The duchess henceforth plans to withdraw to await the birth of her first baby, due in July.
She wore a pale pink Alexander McQueen coat and a matching hat to watch the festivities, the Telegraph reports.
This time she was surrounded by other royals -- earlier in the week she made her last solo public appearance when she christened a cruise ship in Southampton. This time, she arrived in a carriage with her stepmother-in-law Camilla, Duchess of Cornwall, and brother-in-law Prince Harry, and was joined on a balcony at Buckingham Palace by her husband Prince William and various other members of the royal family to watch RAF planes do a flyover.
More than 1,000 soldier and horses took part in the ceremony, which dates from the 18th century and has honored the monarch's birthday since 1748. Someday, decades in the future, it may honor the child the Duchess is carrying, who will be in line for the British throne.