Kate is expecting her second child, Kensington Palace announced this morning.
The Duchess (32) is suffering from acute morning sickness and is currently being treated by doctors at her Kensington Palace apartment.
The royal announcement comes after Kate was forced to pull out of an engagement in Oxford earlier today.
"The Queen and members of both families are delighted with the news that their Royal Highnesses are expecting their second child," the palace said in a statement.
A senior royal source told The Daily Mail that the couple only found out "very recently".
"The Duchess has not even reached the crucial 12-week stage. But when it became apparent that she was no well enough to join the Duke on a joint engagement in Oxford today and that the number of forthcoming engagements could be affected, the Duke and Duchess have taken the difficult decision to be fully open about the pregnancy."
Kate also suffered from severe morning sickness, or hyperemesis gravidarum, while pregnant with Prince George, now 13 months. In early December last year, she was admitted to hospital, prompting the palace to announce she was expecting.
PM: Many congratulations to the Duke and Duchess of Cambridge. I'm delighted by the happy news that they're expecting another baby. — UK Prime Minister (@Number10gov) September 8, 2014
Additional sources: theguardian.com, dailymail.co.uk