The 27 Dresses star and husband Josh Kelley were informed that the little one would need surgery to correct a congenital heart defect before leaving her native Korea in 2009.
The couple were apparently urged to reconsider the implications of adopting Naleigh, but Heigl said that they were not in any doubt about their devotion to the child.
"[Adoption workers] wanted us to take the time to [ask] our own paediatrician and our doctors about it and, you know, then decide," she recalled to Scholastic Parent & Child. "And we just said, 'Look... We're not going to say no. Whatever we're walking into, we're walking into'.
"The beauty is that Josh's father is a heart surgeon, and we were able to call him immediately and tell him about the condition... it wasn't about whether or not we were going to embark on this journey. We were definitely doing it. But we wanted to know what we were dealing with."
Heigl and Kelley were united with Naleigh four months after her procedure, by which point she had "a perfectly healthy heart".
Reflecting on the moment she discovered that she was about to become a parent, Heigl added: "It was a really crazy time. You're so amped up, you're so excited, you're so impatient - you know there's a child out there waiting for you, and you just want to get your hands on her."
The couple have said that they would like to adopt again in the near future.
> Katherine Heigl: 'I was failing as a mother and an actress'