"Baby's first vacation," she captioned the first picture of Ryan in a pink onesie.
She followed it up with a close-up of her grinning bundle of joy, writing, "Our sweet baby girl #RyanAvaErhard Rosenberg is 16 weeks today."
Haylie has clearly been enjoying life as a new mom, but last month, she and her younger sister, 27-year-old Hilary Duff, opened up to ET about combating mom-on-mom bullying
with their Sisterhood of Motherhood campaign.
"Becoming a mother and that whole process is so special and so magical really, but it's also very isolating," Hilary explained. "I think a lot of the times you feel like you should have all the answers and you feel like you should know how to do everything and nothing should go wrong."
"You put so much pressure on yourself, and it's really kind of a lonely place to be," Hilary said, also revealing that she felt judged when she chose to have an epidural, and when she chose to start supplementing with formula for her now 3-year-old son Luca. "So I think women and families need to boost each other up instead of, you know, bullying one another."
Though if we're judging by Haylie's Instagram, her life as a mom with Matt by her side couldn't be more perfect. "You are an incredible new dad," she gushed about her handsome fiancé on Father's Day. "I feel blessed to be on this journey with you. Ryan & I love you so much!"