The former Lizzie McGuire star announced in August that she and husband Mike Comrie are expecting their first child together early next year.
Duff confessed that while she initially expected that she would be uncomfortable with her figure prior to giving birth, she has found that she feels more confident when dressing to show off her new curves.
"In the beginning [of my pregnancy] I was thinking, I'm going to wear really loose things, because you don't really look pregnant at first - you just feel a little bigger than your normal self," she explained to US Weekly.
"And now that you start to show I think it's better to wear tighter things, but it's kind of a challenge to find things!"
Duff recently revealed that biting into a bagel caused her to lose a tooth on her wedding day.