The former Kate Middleton appeared Tuesday at Hope House, a London treatment centre for women recovering from addiction.
The royal couple's first child is due in July.
Photos of a pregnant, bikini-clad Kate were published last week by an Italian gossip magazine. They were taken while the couple was vacationing on the Caribbean island of Mustique.
Kate's visit to the treatment centre came amid a flurry of comments and criticism over a speech by Hilary Mantel, a Booker Prize-winning novelist.
In a London Review of Books lecture earlier this month about royal women and their relationships with the public and the press, Mantel said Kate "appeared to have been designed by committee and built by craftsmen, with a perfect plastic smile and the spindles of her limbs hand-turned and gloss-varnished."
"Kate seems to have been selected for her role of princess because she was irreproachable: as painfully thin as anyone could wish, without quirks, without oddities, without the risk of the emergence of character," Mantel added.
She later said: "We are happy to allow monarchy to be an entertainment, in the same way that we license strip joints and lap-dancing clubs. Adulation can swing to persecution, within hours..."
Some of the British press assumed Mantel was directly attacking Kate. The Daily Mail accused Mantel of launching "an astonishing and venomous attack" on her, while the Sun called it a "bizarre rant."
On a trip to India, British Prime Minister David Cameron told the BBC, "I think [Mantel] writes great books, but I think what she's said about Kate Middleton is completely misguided and completely wrong."