First-time mom January Jones got back to work on the Los Angeles set of Mad Men on Monday.

It's the first time the Emmy-nominated actress, 33, has been photographed since welcoming son Xander Jones Sept. 13. In period costume as Betty Draper, the emotionally reserved ex-wife of protagonist Don Draper (Jon Hamm), Jones covered up her post-baby bod in a floral 1960s-style mumu and flats. See more pics of January back on set.

Jones has yet to reveal the identity of the baby's father -- even leaving the father's name off the baby's birth certificate.

Her past exes include Saturday Night Live's Jason Sudeikis, singer Josh Groban and Ashton Kutcher.

Mad Men costar John Slattery gushed about little Xander two weeks after his birth. "I've seen a picture of the baby. He's beautiful," Slattery told Us at the Rape Treatment Center's Annual Brunch. "She's in love with that boy."