In an announcement from Kensington Palace, the royal family is "delighted with the news".
It's already a big week for the family as they prepare for George to start school at Thomas's Battersea in south London on Thursday.
"Their royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their third child", Kensington Palace shared in a statement posted to Twitter.
Awareness of her tendency to the condition meant her second pregnancy was announced relatively early, rather than waiting for the 12-week milestone, as she cancelled public duties due to sickness.
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Together with Prince Harry, William and Kate have taken a leading role in marking the 20th anniversary of Diana's death.
George and Charlotte have since appeared on the front covers of magazines around the world and traveled on official royal tours of Poland and Germany with their parents.
It was always thought that the Duke and Duchess of Cambridge would go on to have three children.
Catherine is once again suffering from Hyperemisis Gravidarum which causes severe nausea, vomiting, electrolyte disturbance and weight loss, and this may lead to fewer royal engagements.