Williams has since deleted the post and didn't say why, but fans are still going crazy over the picture. The 35-year-old donned a yellow swimsuit posting that she's "20 weeks" along.
Williams has been spending a lot of time in San Francisco since getting engaged to Reddit co-founder Alexis Ohanian in January.
RELATED: Serena Williams surprises two guys to match at SF park
Williams hasn't played since winning the Australian Open in late January. She pulled out of her next two scheduled events -- Indian Wells and Miami -- citing a left knee injury. Her only other tournament this year was in Auckland, New Zealand, where she lost in the round of 16.
Tennis' next major event is the French Open, which begins on May 28. If Williams is, in fact, 20 weeks pregnant, she would be due in early September.
ESPN has contributed to this report.