The expectant royal rode an open-topped carriage and later joined three generations of the royal family on the balcony at Buckingham Palace.
On Kate's last official engagement before she heads off to prepare for life as a mother, she paired a pale pink coat from Alexander McQueen with an older, favorite headpiece – a Jane Corbett hat trimmed with lace that she had worn at a palace garden party last year.
Kate smiled and waved at the crowds as the carriage she shared with brother-in-law Prince Harry took them along the Mall to Horseguards Parade where the marching and drills took place.
As enthusiastic crowds along the route cheered her, Kate, 31, was seen chatting with Harry, 28, in the open-topped carriage.
Her husband William donned his ceremonial Colonel of the Irish Guards uniform as he rode alongside his father Prince Charles just behind the Queen, who was in the Glass Coach alongside her cousin, the Duke of Kent.
The Queen's husband, Prince Philip, 92, was noticeably absent. He remains hospitalized, recovering from abdominal surgery. On Friday, his royal relatives, including William and Harry, paid him a visit.
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Flanked by William and Harry, she turned to her husband as some Tornados flew over. The spectacular aerobatics team of the Red Arrows ended the show.
Watched by tens of thousands of people in the Mall below, the family headed back inside – and for Kate it was a farewell to the public before she becomes a royal mum.