The 30-year-old star is due on October 19, reported Us weekly.
"I have never been more excited. The reality is sinking in that we are going to have a child! I've read every pregnancy book, and now I realise I don't know anything," she said.
While Pratt added, "the click-click-click of the roller coaster going up. That's how I feel right now."
The two tied the knot in April 2009.