Although Simpson announced her pregnancy on Christmas Day, the Twitpic shows that she has been hiding the news for quite some time.
"Bumpin' and proud!" she tweeted with her picture, which shows her reflection in what looks like a hotel mirror during her vacation in Hawaii.
Simpson was dressed in a floral robe, which hung wide open to reveal a busily patterned bikini, along with her cleavage and baby belly, topped off with a matching headband.
Simpson also took to Twitter to reveal her pregnancy five days earlier. She tweeted a picture of her daughter Maxwell Drew Johnson on a patch of sand with the words, "Big Sis" written in it.
The 32-year-old singer and actress' new endorsement company, Weight Watchers, has also been supportive of the news.
“Congrats to @JessicaSimpson, Eric and big-sister-to-be Maxwell!” the company tweeted. “Your WW family is so thrilled for you. what an amazing year you’ve had!”