In a previously unannounced appearance, she joined the royal brothers to salute 150 apprentices who've graduated from Coach Core, a mentoring program founded by the royal trio.
The tennis coach said: "To have the backing of the Princes and the Duchess is unbelievable and a huge inspiration for all the young people".
The Duchess was attractive in a blue double-breasted Philosophy Di Lorenzo Serafini blazer, paired with a pair of black trousers, black suede booties with chunky heels and Kiki McDonough laurel leaf earrings.
Middleton accessorized her look with a black turtleneck and matching booties.
In September, the royal couple announced their third pregnancy.
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The club's captain Mark Noble said: "We know that William's an Aston Villa fan so whether it gets put on or not I don't know". She'll think I'm more of a great dad now than I was yesterday.
Apprentices from all over the United Kingdom were honoured at West Ham's football stadium - the former Olympic stadium in London.
"For me, getting kids doing sport instead of sitting in front of computers is great".
Kensington Palace announced: "The Duke and Duchess of Cambridge will undertake an official visit to Norway and Sweden at the request of the FCO in early 2018". "They have each been on a unique journey, created to build on their existing skills to equip them with the tools to be effective role models and mentors in their communities".
A new ensemble for the future prince or princess.