Born on Saturday, the little one is the first child for the couple, who have been married since 2002.
"The family is over the moon," said her spokesman.
Just before welcoming her little superheroine, Sarah was the guest of honour at a fun-packed baby shower thrown in her honour by Shannen Doherty and her former Buffy co-star Michelle Trachtenberg.
The 32-year-old stayed active well into her pregnancy. Just one day before giving birth the actress was seen scooting off to a pilates class dressed in sports gear and trainers.
Earlier this year, she also filmed and executive produced The Wonderful Maladys, a US series about a trio of dysfunctional adult siblings still struggling to come to terms with losing their parents when they were young.
Meanwhile Freddie – whom she met on the set of I Know What You Did Last Summer - can be seen in 24.