Jamie-Lynn Sigler takes newborn son, Beau Kyle Dykstra for a checkup in Beverly Hills September 27th, 2013 Beverly Hills, CA Credit: National Photo Group

A milestone for new mom Jamie-Lynn Sigler: She's back in her skinny jeans! The Guys With Kids actress, who gave birth on Aug. 28 to baby boy Beau, is already on the slim-down.

The former Sopranos star, 32, was spotted taking her one-month-old bundle of joy to a checkup on Sept. 27 in Beverly Hills. Sigler rocked a loose-fitting oatmeal-colored sweater and a pair of skinny jeans, while her baby boy sported a royal-blue top, black sweatpants, and a full head of brown hair!

The svelte mom has been getting back in shape, in part by eating healthy. She recently Instagrammed her breakfast, lunch, and dinner by Paleta, a farm-to-table meal service in L.A. "Let's do this!" she captioned the shot. Fiance Cutter Dykstra, 24, very cutely commented, "Get it in ya."

In another food shot, which shows a carb-less meal of steak, asparagus, tomatoes, and some root vegetables, Sigler wrote, "My yummy dinner from @eatPALETA! By candlelight with the tv volume down.. #notromantic#dontwakeupthebaby."

Her diet plan is obviously working. Not even a month after giving birth, on Sept. 20, Sigler looked gorgeous and slim in a navy and white speckled blouse and gray skinnies for a date night with her baseball player fiance.