Editor's note: Mark Saunders is a longtime reporter on the royal family and has covered more than 200 official royal engagements. He the author of several books on the royals, including "Prince Harry: The Biography."
(CNN) -- The news the Duchess of Cambridge is expecting a child marks a dramatic departure from previous royal pregnancies. For it is believed even the queen may not have been given the happy news.
Buckingham Palace were forced to make the announcement only after Catherine was taken to hospital suffering from acute morning sickness. Usually proclamations of royal births are posted on the gates at Buckingham Palace only after the 12th week of pregnancy.
But knowing it would be impossible to keep Catherine's visit to hospital a secret, Buckingham Palace took the unusual step of pre-empting press speculation and revealing the news immediately.
The announcement is in stark contrast to Princess Diana's first pregnancy, which was announced some six weeks after the queen had been informed, and after months of intense press speculation.
Royal advisors are being surprisingly open about Catherine's condition, despite recent intrusions into her private life, including the publication of topless photos, which were condemned by Buckingham Palace. It is believed a new spirit of openness has been embraced by William and Catherine in an attempt to prevent the massive press interest and speculation that surrounded Princess Diana.
Prince William drove his wife to hospital earlier today and spent hours by her bedside. He left tonight looking tense but happy. Catherine is expected to stay in hospital for several days.
Sources said the couple are "incredibly happy but nervous." In a statement on their website William and Catherine said they were "delighted with the news."
It is unclear when senior members of the royal family, including the queen, were told about the pregnancy -- but a palace insider says Catherine's parents were already aware their eldest daughter was expecting because she was taken ill whilst staying at their family home in Berkshire this weekend.
Catherine's pregnancy could turn out to be one of the most significant in the history of the British royal family, for the child will accede to the throne even if she is a girl.
The child will not be subject to the centuries old law of primogeniture, which puts male heirs ahead of women.
Britain's Prime Minister David Cameron has agreed a deal to change the rules on the royal line of succession, which means male heirs will no longer be given priority.
News of the pregnancy was met with joy throughout the UK. Within minutes of the announcement being published on the royal couple's website it had crashed under the weight of traffic.