Serena Williams has resisted saying publicly that she has retired from tennis and the finality that would bring, but now she has another reason to stay away from competition in the near future.

Amid the costume-themed glitz of the Met Gala on Monday night Williams announced, both on the red carpet and on social media, that she was pregnant with her second child.

She attended the event with her husband, Alexis Ohanian, a co-founder of Reddit, and said in a post on Instagram that she “was so excited when Anna Wintour invited the 3 of us to the Met Gala.”

On the red carpet before the event, Ohanian put his hand on his wife’s stomach in a gesture indicating a pregnancy. Williams also posted a series of portraits that included an image of her cradling her stomach with both hands.

Williams was not the only pregnant celebrity at the gala. The singer Rihanna, who wore a white Valentino gown on Monday night, made her pregnancy public during her halftime performance at the Super Bowl in February.

Williams and Ohanian had their first child, Alexis Olympia Ohanian Jr., in 2017. Williams had won the Australian Open that year while roughly two months pregnant.

She returned to tennis and made four more Grand Slam finals in 2018 and 2019, although she did not add to her total of 23 Grand Slam singles titles.

In August, Williams said she would step away from tennis after the U.S. Open. But last October she said she was “not retired” and that the chance she would come back at some point was “very high.” Officially, she is on the retired list.

In an essay in Vogue at the time of her retirement, she wrote: “If I were a guy, I wouldn’t be writing this because I’d be out there playing and winning while my wife was doing the physical labor of expanding our family. Maybe I’d be more of a Tom Brady if I had that opportunity.” Brady, the star N.F.L. quarterback, played until he was 45 and announced his retirement in February.

Williams, 41, has won 23 Grand Slam singles events, starting in 1999, when she was 17: seven Australian Opens, three French Opens, seven Wimbledons, and six U.S. Opens. The total is one short of the record of 24, set by Margaret Court, although about half of Court’s wins came in the pre-1968 amateur era.