Most new mothers would say that gaining weight during pregnancy was easy — and, if they’re being honest, they’ll admit that all that eating for two was kind of nice. As actress Mariska Hargitay, a first-time mom at 42, put it, “I was thrilled to sublet my body for nine months.” But once son August Miklos Friedrich Hermann was born last June, the Law & Order: Special Victims Unit star was “ready to take ownership” again.

So before heading back to work on the series, she focused on losing some of the 54 lbs. she’d gained during pregnancy. As the daughter of ’50s bombshell actress Jayne Mansfield and former Mr. Universe titleholder Mickey Hargitay, Mariska had good genes on her side. But she also had a healthy respect for her body’s natural shape. “I wasn’t trying to be skinny,” she said not long after starting her get-fit regimen. “My mom is Jayne Mansfield. I’m allowed to have curves.”

The “I’m Not Dieting” Diet

To preserve those curves while nursing her son and readying herself for a return to Detective Olivia Benson’s working wardrobe of well-fitting T-shirts and pants, Mariska started watching what she ate. “I’m trying to eat well for August and be very selective,” she said last summer.

“So that means a lot of organic fruits and vegetables.” Salads became a focus of her new eating plan and she cut back on dairy, sugar and wheat to make things easier on August’s sensitive stomach. And now she never leaves home without a bottle of water. Without any superhuman effort on her part, Mariska’s weight started coming off, but she didn’t obsess. In fact, she didn’t keep track of how much she was losing: “My number one mommy rule — no scales at home.”

Keeping Fitness Fun

Six weeks after August’s birth by Cesarean, Mariska got the OK to start exercising. At a gym near her home in New York City, she started on a three-day a week, 55-minute plan that included stretching, lunges, push-ups, resistance training and a final five minutes of ab work. Also, when she could, she swam. Growing up in Los Angeles, she was on her high school’s swim team. “Swimming reminds me of being in the womb,” she says. “You hear your breathing and it’s completely relaxing.” And when she had no other options, she ran on a treadmill at home, but only with musical accompaniment.

Within four months, Mariska was 40 lbs. lighter and in great shape physically. She says she plans to lose another 15 lbs., but meanwhile, she relies on a tried-and-true way to look her absolute best: “When you have good posture, you look better and prettier. It shows in my shoulders. They make me feel strong, centered and present — like I am really filling out my space.” Whether that space is a TV screen or a patch of red carpet, it’s no wonder all eyes are on this radiant new mom.

Mariska’s Feel-Good-Fast Tips:

• “I never leave the house without a few lipglosses. Gloss makes me feel instantly polished and pulled together.”

• "I always take baths. My grandma taught me that taking baths gets everything moving. It gives me a natural rosy glow.” A favorite bath-time indulgence: Fresh Nectarine Milk Formula f21c, $35, fresh.com.

• A favorite meal is a “killer salad. I’m not scared of a big salad with the darkest, greenest, freshest vegetables in the market.” To make hers super-nutritious, Mariska adds a shot of Bragg Liquid Aminos — a protein concentrate made from soybeans — to her salad dressing; log on to bragg.com for product info and to order.

Next page: A diet expert weighs in on Mariska's slim-down strategy