Kate Middleton is to make her first public appearance since Italian magazine Chi published pictures of her bikini baby bump.
The Duchess of Cambridge will be visiting Action on Addiction's Hope House treatment centre in Clapham, south London, today.
She became a patron of the charity, which supports people with addictions and their families, in January 2012.
Amanda Thomson, research manager at Action on Addiction, said it will be an emotional visit for the Duchess: "This has to be one of the hardest jobs for her so far.
"These are heart-rending tales of what they have been through. It's a gritty issue - it will be an emotional day.
"The Duchess has a heart and will certainly find it moving - it's impossible not to cry."
Chief executive Nick Barton said he was "thrilled" that Kate supports the charity and their cause: "We are very grateful to the Duchess whose support will enable us to keep the issue of addiction in the spotlight. Addiction is a consuming condition that results in a great deal of harm to individuals, families, communities and society as a whole.
Hilary Mantel calls Kate plastic princess
"Yet, it is not always well understood or responded to effectively. We are thrilled that the Duchess will be supporting us in our mission to free people from addiction and its effects."
Her visit to Action on Addiction is only the third public appearance Kate has made since announcing her pregnancy in December.
Unofficial pictures of her in a bikini, showing her baby bump, were published by Italian magazine Chi last week, to the disappointment of St James' Palace.
The pictures showed Kate and Prince William on the beach on the Caribbean island of Mustique.
It is also her first appearance since Booker Prize winner Hilary Mantel criticised the Duchess for being a "plastic" princess who had "dead eyes", and whose only purpose is to breed and be polite.
Mantel was lambasted for her comments, with Telegraph women's editor Emma Barnett saying the author's comments were "cheap and cruel".
She said we will learn more about Kate through the charities she chooses to support, adding: "As a fully paid-up member of the royal family, she can only respond by doing the very same thing Mantel has criticised her for: staying quiet."