Over the weekend, Duff used Instagram to share a snap of her growing baby bump, with boyfriend Matthew Koma by her side as he displayed his own stomach for the camera.
The couple captured the image in an elevator, with Duff snapping the image in a mirror as she gazed at Koma, who was lifting his shirt over his stomach to try and gain an edge in the competition.
"I win," Duff cracked in the caption.
The 30-year-old announced her pregnancy in June with a photo of herself and Koma together, with the actress wearing a maxi dress that accented her bump.
"Guess what guys!" she wrote. "@matthewkoma and I made a little princess of our own and we couldn't be more excited!!!!!!"
Since then, she's been showcasing her growing belly in several snaps, including a sweet shot with son Luca at the beach.
"My sweet sweet boy," Duff wrote. "Oh the adventures we've had."
Duff shares Luca, 6, with ex-husband Mike Comrie. Her upcoming arrival will be her first child with Koma, who she has been dating on and off for three years.
Speaking to Ryan Seacrest On Air in June, Duff revealed that she had been hoping for a daughter, as she's already mom to a son.
"I was so nervous beforehand because I thought for sure I was going to have a boy," she recalled. "I have a boy already, which would've been great and I'm obsessed with Luca, but I don't know, I just wanted one of each."
"It was funny that I cared so much," Duff added, noting that sister Haylie Duff "has girls and I feel like I'm really girly. But for the past six years I've bought monsters and trucks and planes and Luca's into all the boy stuff, so everything's blue and green and I was just ready for pink and purple stuff."
The Younger star hasn't yet revealed whether she's picked out a name for her baby girl, but she revealed on The Late Late Show with James Corden that Luca has a few ideas for his younger sister's moniker.
"Luca thinks 'Cofant Croissant' is like, the coolest name," Duff shared. "There would be no confusion. And when I just refer to it as 'Cofant' and I'm like, 'That would be so cool, no one at school's going to be named Cofant,' he's like, 'Croissant. Cofant Croissant.' It goes together. There's no separating the two."
Photo Credit: Getty / David Livingston