Marisa Miller is no stranger to skimpy swimwear, and on August 6, the Sports Illustrated model showed off her pregnancy curves in a black bikini.

"Thanks so much for all the sweet birthday wishes! Love you all!" the 34-year-old wrote in a Twitter caption. "I had the best day and spent most of it underwater."

PHOTOS: Pregnant models

Miller and her husband, music producer and talent manager Griffin Guess, will welcome a baby boy in December. It will be the first child for both stars, who tied the knot on April 15, 2006.

PHOTOS: Supermodel moms

The former Victoria's Secret Angel -- who has graced more than 80 magazine covers in her career -- first tweeted a photo of her baby bump on August 2.

"This was a month ago at 17 weeks. My belly seems to have doubled since!" Miller wrote. "Had to paddleboard to get to this cave."

VIDEO: Marisa Miller takes Us behind the scenes at Fashion Week

Miller next appears in the movie R.I.P.D., costarring Ryan Reynolds, 35, and Jeff Bridges, 62. She is also the face of Harley Davidson.

This article originally appeared on See Marisa Miller's Pregnant Bikini Body!