The 33-year-old actress and husband Josh Kelley adopted Adalaide earlier this year as a sister for their three-year-old girl Naleigh, whom they adopted in 2009.
Naleigh was nine months old when she was welcomed into the family, while Adalaide was a newborn, and Heigl said that it was a completely different aspect of motherhood, reported Daily Star online.
"I had never had a newborn before. I didn't really have that experience. So it's all been new. I was prepared for the nights and having to get up and all of that. But the beauty of it is it's really only a few months of your life that you're walking around like a zombie because you haven't slept," Heigl said.
"I think it's far more chaotic than I anticipated. And I think every parent probably says that and every parent tries to warn new parents about it. And we all go, 'Pish posh, it's going to be fine. What's one more?" she added.