Prince William and Kate are expecting their third child.
A statement from Kensington Palace said: "Their Royal Highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting their third child.
"The Queen and members of both families are delighted with the news.
"As with her previous two pregnancies, the Duchess is suffering from hyperemesis gravidarum.
"Her Royal Highness will no longer carry out her planned engagement at the Hornsey Road Children's Centre in London today. The Duchess is being cared for at Kensington Palace."
The announcement comes days before four-year-old Prince George is set to start school.
The new baby will be fifth in line to the throne, after Prince Charles, Prince William, Prince George and Princess Charlotte.