The Duchess of Cambridge has been described as a "fighter" for braving severe morning sickness to attend a Buckingham Palace reception honouring mental health campaigners.
Despite still suffering from the illness, which plagued her three pregnancies, Kate looked relaxed and at ease as she mingled with guests during her first public engagement in almost six weeks.
A slight baby bump could be seen beneath the Temperley dress worn by the Duchess, who is believed to be fewer than 12 weeks pregnant.
Dr Nicola Byrom, founder of the mental health charity Student Minds, said after chatting to Kate: "She's amazing, what a fighter that she's here, that she's putting other people above herself."
Royal fans have not seen the Duchess since the end of August, when she joined the Duke of Cambridge and Prince Harry to mark the 20th anniversary of the death of Diana, Princess of Wales.
They toured Kensington Palace's sunken garden which had been transformed into a memorial space planted with white foliage and flowers in honour of Diana.
A royal aide said: ''The Duchess' condition is improving but she's still suffering from hyperemesis gravidarum. She's delighted to be here tonight.''
It is understood Kate's appearance does not mean she will be resuming full royal duties.
The duke and duchess and Prince Harry arrived via a well-known hidden door in the Palace's white drawing room and waiting to meet them were leading figures from the mental health sector including Mind president and comic Stephen Fry.
The royals had invited 350 guests from the mental health sector including many of those who appeared in videos promoting William, Kate and Harry's Heads Together campaign, which has been encouraging people to talk about their psychological problems or be a sympathetic ear for those in need.
Fry said about the Duchess: "I do understand she's been having this problem more severely than many pregnant mothers and so we're very grateful (she's here)."
Speaking about the lavish palace reception, held to mark World Mental Health Day, he added: "When mental health is discussed here under these chandeliers in front of these portraits it does have a meaning, it does show a change in our society's ability to look at problems like mental health."
And commenting on the Heads Together campaign he added: "I think it's an extraordinary achievement by this generation of the royal family, they've really done a remarkable thing - no-one would realise if they hadn't done it, which makes it all the more remarkable that they have."
Kensington Palace announced on September 4 that Kate was expecting her third child and, like previous pregnancies with Prince George and Princess Charlotte, was suffering from severe morning sickness.
The Duchess' bout of hyperemesis gravidarum forced her to pull out of an event at the beginning of September which prompted the palace to make the pregnancy announcement early.
Those suffering from HG can be constantly sick morning, noon and night, and unable to keep any food or drink down.
During the reception William praised his guests for the work they have been doing for many years.
He said: "Tonight, on the evening of World Mental Health Day, we celebrate that while there is more work to do, the walls of judgment and stigma around mental illness are finally falling.
"Catherine, Harry and I are proud to stand up in support of this community. All three of us have seen that mental health is at the heart of so many of the issues we are passionate about."