According to a press release from Clarence House:
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.
Babies are fun. Throwing up for the length of the pregnancy is probably not.