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You would never guess that Duchess Kate gave birth just six weeks ago!

Princess Charlotte's mommy turned heads all around the world when she stepped out in skinny jeans on Sunday, looking as fit as ever. How did she do it?

According to People magazine, she secretly worked out with a top London trainer throughout her pregnancy.

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The trainer "was brought on board while she was pregnant and is super-discreet," a source told the magazine, adding that the trainer, who remains unnamed, is popular with A-listers, royals and executives. "It's a lot of money, but you get a lot for the price."

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Post-pregnancy, Kate has continued to work with the trainer who tailors a fitness routine to her liking and checks in regularly.

"The trainer checks in remotely with Kate by phone every day as well as seeing her in person once a week," says the source.

Of course, Kate also burns a fair share of calories chasing around her "live wire" son, Prince George!

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As for her diet, Kate has relied heavily on lean meats and organic fruits and veggies — something she apparently has done for quite some time and something her trainer encourages.

"The food is very elegant and actually feels like food you would eat at a restaurant," says People's source. "The whole program is very sensible -- it's not fad-based and is really focused on your overall health and well-being."

Well, it's working.