Serena Williams says she revealed her pregnancy by accident, after mistakenly uploading a photograph on Snapchat.
The 23-time Grand Slam winner posted a picture on the social media app, posing in a mirror with the message: "20 weeks", before deleting it, with her publicist later confirming the news.
Williams, 35, said she took photographs every week to track the pregnancy.
"I was just saving them [for myself]" she said. "I've been so good about it, but this was the one time it slipped."
The world number one, who is due to give birth in the autumn, said she discovered she was pregnant just two days before the Australian Open in January.