Her condition, known as hyperemesis gravidarum, forced her to be hospitalised in 2012 while pregnant with George, aged four. In the first few months, the Duchess had to retire from her royal functions because she was too ill to be able to fulfill her commitments.
At the time of Kate's pregnancy announcement, it was reported she was 11 weeks gone, which would mean she is around the 15-week mark today. She will take part in the event along with Prince William and Prince Harry to celebrate and express gratitude towards the people from across the United Kingdom devoted to the mental health field.
At an event earlier in the day, Prince William explained in a speech that it was his wife who first decided the trio should join together to focus on mental health. But her health is most important and it's good to see that Middleton's condition is improving.
"And after years of working with the homeless, and having been called out to multiple scenes of suicide as a search and rescue and then HEMS (helicopter emergency medical services) pilot, I understood the damage poor mental health was causing to our communities and for our families".
Kate's previous public appearance was on was August 30, when she toured the Princess Diana garden at Kensington Palace with William in advance of the 20th anniversary of Diana's death.
Hyperemesis gravidarum can cause severe nausea and vomiting and requires supplementary hydration and nutrients.
Just hours before his wife made her appearance, Prince William told guests at a St James's Palace reception that it was Kate who was the brainchild behind the campaign.