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.