The 35-year-old posted a picture of herself in a yellow swimsuit with a prominent bump - she captioned the image "20 weeks".
The younger Williams sister is just one Grand Slam short of Margaret Court, who collected many of hers in an era when the difficulties of global travel often diluted the fields.
Williams claimed her first Slam - the US Open - 18 years ago. She later returned after the birth of her daughter and subsequently won at the Flushing Meadows, which was only the third tournament into her comeback.
With the Serena Williams pregnancy news on the rise, the tennis champ is set to return to the court in 2018. Any return would not be until well past her 36th birthday, which is bound to raise the question about whether a truly great career is at an end.
Serena Williams is the oldest woman to win a Grand Slam singles title and still dominates the tour at an age when most players have retired.
Serena Williams has not played a tournament since that victory. Serena Williams has not played since then, citing a knee injury. "I will keep moving forward and continue to be positive".
Williams, a 23-time major champion, was last seen participating in the Australian Open in January which she ultimately won.
Bill O'Reilly is Officially Out at Fox News Channel
Fox News has cut ties to "The O'Reilly Factor" host Bill O'Reilly, 21st Century Fox confirmed in a statement Wednesday . Martha MacCallum, who has been hosting a seasonal special called The First 100 Days , is expected to continue at 7 p.m.
Defense to plead for leniency in death of ex-New Orleans Saint
The court heard hours of impact statements from Smith's friends and family yesterday, including Smith's widow Racquel. "Like rare. And with tears and anger, she lashed out at the man who shot her and killed her husband during a traffic dispute previous year .
Court rulings blocks Arkansas plan for multiple executions
The Innocence Project previously asked the state's circuit court to grant Johnson new DNA testing, CBS affiliate KTVH reports . Asa Hutchinson to complain that state judges aren't honoring the decisions jurors made when sentencing the prisoners to death.