"We are growing!!! Mostly me …" the actress wrote alongside a boomerang of Koma rubbing her belly.
Koma, 33, posted the same clip, making a little joke about what he and his wife have been up to during time at home amid the coronavirus pandemic.
"Lol quarantine was fun," he captioned the post. "Baby #3 -- 2021."
Duff and Koma, who married back in December 2019, share a 2-year-old daughter, Banks, while Duff also has an 8-year-old son, Luca, with her ex-husband, Mike Comrie.
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Back in July, Duff opened up to E! News about how she and her family were finding a new "normal" routine during lockdown.
"I obviously want things to go back to normal, but I'm not going to fight it," she told the publication at the time. "It's actually become the norm -- 'here's what we are doing,' you know, 'we're still in this' -- and it's not as much of a panic like, 'I can't do it anymore.' Like, we're in this for a while, and I think some days are better than others. It's hard to complain because we have a pool, and a backyard...and we have everything we need. We have food. But everyone's life has changed, so it's definitely still challenging at times."
The "Lizzie McGuire" star said she's enjoyed having more time to spend with the family.
"My kids, it was different having them so far apart," Duff said, "and it's been a beautiful thing to watch my son play with my daughter every day and, like, actually want to, you know? I was worried that they wouldn't have anything in common, but he's so good to her and she's obsessed with him. So, that's been a bonus. I'm not working and as tired as I get by them by the end of the day....there's so many moments where I feel so grateful to watch their bond and have uninterrupted family time."