ABC News' Mary Pflum and Katie Kindelan report:

Royal bump watchers got a new glimpse of the pregnant Duchess of Cambridge today as Kate Middleton stepped out to speak to women recovering from addiction.

Middleton, 31, appeared at Hope House in South London today wearing a grey wrap dress that showed the slightest hint of a growing baby bump. The duchess and her husband, Prince William, 30, announced in December that they were expecting their first child.

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Today's appearance by Middleton was one of the few she's made since being hospitalized early in her pregnancy for peremesis gravidarum, an acute morning sickness that requires supplementary hydration and nutrients.

Middleton and Prince William recently returned from vacation on the exclusive Caribbean island of Mustique in St. Vincent and the Grenadines, where the photos of Middleton wearing a blue bikini that showed a barely rounded lower belly were published by some gossip magazines, much to the palace's displeasure.

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Back in London today, Middleton was ready for the cameras as she met with clients and staff at Hope House, an all-female rehabilitation center that is supported by Action on Addiction, one of the several charities to which Middleton committed her time as a patron in 2011.

Middleton was compared to her mother-in-law, the late Princess Diana, when she made such a public commitment to charitable work then, and today she is again being compared to Diana as she begins her journey into motherhood.

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Fashion watchers, who say Middleton's dress today was by designer MaxMara, say the duchess is taking fashion cues from Diana, who was known to don fashion-forward frocks throughout her pregnancies with both Prince William and his younger brother, Prince Harry.

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"Kate really has an easy glam style that a lot of people feel that they can imitate," said Charlotte Triggs of People magazine. "She doesn't choose the most expensive items all the time. She has a mix of high and low…and that's why people are so interested in seeing what she wears."

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Middleton stepped out in a dark green gown reportedly by Sarah Burton of Alexander McQueen, the same designer responsible for her ivory and lace wedding gown, in December and, just a few weeks ago, donned a stylish cape by the mass retailer Zara that has since sold out globally.

Likewise, Princess Diana, paid close attention to her fashion look while pregnant, as revealed in a new fashion exhibit devoted to one of her favorite designers, Belville Sassoon.

Included in the "Glamour of Bellville Sassoon" exhibit, opening this September at the Fashion and Textile Museum in London, are a series of sketches made for Diana that include hand-scribbled notes from the late princess indicating her likes and dislikes.