The 49-year-old actress - who gave birth to son Benjamin, her third child with husband John Travolta, just over a year ago - revealed as an older mother she was worried about losing the baby weight but is thrilled with her new physique.
She told People: "When I got pregnant at 48, I didn't know if my body would bounce back."
"I lost 39 lbs, have more energy than I had 20 years ago - and I feel amazing. I'm going to be turning 50 next year - holy s**t! Hey, 50 is the new 30."
Along with her delight over her weight loss, Kelly revealed the couple are besotted with Benjamin and can't believe he is already one year old.
She said: "We just celebrated Ben's first birthday. I can't believe it's already been a year."
Kelly and John's eldest child, 16-year-old Jett tragically passed away three years ago when he suffered a seizure while on a family holiday in the Bahamas and the actress - who also has a daughter Ella Bleu, 11, with her spouse - has previously revealed Benjamin has helped the entire family to cope with their loss.
She said: "It's been wonderfully healing. Of course, you know, we still - it's still every day. But it's been, I think, a really nice gift for a lot of people throughout the world.
"We've gotten the most beautiful letters. And the most beautiful notes of happiness. And I think - just sort of a sigh of relief, a bit. And that's been really nice to sort of share that with people."