The report came after a beaming Jolie appeared with Pitt at the Independent Spirit Awards on Saturday in a black dress that revealed a clear baby bump.

The source told People that Jolie and Pitt were "thrilled to be adding to their brood" and were "very, very happy."

Pitt, 44, and Jolie, 32, also are parents to adopted Cambodian son Maddox, six; four-year-old Vietnamese son Pax; three-year-old Ethiopian daughter Zahara and biological daughter Shiloh, who was born in Namibia in May 2006.

Pitt and Jolie met on the set of their 2005 film Mr and Mrs Smith and later began a romantic relationship.

Pitt was previously married to Friends star Jennifer Aniston, while Jolie has been married twice before.