Rhett's wife announced the gender of their baby on Instagram by posting a series of photos and videos.
Lauren, 27, shared a picture of the cake, complete with a gold crescent moon, gold stars, blue and pink frosting, as well as the first lyrics of the lullaby.
"...How we wonder what you are... Guess what we're doing tonight ?? @crumbdelacrumb," she wrote.
Hours later, the couple posted videos, which featured the "Crash and Burn" hitmaker cutting the cake to discover the color pink.
"Sorry our family is nuts BUT WE'RE HAVING A BABY GIRL," Lauren wrote.