Earlier this month, a spokesperson denied speculation that the actress and second husband Jim Toth were expecting.
"Reese is right around 12 weeks," one said, adding that the 35-year-old is "not planning to announce it".
Another source close to the star said: "The timing is right. She's so happy!"
Witherspoon has two children with ex-husband Ryan Phillippe; Ava, 12, and Deacon, 8.
Speaking last month, the This Means War actress said that she is at a "really good place in my life" and "would love" to expand her family with Roth, who she married last year.
"It's great to have older kids," she told ITV's Daybreak. "There's a little sadness as they're growing up and walking away from you, but it kind of changes the whole dynamic. We have really funny dinner conversations now, and they know more about certain topics than I ever, ever knew.
"This is the time where I start asking all my friends who have kids who are older, 'What do you do about a cell phone? What are your rules about the computer? Is she allowed to go out with friends or is she not?' So I learn a lot from other mothers."
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