Gorgeous supermodel Marisa Miller is expecting a baby boy, and she hasn't been afraid to show off her baby bump. The model and actress recently tweeted a photo of her growing baby bump on her birthday, in an underwater photo. "Thanks so much for all the sweet birthday wishes!" Miller wrote to fans. "Love you all! I had the best day and spent most of it underwater."

Miller, 34, announced in June that she and her husband, Griffin Guess, who were married in 2006, were expecting a child together. And in July, they happily announced that it would be a boy, and that Miller's due date was some time in December.

"We just found out that we are having a ... BOY!!! We would be excited either way, but it was amazing to know for sure," Miller told  People Magazine.

"Griffin and I have spoken so much about having babies throughout our marriage. We already have enough names for three kids, haha!" said Miller, in a blog post for People. "It's always been a time we really looked forward to. I know you can't plan everything in life, but I knew that I wanted to really enjoy my pregnancy and have time to revel in motherhood as well."

But Miller, who is hailed in the industry for her toned, athletic body, admitted that although starting a family was a longtime dream for her and Guess, she postponed it due to the physical demands of her job. "My career is so focused on the physical that I really didn't want the pressure on or to have to worry about anything that happened to my body during that time," wrote Miller.

"There was so much freedom in letting go of my appearance because the reason for all these symptoms was this miracle growing inside me. I really could care less about what I looked like."

Miller's career -- she began modeling at 16 and appeared in the magazine Perfect 10 in 1997 -- didn't really begin to take off until 2001. That year she was discovered by famed photographer Mario Testino, and soon after began getting offers to do high profile jobs. At the time, however, she was still operating a surf school in Manhattan Beach, Calif.

Miller has since appeared on more than 80 magazine covers, including Sports Illustrated, Maxim, GQ, and FHM, who named her the Sexiest Woman in the World in their August 2010 issue. Miller is also set to appear in the film "R.I.P.D." with Ryan Reynolds and Mary-Louise Parker in 2013.