Middleton - wearing a summery Max Mara dress - looked radiant as she cupped her hands on her stomach, as dozens of eager photographers and TV crew gathered in the street to snap her picture.
In her first official engagement since Christmas, Middleton, 31, spent her afternoon visiting with recovering alcoholics at Hope House, which is a residential center for women run by Action on Addiction; Kate is a patron of the center.
"Nice to see you again, nice to be back", the Duchess told the charity's chief executive Nick Barton. She added as she looked up at the sky: "It's a lovely day for it."
As she talked with some women in an art therapy class, she told one of them: "Well done for getting sober."
Ahead of Kate arriving at the center, Barton defended the Duchess after British novelist Hilary Mantel mocked her as a “machine-made” princess with a “plastic smile”
Barton said: "Having met the Duchess several times I find her to be engaging, natural and genuinely interested."
Despite suffering from extreme morning sickness in the early stages of her pregnancy, the Duchess looked tanned after her recent vacation in Mustique with husband Prince William. During that trip, she was photographed wearing a bikini and the pictures were published in some foreign magazines.
She’s due to give birth in July.