Fergie and Duhamel both tweeted a photo of their newborn son on Saturday, reported Radar Online.
"Hello world!," they captioned the picture (a close-up of baby Axl - the first image they've shared showing the baby with his eyes wide open), adding hashtags "daddy and mommy made me do this" and "what's a hashtag?"
On Duhamel's version of the photo, he added, "Go Bison!" - a reference to the college football team from his home state of North Dakota, who will play Delaware on Saturday.