"I'm nearing the end of my first trimester and I'm so excited – I've been bursting to tell people," says McKellar, 35, who played Winnie on The Wonder Years, tells PEOPLE exclusively.
However, she originally put off confirming her suspicions. "I was in the middle of a really intense deadline for my book," explains McKellar, whose third tome, Hot X: Algebra Exposed, comes out in August.
"I wanted to put off the [pregnancy] test because I didn't want any distractions. That only lasted a day – I couldn't stop thinking about it and I caved! You can't try to put that off."
Initially, even McKellar herself couldn't believe the news. "When you do take the home pregnancy test, it doesn't quite seem real," notes the mathematician and mom-to-be. "But when you see the baby and the heartbeat on the ultrasound, it's so incredible."
"Me and Mike looked at each other and we could not stop the tears," she says. "We are so happy."
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