The couple, who married in 2007, also have a 4-year-old son, Alastair. Stewart, 65, has five other children from previous relationships.
"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 (in March)," Stewart and Lancaster said in a statement issued to People.com.
The baby's gender was not disclosed, the Web site said.
Celebrity Breakups and divorces of 2014 [PHOTOS]