Earlier in the day William and Harry hosted a reception at St James' Palace to celebrate everyone who has helped with their mental health campaign Heads Together.
Despite still suffering from severe vomiting (hyperemesis gravidarum), she looked relaxed and was showing signs of a tiny baby bump.
A palace rep confirmed that Catherine is still affected by HG, but said her condition is improving. "The Duchess is delighted to be here tonight".
Thankfully, a new report claims that the worst has passed and the Duchess has not only been able to finally enjoy her pregnancy (she just entered her second trimester), she is also now ready to attend to her royal commitments.
One of their initiatives that's close to the duchess' heart is Heads Together, a mental health campaign whose aim is to challenge mental health stigma and change how it is perceived.
Kate made an unusual appearance without William by her side in a video released last month aimed at educating young children about mental health.
If Kate Middleton is absent since the announcement of her pregnancy, her return has recently been announced.
Kate also suffered from the condition when she was expecting Prince George and Princess Charlotte.
Kate Middleton stepped out for the first time since announcing her third pregnancy. "Things that can't really be seen but that affect us every day, and talking about them can feel hard", she says, introducing a short animated film on the topic for children.
"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".