The Duke and Duchess of Cambridge are expecting their second child.
However, she has not been taken to hospital and is being treated at Kensington Palace. Depending on her level of sickness, she may require supplementary hydration, medication and nutrients.
A statement said the Queen and members of both families were delighted with the news. It isn't yet known when the baby is due.
Clarence House said: "Their Royal Highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting their second child."
Prime Minister David Cameron tweeted he was 'delighted by the the happy news that they're expecting another baby'.
The Duchess will no longer accompany Prince William on their planned engagement in Oxford on Monday, Clarence House said.
You can read more details of Kate's pregnancy news here.
More on Parentdish: Prince William says he's 'immensely thrilled'
More:Pregnancy Week By Week
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