The couple, who married in September last year, shared a joint Instagram post announcing the happy news.
”Baby Duhamel coming soon,” the pair captioned a photo that showed a sonogram image.
The news was shared a day after Audra shared a sweet post on Instagram for their one-year wedding anniversary.
“Happy first anniversary sweet man. Oh how I prayed for you,” she captioned a post.
Duhamel also shares son Axl, 10, with ex-wife Fergie, 48.
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Fergie and Duhamel announced their split in 2017 but finalised their divorce two years later.
Duhamel and Mari tied the knot last year in Fargo, North Dakota, their hometown.
Instead of getting a good night’s rest before his wedding, the actor found himself in the emergency room after busting too hard of a move.
His bride told Vogue Australia her husband-to-be was “pulling some wild dance moves on the party bus,” when his back suddenly gave out.
The actor was rushed to the hospital where it was discovered that he had a slipped disc – but this didn’t worry the soon-to-be newlyweds in the least, with their reaction being, “Cue the cortisone shot, and let’s get married!”
The bride wore a dress from Australian designer Leah Da Gloria, a long-sleeved, full-skirted number called “Linden”.
He proposed on the beach at Little Palm Island after writing a message in a bottle.
“It said, ‘Will you marry me, Audra Diane Mari?’ I cried, he cried—I’m not sure I even answered his question, I was in such shock!” Mari recalled.
“He stared at me a little bit longer and said, ‘Well…?’ And I looked at him and said, ‘Well, duh!’. It was the easiest ‘yes’ there ever was.”