Serena Williams announced her pregnancy by accident when she accidentally shared a picture publicly, she has revealed.

The world number one tennis star posted an image of herself on social media app Snapchat with the caption "20 weeks", before later deleting it.

But she has since told how she had meant the picture to be kept as a personal memento, planning to officially announce the news days later.

Speaking at a Ted Talk, the 23-time Grand Slam winner said: "I was on vacation, taking time for myself, and I have this thing where I’ve been checking my status and taking a picture every week.

"I’ve been just saving it, but you know how social media is — you press the wrong button and…

"My phone doesn’t ring that much, and thirty minutes later, I’d missed four calls. So I picked it up and realised, ‘Oh no.'”

Williams, 35, told how she found out about her pregnancy two week before the Australian Open, which she won in January.

The star said: "Pregnant or not, no one knew. Every tournament where I show up, I’m expected to win. If I don’t win, it’s actually bigger news.”

Williams went on to say that she "definitely" plans to return to tennis after the birth of her first child, due to be born in the autumn.

She added: "This is just a new part of my life. My baby is going to be in the stands, hopefully cheering for me and not crying too much."