The 28-year-old fashion designer, who already has two sons, Camden, two, and Jaxon, 14 months with her Chicago Bears quarterback husband Jay Cutler, 32, confirmed the news on her her app, Kristin Cavallari Official, reported People magazine.
The former "Hills" star posted a photo of herself with her arms over Cutler's shoulders and dangling from her hands is a pair of hot pink baby Converse sneakers.
"Our new addition! The boys are so excited to have a little sister on the way!!!"
Cavallari announced in May that she was pregnant with the couple's third child.