The Duchess of Cambridge said she was "delighted" to support , despite still suffering from hyperemesis gravidarum, which some describe as severe morning sickness.
Kate — who showed just a tiny hint of a baby bump in a dusky blue lace — made her first public appearance six weeks since she and Prince William announced that they are expecting their third child together.
She was joined by William and Prince Harry at Tuesday night's (October 10) reception at Buckingham Palace to celebrate the contribution of those working in the mental health sector across the UK.
However, despite several well-wishers taking to social media to show support and admiration for the royal, some users questioned whether she was, in fact, pregnant and why her bump was so small.
"She hardly looks preg!" wrote one Instagram user. "I can't believe she's prego, she looks too thin," wrote another.
Others commented on her slender frame, criticizing the pregnant mum-to-be for looking like a "malnourished anorexic" and describing her as "emaciated."
Fortunately, Kate's supporters came to her defense, praising her for coming out in public while suffering from hyperemesis gravidarum and pointing out the cruelty and ignorance to criticize the size of a woman's pregnancy bump — big or small.
"Wonderful to see the princess! Hope she is feeling better!" wrote one fan. "So happy to see that the Duchess was feeling well enough to attend!" commented another.
An aide confirmed at the event (via ): "The Duchess' condition is improving but she is still suffering from hyperemesis gravidarum.
"She is delighted to be able to be here tonight."
Stephen Fry, who is president of mental health charity Mind, was also in attendance and praised the Duchess for attending despite battling hyperemesis gravidarum.
"I do understand [Kate] been having this problem more severely than many pregnant mothers and so we're very grateful [she's here]," Fry said.
"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," he added about the event.
Mental health is a subject close to Kate's heart, as she, Prince William and Prince Harry founded the charity campaign Heads Together — a cause intended to end the stigma surrounding mental health issues.
Earlier in the day, the Duke and Prince Harry hosted a reception at St James's Palace to celebrate the impact of their Heads Together campaign, which aims to get the country talking about mental health.