Gwyneth Paltrow opens up about how she lost her extra weight and about her 40th birthday with her family in a cover interview with Redbook.

Frequently cited on the world's "Best Dressed" lists, Paltrow recalls the extra weight she gained after giving birth to her second child, Moses, in 2006 and how her personal trainer Tracy Anderson helped her get back in shape.

"Gwyneth had extra weight she couldn't drop from her pregnancy with Moses, and she needed to get in shape for her first action film, 'Iron Man'," Anderson said.

"With my daughter it had been easier, but this time, no matter what I did, I felt stuck. I couldn't shift the weight," Paltrow said. "But I met Tracy and she was this force from the second I met her."

Paltrow recalled an embarrassing moment when Anderson saw her naked.

"She pulled my pants off," the actress said. "I'll never forget it. She was like, 'Oh, my God, wow, I just am so surprised, because you look so good in clothes. I wasn't expecting this.'"

Paltrow spent her 40th birthday with her husband, Chris Martin and their two children. Paltrow, who was woken up with "hugs and gifts" said it was the best birthday of her life. 

"I feel really happy being 40, now that I'm on the other side of it," she told the magazine. "There was definitely some uncomfortable anticipation. But I'm amazed at what I've been able to accomplish, that I've maintained my important relationships and have such incredible women friends, that I have two funny, beautiful children who can give me a real life. In a way, I feel like no one should make big life decisions until 40. It's when it all starts to settle and become clear."

The January edition of Redbook magazine will be on sale Dec. 18.