Viewers of the BBC soap will see Kat share the happy news with her former husband over the Christmas season, Digital Spy reports.
The on-off couple – played by Shane Richie and Jessie Wallace – reconciled last month after Alfie fled his wedding to Roxy Mitchell to chase after Kat as she attempted to leave the country.
Their reunion after 11 months apart was watched by 8.1 million viewers.
The baby will be Alfie’s first child, although he has helped to raise Tommy, Kat's son with his cousin Michael Moon.
Christmas is always a dramatic time in Albert Square, and this year is no exception.
Other festive storylines include Carol Jackson’s passionate kiss with her former flame David Wicks, and new landlord Mick Carter (Danny Dyer) taking over the Queen Vic.