The couple said in a statement Monday that they learned the "happy news" in June as they celebrated their third wedding anniversary.
Stewart and 39-year-old Lancaster have been a couple for a decade and have a 4-year-old son, Alastair.
The Rock and Rock Hall of Famer also has five children from previous relationships.
The couple said they were "thrilled and delighted to be able to tell Alastair that he was going to be the big brother to a little baby, expected just before Mummy's 40th birthday."
