The Duchess of Cambridge took on the title of "Dancing Queen" when she was caught on camera twirling with Paddington Bear on Monday morning.
A statement issued by Kensington Palace said: "The Duke and Duchess of Cambridge are delighted to confirm they are expecting a baby in April 2018".
The mother-of-two has been keeping a low-profile in recent weeks as she battles severe morning sickness.
The Duke and Duchess of Cambridge's third royal addition will arrive in April 2018, Kensington Palace confirmed on Twitter Tuesday. The Duchess was forced to put on a smile and play along even though she's been feeling miserable at home with her pregnancy.
Middleton made the surprise appearance on Monday at Platform 1 at Paddington station in London alongside the cast members of the upcoming "Paddington Bear" movie.More news: Gal Gadot in Talks to Star in Revenge Thriller Ruin
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Now we know when Kate's baby is due, the attention will inevitably turn to gender! Mahira, who described the royal encounter as "mind-blowing", said: "I asked her what's it like to be a princess, because I'd like to be one, and she said her favourite thing was meeting special people like us".
Will it be a boy or a girl?
Prince William and Duchess Catherine - who are already parents to Prince George, four, and Princess Charlotte, two - have confirmed 35-year-old Catherine's due date following them announcing that she was pregnant again in September.
The baby will become fifth in line to the throne, behind big sister Charlotte, bumping William's brother Prince Harry further down the line of succession.