The royal household broke the happy news via their Twitter, writing: “Queen & members of both families are delighted with the news that Their Royal Highnesses are expecting their second child.”
They wrote in an official statement: "Their Royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their second child.
"The Queen and members of both families are delighted with the news.
"As with her first pregnancy, The Duchess is suffering from Hyperemesis Gravidarum. Her Royal Highness will no longer accompany The Duke of Cambridge on their planned engagement in Oxford today. The Duchess of Cambridge is being treated by doctors at Kensington Palace."
The news comes following speculation that Kate had fallen pregnant.
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The baby will be a younger brother or sister for their son Prince George, who was born last summer.
The child will be fifth in line to the throne after Prince Charles, Prince William and Prince George.
According to reports, Kate is under the twelve week mark.
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Kate was pictured hiding what appeared to be a budding baby bump during an recent royal appearance alongside husband William.
The birth of their first child sparked a global media frenzy, with reporters from around the world setting up camp outside St Mary's Hospital in London as they waited to catch the first glimpse of baby George.
Both Kate and William have yet to make a statement on their happy news.
Congratulations Kate and Wills!