Drew Barrymore and Will Kopelman knew they were going to name their firstborn Olive six months before before her Sept. 26 arrival.

"I never would have guessed that would of been the name. But I was reading a book with my husband, and I was three months pregnant. They said your baby is the size of an Olive and that was it," the new mom said on The Ellen DeGeneres Show (airing Thursday, Dec. 6). "We never looked back."

Host Ellen DeGenres joked that "it could have been the size of a peanut or a grapefruit" or even "a kidney bean."

"It could have been. It was a lot of fruits and vegetables," Barrymore, 37, laughed. "Kidney's not such a fun name."

The Big Miracle actress added, "We were work shopping a few [names], as all people do who are so blessed to get to do this do. And then once the light was shed, there was just no turning back."

Barrymore says she's fortunate that her 10-week-old daughter is "a good baby. I count my lucky stars every day because she is . . . really interactive and happy and like nice to people. She's a nice person."

"She is selling it," the star said of her daughter's cuteness. "She is working it! It's so great."

An exuberant Barrymore -- who wed art dealer Kopelman, 34, at her home in Monteceito, Calif., in June 2012 -- repeatedly praised her child's calm demeanor. "I have met babies who are screaming and they're crying and they're trying to convey all these things and you get so terrified and overwhelmed. I'm very grateful to her," she said. "She really has it together! She's really impressing me."

Married life, Barrymore added, is everything she ever imagined. "I'm so lucky. It's amazing!" she smiled. "I never take it for granted."

This article originally appeared on Usmagazine.com: Drew Barrymore: Why I Named My Daughter Olive