Stewart, 65, and 39-year-old Lancaster have been together for a decade and have a 4-year-old son, Alastair.
"We 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," the couple said.
Stewart also has five children from previous relationships: two children with his first wife Alana Hamilton, one child with Kelly Emberg, and another two kids with former wife Rachel Hunter.
The Associated Press contributed to the report.
