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The Duke and Duchess of Cambridge had hoped to keep news of their expected first child under wraps until Christmas Day, UK media report.
Today, Prince William and 30-year-old Catherine - widely known as Kate - drove to King Edward VII Hospital in central London hospital after Kate fell ill with acute morning sickness.
The Daily Mail reported she was on a drip after being admitted in the afternoon (local time). William spent hours at his wife's side and left the hospital to a hail of flash photography.
Weeks of speculation and rumours were put to rest this morning when St James's Palace confirmed the royal couple were expecting a baby. It also said that Kate was admitted to hospital with Hyperemesis Gravidarum, an acute morning sickness which causes severe nausea and vomiting and requires supplementary hydration and nutrients.
She is expected to remain in hospital for several days.
The Daily Mail said the couple had hoped to keep the news to themselves until December 25, but were forced to bring the announcement forward after Kate was hospitalised.
William drove his wife to the hospital just after lunch (local time) and she was put on a drip immediately, the Mail reported.
TWINS A POSSIBILITY
"Their Royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting a baby," the prince's office said in a statement, adding that Queen Elizabeth and the royal family were delighted.
Kate is thought to be eight weeks pregnant. Eight weeks ago she was visiting the Solomon Islands with William.
The Daily Mail also reported the couple only found out they were expecting ''very recently'' and added the Queen was not told about the pregnancy until Kate was admitted to hospital.
The couple, officially known as the Duke and Duchess of Cambridge, married in April last year, amid a global media frenzy and there has been much speculation, particularly in US gossip magazines, about a possible pregnancy.
"It's only been a matter of time. Everyone has been waiting for Kate to announce that she was pregnant," Claudia Joseph, who has written a biography of the duchess, said.
Professor Tim Draycott, a consultant obstetrician at the University of Bristol, said the condition was common in the early weeks of pregnancy but did not put the baby at any increased risk, although in extreme cases it can lead to the baby being born with a slightly low birth weight.
Draycott said it may also indicate more than one royal baby may be in the offing.
"Hyperemesis is slightly more common with twins," said Draycott, explaining that the condition affected about one in 100 to 200 pregnant women.
There was no detail about when the baby was due, although the prince's spokesman said Kate was less than 12 weeks pregnant.
"I'm delighted by the news that the Duke and Duchess of Cambridge are expecting a baby," Prime Minister David Cameron said on his Twitter website. "They will make wonderful parents."
New Zealand Prime Minister John Key and Governor-General Sir Jerry Mateparae have both offered their congratulations.
WAS THE SECOND HONEYMOON KEY?
It is all a question of timing and discretion, perhaps, but more than a few people have worked the numbers - and the next heir to British throne was possibly conceived in the South Pacific.
And for the really intrigued, there are pictures already of the bed that the Duke and Duchess of Cambridge slept on two months ago.
The royal couple were at that time touring the Solomon Islands and Tuvalu celebrating Queen Elizabeth's Diamond Jubilee.
On September 17 they went to the lost and little known Tavanipupu Island at the south eastern end of Guadalcanal. It was billed by Fleet Street as a second honeymoon for the two.
Palo Carr, manager of the Tavanipupu Private Island Resort, is saying little.
"Fantastic news for the couple, and of course we at Tavanipupu would hope this Island and its very romantic settings is responsible is some way," he said.
The island was a 19th Century coconut plantation owned by Norwegian traders - they bought it for a handful of ammunition.
Tavanipupu is a dream created by English interior decorator Dennis Bellote and his late partner Keith Paske in the 1970's.
During the ethnic tensions of the 1990s which saw civil war threaten on Guadalcanal, a small resort was little used and almost forced to close.
The royal couple stayed in a villa complete with a king sized bead, on the edge of Iron Bottom Sound.
For ordinary folk it costs A$1200 (NZ$1523) a night.
BABY WILL BE KING, OR QUEEN
William, Queen Elizabeth's 30-year-old grandson, is second in line to the British throne, and their first child will become the third in succession when he or she is born.
Last year Britain and other Commonwealth countries which have the queen as their monarch agreed to change the rules of royal succession so that males would no longer have precedence as heir, regardless of age.
The agreement also means an end to a ban on a future monarch marrying a Catholic, a stipulation dating back some 300 years.
Britain's royal family are currently riding the crest of popularity on the back of William and Kate's wedding and the queen's diamond jubilee this year which has witnessed nationwide celebrations.
"It's something everyone can look forward to, just like their wedding brought the whole nation together," said Johanna Castle, 25, a sales assistant in an east London homewear and fashion store.
The young royal couple have become global stars after some two billion people tuned in to watch their glittering marriage ceremony and the sumptuous display of pageantry that accompanied it, and barely a day goes by without a picture of Catherine appearing in the pages of Britain's royalty-obsessed newspapers.
The duchess, the first "commoner" to marry a prince in close proximity to the throne in more than 350 years, is now a fashion icon, with her attire scrutinised every time she steps out in public and followed by legions of women around the world.
US President Barack Obama and his wife Michelle were one of the first to send congratulations, an indication of the young royals' popularity across the Atlantic.
"I know they both feel that having a child is one of the most wonderful parts of their lives. So I'm sure that will be the same for the Duke and Duchess of Cambridge," said White House spokesman Jay Carney.
With their fame has come unwanted attention, and there was anger in Britain when topless photos of Kate relaxing on holiday were published in a French magazine in September.
The pictures rekindled memories of the media pursuit of William's mother, Princess Diana, who was killed in a car crash in Paris in 1997 while being chased by paparazzi.
"I will be very surprised if this isn't handled with the utmost tact and sensitivity," said media commentator Steve Hewlett. "Newspapers realise there's a huge amount of goodwill towards Will and Kate, and they take their cue from their readers."
'DADDY'S LITTLE CO-PILOT'
Kate made her last public appearance on Friday when she returned to her old school - a minor event that nonetheless generated live television coverage on news channels - when she looked healthy and joined in a game of hockey with pupils.
Earlier in the week William had hinted at a pregnancy during a visit to Cambridge in central England when they were given a home-made baby suit emblazoned with the words "Daddy's little co-pilot", a reference to William's job.
"When I gave it to him he said 'I'll keep that', and handed it to his aide," said Samantha Hill.
Joseph, author of "Kate: The Making of a Princess", said she believed the couple, who currently live in north Wales where the prince is based as a search and rescue pilot, had been waiting for the right moment to have a baby.
"My feeling has always been that they were not going to take the spotlight away from the queen in her Jubilee. But now 2013 is going to be William and Kate's year," she said, adding the couple would make wonderful parents.
"We have seen her with children and she is lovely with them, she's got the natural touch, and her parents run a party business and she has spent a lot of time with children," Joseph said. "(William) he has always talked about wanting children, so I am sure he is delighted."
NAME GAME STARTS
Bookmakers were quick to accept bets on possible baby names, with Elizabeth, Frances, John, Charles and Diana the early front runners among punters.
Bookmaker William Hill has odds of 9/1 for Frances and John with Charles, Victoria and George at 10/1. Ladbrokes, meanwhile, had Elizabeth, after her great-grandmother, the Queen, as the 8/1 favourite, with Frances, John and Charles all on 10/1.
William and Kate currently live in north Wales where the prince is based as a Royal Air Force search and rescue pilot.