The Duchess is understood to have found out last month that she was pregnant, and had not intended to go public with the news until she was at least 12 weeks pregnant.
But after being admitted to hospital with a bout of acute morning sickness, the Duke and Duchess decided to make the announcement early to avoid any speculation about her condition.
Royal sources refused to say how many weeks pregnant the Duchess is, but confirmed she has not yet reached the 12-week stage.
The Duchess, 30, is suffering from hyperemesis gravidarum, which means she needs to be given extra fluids and nutrition, and is likely to remain in hospital for several days.
She was taken ill over the weekend while staying with her parents in Bucklebury, Berks. The Duke was not with her at the time, as he was spending a weekend privately with friends, but he rushed to Bucklebury to join his wife earlier today and travelled with her by car to the King Edward VII hospital in London.