PIC: Jessica Simpson Gives First Glimpse of Post-Baby Bod

Jessica Simpson leaving a gym in Los Angeles on June 12, 2012. Credit: Juan Sharma/PacificCoastNews.com; Alberto E. Rodriguez/Getty Images

Go, Jessica, go!

Jessica Simpson isn't kidding about dropping the pounds gained during her very first pregnancy with daughter Maxwell Drew Johnson, now one-and-a-half months old.

On Tuesday, the 31-year-old Fashion Star mentor was once again spotted hitting a gym in Los Angeles -- and this time, after the workout was done, photographers caught the very first glimpse of Simpson's shrinking post-baby figure.

Heading back into a waiting SUV, Simpson (engaged to Maxwell's dad Eric Johnson) wore black leggings, tracksuit hoodie top and cross-trainers.

The singer and fashion mogul was first spotted hitting the private gym of celeb trainer Harley Pastermak (who has worked with Halle Berry, Hilary Duff and others) last week. The 5-foot-3 star told ELLE magazine that she weighed 170 pounds at the time of her iconic pregnant nude photo shoot for its April cover. After Maxwell's birth (the little girl weighed 9 lbs 13 oz), Simpson needed to heal from her C-section -- and then it was back to the gym.

An extra motivation: A $4 million deal with Weight Watchers.

"She hasn't even thought about working out for a year!" a source told Us Weekly last month. "But if she wants this, she'll need to work out a good five days a week."