The two-times Oscar nominated actress, whose film hits include Minority Report and In America, is engaged to film editor Harry Holm and the baby - due January 2008 - will be their first child together.
The 30-year-old is already mother to Esme, seven, from a previous relationship with British actor Charlie Creed-Miles.
Morton's publicist, Karon Maskill, confirmed: "It is wonderful news, they are both very excited and looking forward to having their new baby."
Morton is currently filming drama Synecdoche, New York alongside Philip Seymour Hoffman, Michelle Williams and Emily Watson, which is due to be released at the end of 2007.