The 30-year-old singer and actress shared a cute photo on Instagram of herself and Koma comparing midriffs.
The picture shows Duff snapping a mirror selfie as she and Koma take a ride in an elevator. Duff flaunts her baby bump in a formfitting dress, while Koma lifts up his shirt to bare his stomach.
"I win," Duff captioned the post.
Duff announced in June that she's expecting a daughter with Koma, whom she started dating again in the fall.
"Guess what guys! @matthewkoma and I made a little princess of our own and we couldn't be more excited!!!!!!" she said.
The Younger star is already parent to 6-year-old son Luca Cruz with ex-husband Mike Comrie. She shared Luca's idea for a baby name during an interview with The Late Late Show with James Corden in June.
"Luca thinks Cofont Croissant is, like, the coolest name," Duff told host James Corden.
"When I just refer to it as 'Cofont,' and I'm like, 'That would be so cool, nobody at school's gonna be named Cofont,' and he's like, 'Croissant. Cofont Croissant.' It, like, goes together. There's no separating the two," she said.