"When I was throwing my body around fearlessly, and going for gold for our country, I was pregnant, and today I'm 11 weeks pregnant," Walsh Jennings told Matt Lauer.
Their baby, due April 9, will join the couple's sons, Joey, 3, and Sundance, 2.
Walsh Jennings, 34, told TODAY.com she and Casey started trying for a third before the Olympics. But she didn't expect it to happen so quickly. She said she felt "moody and touchy" in London -- more than could just be explained by competition stress -- and her period was four days late.
"You're probably pregnant," her beach volleyball partner, Misty May-Treanor, said. And sure enough, her teammate was right.
"It's just so exciting," Walsh Jennings told TODAY.com.
Image: Volleyball, via Shutterstock
CORRECTION: Kerri Walsh Jennings became pregnant before, not at, the Olympics. This headline was edited to reflect that correction on 9/28.