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.
Pro-pot activists plan to hand out joints outside Capitol
Yet, Kelly's comments come off as relatively lenient once contrasted against ones made by attorney general Jeff Sessions. They plan to smoke marijuana at that location, on federal land, where using and possessing marijuana is a crime.
Evacuations resume in Syria
Some 300 evacuees from rebel-held towns were similarly held up at a staging point at Ramussa in government-held territory. The evacuation of the towns, along with more rebel villages along the border, are expected to continue for some time.