Jolie ignored reporters' questions on the awards show carpet, but People magazine reported on its Web site that a source close to the couple confirmed that she and Pitt are "thrilled to be adding to their brood."
Pitt's publicist, Cindy Guagenti, declined comment to The Associated Press.
Jolie and Pitt are parents of a 1-year-old daughter, Shiloh, and three children adopted from Cambodia, Ethiopia and Vietnam. Their adopted children are Maddox, 6; Pax, 4; and Zahara, 3.
Jolie's appearance helped give a theme to Saturday's Spirit Awards, where the big winner was the teen pregnancy comedy "Juno," taking awards for best independent film, best actress for Ellen Page and best first screenplay for Diablo Cody.