|Fox at the Friends with Kids premiere|
The 'Friends With Kids' actress - who gave birth to her first child, Noah, with husband Brian Austin Greene in September - only gained 23 pounds while expecting by keeping her exercise routine consistent.
The 26-year-old beauty's personal trainer Harley Pasternak told People.com: "We worked out throughout her entire pregnancy. She was a model of health through the whole process. She really educated herself big time and had a great relationship with her physician so she really understood the safe boundaries of how to exercise and eat well during pregnancy. As a result, she knew how active she could be and she was very consistent and didn't overdo it.
"As she progressed from trimester to trimester, we'd modify things. As a result, she was so fit at the point of delivery that it actually went more easily and she was able to get medical clearance relatively quickly to come back and keep working out."
The celebrity fitness expert - who also trains Jessica Simpson - also revealed that Megan was back working out within weeks of delivering her baby boy but relies on his video game when she wants to spend more quality time with her son instead of going to the gym.
He said: "On the days when she doesn't want to leave her baby and come to the gym to see me, she does my video game Harley Pasternak's Hollywood Workout. I actually developed it for a lot of my new mom clients from Jessica Simpson to Hilary Duff, Megan and Milla Jovovich. It's a great, intense workout."
Megan recently revealed she isn't concerned about her post baby weight and only has 10 pounds more to lose.
The 'This Is 40' star is finding it difficult to adjust to parenthood but says it is worth it because she is so besotted with her son.
She added: "It's exhausting, but it's amazing and you can't, until you have kids, you can't imagine how much you could possibly love a human being. It's really intense and really overwhelming and amazing. He's super cute."