"I'm happy to confirm Serena is expecting a baby this Fall," Los Angeles-based publicist Kelly Bush Novak said in a statement emailed to AFP.
While Williams won't play again this year, "she looks forward to returning in 2018," Bush Novak said.
Williams' Snapchat post, quickly highlighted in US media, showed the 35-year-old in a yellow swimsuit that revealed an apparent baby bump.
But the post was quickly taken down, lending an air of mystery to its meaning.
"The GOAT herself @serenawilliams took to Snapchat to announce she's 20 weeks pregnant! Congratulations Serena!" the US Tennis Association, organisers of the US Open – which Williams has won six times – tweeted, using the acronym for "Greatest Of All Time."
But the Women's Tennis Association deleted a congratulatory tweet and acknowledged in a statement they had not had confirmation of the news.
