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Just call her one-piece Reese!

Less than a month after Us Weekly broke the news that Reese Witherspoon is expecting her first child with husband Jim Toth (joining Ava and Deacon, her kids with ex Ryan Phillippe), the 37-year-old actress showed off her growing baby belly in Costa Rica Wednesday.

Wearing a plunging black swimsuit, the star of "This Means War" frolicked in the water with Ava, 12, as Toth, 41, and Deacon, 8, steered a paddle boat.

PHOTOS: Pregnant bikini bodies

"Deacon was helping Jim pedal the boat, but then stopped and had fun jumping off the side of the boat and into the ocean," an eyewitness tells Us. "He did it a few times and Jim seemed amused as Deacon was having so much fun."

PHOTOS: Reese Witherspoon's body evolution

It's a sure sign that CAA agent Toth is ready to embark on a new journey with Witherspoon, whom he wed on March 26, 2011. "Before Jim met Reese, he wasn't sure about having kids," a Witherspoon source recently told Us. "Being around her kids has really shown him what it's all about."

PHOTOS: Reese Witherspoon's life as a newlywed

The time is right for a third child, a source close to the pair explained. "Reese's kids are at an age where they are busy with friends and lives of their own. It will be nice for her to be able to do it all over again with her new husband."

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