The photo shows the Buffy star holding baby Charlotte in her arms.
"'Sometimes when I need a miracle, I look into my daughters eyes, and realize I've already created one,'" she wrote, referencing a popular parenting quote. She added the hashtags, "#happybirthdayCharlotte #thisis7."
Whatever the celebrations were for Charlotte and her little brother Rocky, who also has a September birthday, it looks like they included loads of balloons. Gellar, 39, posted a picture of her balloon-packed car on Sunday.