The actress says she would have shared the photo earlier, but she's got her hands full as a new mom of three.
Duff gave birth to Mae on March 24, and announced the news a couple days later.
"Most people post this on the first day..... but you’re a 3rd child Mae Mae .... soooo it’s happening on the 7th," the Younger star wrote on Wednesday alongside a cute black-and-white pic of her baby girl with her birth details. "Happy week earth side little bit! We love you so...."
Duff and her husband, Matthew Koma, are also parents to 2-year-old daughter Banks, while the actress is also mom to 9-year-old son Luca from her previous marriage to Mike Comrie.
Little Mae has made several appearances on her mom's Instagram since her birth, including in several precious snaps over the weekend.
Along with photos of Mae meeting her family, Duff gave a shout out to her "birthing dream team" on Sunday. "Mountains of love for this birthing dream team!" she wrote alongside a pic of the crew that helped welcome Mae to the world, as the infant snuggled up in her dad's arms.
See more on Duff in the video below.