The first lady sat down with People Magazine for an exclusive interview to talk about how she feels about that big milestone, saying she is "more self-assured now."
Mrs. Obama says she doesn't imagine resorting to plastic surgery or Botox but that she's also learned to "never say never."
"Women should have the freedom to do whatever they need to do to feel good about themselves," she said in the interview.
The first lady has never missed a health checkup, including a mammogram or Pap smear, and has had a colonoscopy.
As for diet and exercise, Mrs. Obama says she doesn't "obsess" about what she eats but makes sure to include fruit and vegetables.
Her workouts have also evolved to include things like yoga that she says will keep her flexible.
A White House birthday party is planned for Saturday evening. Guests have been told to come ready to dance and to eat before they come.
Mrs. Obama started the celebration early by staying back in Hawaii to hang out with girlfriends after her family's two-week holiday vacation there. That extended break from Washington was an early birthday present from the president.
She has three years left as first lady and says that not having another election to worry about gives her "a little more room to breathe."
Some of those who watch Mrs. Obama's every move are wondering if she's now ready to test the limits of being first lady.
(The Associated Press contributed to this report)