Serena Williams announces pregnancy on Snapchat. Picture: Snapchat

Tennis star Serena Williams announced that she's pregnant. Williams, 35, got engaged to Reddit co-founder Alexis Ohanian in December. Williams posted a photo on Snapchat with the caption "20 weeks."

Williams has won more Grand Slam titles than any other player in the Open Era, earning her 23rd at the Australian Open in January.

In early March, she pulled out of Indian Wells and Miami, she cited a knee injury. Her last competitive match was the Australian Open final, a 6-4, 6-4 victory over her sister Venus.

Fans immediately began counting the weeks and determined Serena must have been at least three weeks pregnant when she beat sister Venus in Australia but then who's counting.