In the black and white photo, Simpson poses makeup-free, wearing comfy sweats next to her bright-eyed boy. She posted the pic without a caption, but wrote "3 MONTHS" on top of the image.
The mom of two first debuted Ace in August, alongside big sister Maxwell, 17 months, on the cover of Us Weekly. Simpson told the magazine, "With two kids, we have our hands full, but every day is a new adventure. It's fun! I feel very at peace with being a mom."
In September, Simpson flaunted her post-baby body in Calabasas, wearing a snakeskin print top and Daisy Dukes. Check out the pics here!