Middleton will be out and about as early as this Tuesday, Oct. 10 for World Mental Health Day.
William said: "It was Catherine who first realised that all three of us were working on mental health in our individual areas of focus".
The royal mom walked into The White Drawing Room with William and Harry in a pretty blue Temperley London dress, which showed off a hint of a baby bump.
Princess Charlotte was born in 2015, Wills and Kate's second child after Prince George.
She's been battling severe morning sickness.
She was too ill to take her eldest child George to his first day at school last month and her last public appearance had come at the end of August when she joined William and Harry on a visit to a public garden at Kensington Palace dedicated to their late mother Princess Diana.
If Kate Middleton is absent since the announcement of her pregnancy, her return has recently been announced.
If the Duke and the Duchess of Cambridge are delighted with the arrival of this new royal baby, the young woman unfortunately suffers from hyperemesis gravidarum which results in violent nausea and other very disturbing symptoms.
During the St James's Palace reception, William told guests that the British tradition of the "stiff upper lip" was a legacy of the trauma of the First World War.
"I think it really made an impact on people, seeing them be so emotionally literate".
"She had seen that at the core of adult issues like addiction and family breakdown, unresolved childhood mental health issues were often part of the problem".
Harry, who on Monday launched a new initiative that will see Armed Forces personnel receive training created to make them mentally as well as physically fit for combat, said the campaign would also be announcing new programmes in schools and workplaces in the coming year.