The 31-year-old royal wore a patterned wrap Max Mara dress as she visited Hope House, an addiction treatment centre in Clapham, South West London.
The Duchess, who is a patron of Action on Addiction which runs the residential facility, was greeted by the charity's chief executive Nick Barton.
She told Barton as she cradled her bump: "Nice to see you again, nice to be back. It's a lovely day for it."
Prince William and the Duchess confirmed that they were expecting their first child after Kate was admitted to hospital for severe morning sickness.
See pictures commemorating the Duchess's recent 31st birthday below: