The Buffy the Vampire Slayer star and her husband Freddie Prinze Jr. welcomed baby Charlotte Grace in September (09).
But the actress admits her daughter almost had to be delivered by her trainer when she went into labour a week earlier than expected.
She tells People magazine, "I went into labour at the gym! I didn't really believe it. I just felt a little nauseous, and my trainer said to me, 'Um, isn't your baby coming next week? Maybe it's time now?' I said, 'Oh no, I doubt it...'
"...I went home, and I said to Freddie, 'This can't be labour.' He kept saying, 'Sarah, are you sure?'"
The actress put her stomach pains down to a dodgy dinner she'd had the night before, until the aches intensified.
But she still managed to keep her calm throughout her labour and even in the delivery room.
Prinze Jr. says, "Later that night, she was like, 'OK, I think I might be in labour, but you should get some more sleep.' She was very calm, very centred, very prepared. I'm telling you, this girl is like Gandhi. She doesn't let anything rattle her...
And the actor is surprised at how smoothly the birth went: "It was like, three pushes and the baby was out."
Little Charlotte Grace is the couple's first child together. They married in 2002.