The 31-year-old 'American Idol' alum announced the news last evening with an adorable picture on Instagram, reported Us magazine.
"In honour of 'Labour' Day, Ace and Penny would like to make an announcement. Their parents couldn't be happier...," the country crooner captioned a photo of herself holding her two dogs, who wear "I'm going to be a big sister" and "big brother" outfits.
Underwood also took to Twitter and wrote, "We haven't picked names yet but it's looking like Fly is gonna fly. #boyorgirl #flyfisher."