Jamie Lynn Spears is desperate to regain her fit physique after she gives birth next month, but she’s determined not to harm her unborn child by exercising while she’s pregnant.

The 17-year-old sister of pop superstar Britney Spears has gained an estimated 20 pounds in weight while pregnant.

And she’s keen to turn back the clock and slim down as soon as it’s safe for her to do so.

She tells OK! magazine, "I try to stay active, but you can’t do too many exercises because that’s not good for the baby. No running on the treadmill.

"I want to get back to my old weight, but you can’t work out right away. I don’t want to stay this way. I’ll just stick to my treadmill and weights, and I won’t eat cheese every day."

But Jamie Lynn insists she won’t hire an expensive personal trainer, like her big sister: "You don’t have to pay somebody a bunch of money to make you skinny again.

"If you want to be skinny, you have to do it yourself. And I love to swim, so I know I’ll be doing a lot of that."