -- if it wasn’t for SickKids. They gave us everything,” said Karen, whose daughter Charlize is now a brand ambassador for the hospital.

The Lefave family took part in the SickKids Radiothon on Thursday, which helped raise money for the Toronto children’s hospital.

With files from CTV’s Canada AM