"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, 65, has five other children from previous relationships, in addition to a 4-year-old son, Alastair, with Lancaster, 39.
No word on whether the upcoming arrival is a girl or a boy. Either way, Stewart is likely to encourage him or her to play soccer.
"I did take [Alastair] out on the football field the other day," he told the British talk show GMTV in November. "I said, 'Come on, let's get running out there,' and he went, 'Oh, there's some flowers for mummy – must collect them.' He spent about two hours collecting flowers in a basket for his mum!"
Stewart and Lancaster wed in Portofino, Italy, in 2007.