Witherspoon, 36, is best known for her work in the films "Walk the Line," "Legally Blonde" and "Election." She has an 8-year-old son and a 12-year-old daughter with her ex-husband, actor Ryan Phillippe, who she divorced in 2006.
She married talent manager Jim Toth last year after dating him for more than a year.
"Reese is right around 12 weeks," one insider told UsMagazine.com, adding she is "not planning to announce it."
"The timing is right," another source said. "She's so happy!"