The baby is due early next year.
The two were married in 2005. They met on the set of the film "Gothika," which Susan co-produced.
Downey, 46, has a teenage son, Indio, with his ex-wife Deborah Falconer.
He received Oscar nominations for his roles in "Chaplin" and "Tropic Thunder." His film credits also include "Sherlock Holmes" and the "Iron Man" films.
The Associated Press contributed to this report.
