Kate Middleton and the king are both currently hospitalized at London Clinic.
The Princess of Wales turns 42 on Tuesday.
The 38-year-old grandson of the queen was photographed leaving Windsor Castle.
The moment was a rare, candid one for the royal couple.
This marks the royal's first birthday since her coronation in May.
The event in Scotland came two months after he was crowned at Westminster Abbey in London.
King Charles III and Queen Camilla surprised 'American Idol' audiences during the May 7 episode.
Designer Bruce Oldfield spoke with ET about the symbolism and design of the historic gown.
Katy Perry, Tom Cruise, Kermit the Frog, Miss Piggy, Winnie the Pooh and more celebrities appear at King Charles' coronation concert.
Prince Harry didn't speak with King Charles or Prince William at the coronation, but the king did acknowledge Archie's birthday.
Ryan Seacrest and Luke Bryan chat with ET after 'American Idol's live taping about Katy Perry going viral at the king's coronation.
The 'Idol' host opened up to ET on Sunday about the unexpected royal appearance.
Ryan Seacrest chats with ET's Denny Directo after 'American Idol's live show on Sunday, which aired on ABC.
King Charles celebrates his coronation with a star-studded concert, including performances from Katy Perry, Lionel Richie and more.
