Serena Williams had the best accessory at the 2017 Met Gala: a baby bump!

The mom-to-be ascended the steps at the Metropolitan Museum of Art in New York City Monday wearing a custom-made emerald green Atelier Versace gown that featured vertical beaded accents and a dramatic train. Williams, 35, accessorized with a diamond Piaget watch. Italian fashion designer Donatella Versace also dressed Luciana Barroso, Halle Berry, Matt Damon, Céline Dion, Kylie Jenner and Ryan Reynolds for the most glamorous night in fashion.

André Leon Talley, contributing editor at Vogue, spoke to Williams and her fiancé Alexis Ohanian shortly after they made their way inside the museum. Ohanian said they're feeling "really good" as they prepare for the arrival of their firstborn. Williams added, "We have time."

"Do you know what it is?" Talley asked. "Do you know if it's a boy or a girl yet?"

"We're waiting. [It's] a surprise," the athlete replied. "We call it 'baby.'"

Ohanian, the co-founder of Reddit, is based in San Francisco—and Talley was curious if that's where the couple will settle down to raise their baby. "That's the question of the hour, right?" Williams asked as her fiancé flashed a big smile. "I couldn't be stuck in a more beautiful place."

Once they made their way inside, a source tells E! News Ohanian "was being very attentive" of Williams, "making sure she was comfortable, having a good time." The source adds, "You could tell the fashion world is very new to him and she seemed excited to be introducing him to it."

Brandon Stanton, creator of Humans of New York, photographed the couple and asked Ohanian to share something personal about Williams. "She has the biggest heart. Everyone sees her success as an athlete, but all of that is layered on the size of her heart," he said. "She gives 100 percent of herself to everything she does: as a friend, as a lover, and soon, as a mother."

The Costume Institute Ball marked Williams and Ohanian's first major red carpet appearance since she accidentally announced her pregnancy via Snapchat in April. Late last month, the tennis champ explained how her "belly bump" photo went viral during a TED2017 talk. "I was on vacation, taking time for myself, and I have this thing where I've been checking my status and taking a picture every week. I've been just saving it, but you know how social media is — you press the wrong button and…" she told Gayle King with a shrug. "My phone doesn't ring that much, and 30 minutes later, I'd missed four calls. So, I picked it up and realized, 'Oh, no.'"

The athlete said she plans to return to tennis after giving birth.

"My story isn't over...This is just a new part of my life," Williams said of becoming a mother. "My baby is going to be in the stands—hopefully cheering for me, and not crying too much!"

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