Hilaria Baldwin's determination to constantly stay active during her pregnancy has obviously paid off.

The fitness and wellness guru gave birth to her fourth child with husband Alec Baldwin, son Romeo, on May 17, and on Thursday, she took to Instagram to show off her post-baby body. Hilaria, 34, sports black lacy lingerie while taking a selfie in front of the mirror, and posted the picture next to a photo of herself nine months pregnant.

Hilaria Baldwin Reveals Name of New Son Along With a Sweet Pic

"9 months to 12 days postpartum (I took the photo on the right two days ago)," she wrote. "Every body is different, but I believe in staying as active as possible. There were so many moments when I didn’t want to work out this pregnancy. Showing up is the most difficult challenge. I had to drag myself out and do even a little. Consistency helped me so much though...because the body wants attention and wants to be moved. So important for your circulation, your health, your happiness, and for your strength."

Hilaria, who has documented her impressive post-baby body before, also said she started working out again on Wednesday.

"Slow and steady. Nothing crazy," she noted. "Each birth is different and your doctor will be able to advise you on when you can begin. When you can, be patient with your body. You have been through so much and created something so precious. Know that day by day, movement by movement, you will regain your strength and feel amazing. #wegotthis2018."

Hilaria Baldwin Compares Her 4 Pregnancies With Bare Baby Bump Pics

Aside from baby Romeo, Hilaria is also a mom to 4-year-old Carmen, 2-year-old Rafael and 1-year-old Leonardo. Last month, Hilaria compared her four pregnancies with bare baby bump pics, and said she felt she had the biggest baby bump with her latest pregnancy.

Last week, both Hilaria and 60-year-old Alec took to Instagram to share the first adorable photo of Romeo.

ET spoke to the happy couple earlier this month, when they revealed that they hadn't come up with a baby name just days before Hilaria gave birth.

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"This is the first one that we really don't know going into it, and we're going to see what he looks like and take a good look," she explained.