Baby on board!

Reese Witherspoon showed off her baby bump in a black one-piece swimsuit Wednesday during a vacation in Costa Rica with her family.

The “This Means War” actress was joined by husband Jim Toth, 41, and her two children – Ava, 12, and Deacon, 8 – from her previous marriage with actor Ryan Phillippe.

Witherspoon, also wearing a baseball hat and sunglasses, appeared relaxed as she dipped her toes into the ocean water while her husband and son were trying to maneuver a paddle boat nearby.

“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 told Us Weekly. “He did it a few times and Jim seemed amused as Deacon was having so much fun.”

Married since March 2011, Witherspoon, 36, and Toth, 41, reportedly tried to expand their family soon after saying “I do.”

“Before Jim met Reese, he wasn’t sure about having kids,” a source close to the actress told Us. “Being around her kids has really shown him what it’s all about.”

According to E! News, Witherspoon is due “in the fall.”

By Cristina Everett / NEW YORK DAILY NEWS