"I'm nearing the end of my first trimester and I'm so excited — I've been bursting to tell people," McKellar told the magazine.
The 35-year-old former child star said she tried to put off the pregnancy test because she didn't want any distractions while she was in the middle of "a really intense deadline" for her new book, Hot X: Algebra Exposed, which hits shelves in August.
That only lasted a day — I couldn't stop thinking about it and I caved! You can't try to put that off," she said.
The baby, due in the fall, is the first child for McKellar and Mike Verta, her husband of one year.