The British pop rocker, who is also a member of supergroup MCBusted, announced the pregnancy news in a YouTube.com video which shows the couple's 17-month-old son Buzz preparing to play a video game with his parents.
They select Buzz as 'Player One' and when the option for 'Player Two' is highlighted, the video cuts back to Giovanna's belly and a sonogram picture resting on it. The footage ends with the words, "Player Two... Loading".
Following the video's release, Fletcher wrote on Twitter.com, "Thanks for all the lovely messages about our new Fletcher on the way! Can't tell you how excited I am!"
Fletcher, 30, married Giovanna in 2012.
His McFly bandmate Harry Judd is also expecting his first child.