Just weeks after giving birth to twins Max and Emme in February, Jennifer Lopez had already lost 40 of the roughly 50 pounds she'd gained while pregnant. And nearly seven months later, the 39-year-old star was making her babies proud while competing with pal Matthew McConaughey in a triathlon.

What's J.Lo's secret? For one thing, her trainer, Gunnar Peterson, tells Life & Style, "She's killing it in the gym three times a week."

For more on J.Lo's weight-loss secrets — and how to drop up to 20 pounds! — pick up the latest issue of Life & Style, on newsstands now!

Photo Credit: Abaca