Meghan Markle took a break from maternity leave to perform on the occasion of the annual Trooping the Color procession in honor of the birthday of Queen Elizabeth II.
Prince Harry, Prince William and his wife Kate Middleton, Prince Charles and his wife Camilla also attended the celebration, which took place on Saturday.
The Duke and Duchess of Sussex waved to the crowds in their carriage, which was shared by the Duchess of Cambridge and Camilla.
Markle wore a naval ensemble by Clare Waight Keller, who also designed her wedding dress, and put on a matching blue hat by Noel Stewart.
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The Duchess of Sussex welcomed her first child, Archie Harrison, on May 6, just five weeks, and this event marks her first public appearance since his birth.
At the Trooping of the Color, 1,400 soldiers marched past the Queen in scarlet robes and bearskin hats.
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Thousands of spectators stood on the parade ground and gathered in nearby St. James's Park for the spectacle follow.
The ceremony arose from traditional preparations for the fight.
The colors – or flags – were "flipped" or carried along the lines of soldiers so that they could be seen and recognized in battle. The regimental flag presented this year comes from the 1st Battalion Grenadier Guards.
The AP contributed to this story.