KATE MIDDLETON will spend several days in hospital with severe morning sickness after it was revealed yesterday she is pregnant.
There is also speculation that the Duchess could be expecting twins after she was hospitalised with a condition that is often associated with multiple births.
Months of speculation about a royal pregnancy were ended yesterday afternoon when William and Kate told the world they will be parents for the first time.
But the excitement about the eagerly-anticipated announcement was tinged with concern after it was revealed the Duchess of Cambridge was admitted to a private London hospital for hyperemesis gravidarum - very acute morning sickness. It is a condition often experienced by women expecting twins. Mothers-to-be who suffer from the condition are three times more likely to have a multiple birth than other women.
William was by her side at the King Edward VII Hospital yesterday after she was taken there by car from Bucklebury in Berkshire, where her parents Michael and Carole Middleton live.
The Duke left the hospital shortly after 8.20pm, while Kate is expected to remain for a few days.
The Duchess had a number of engagements this week, including a visit to London's Docklands tomorrow for a charity fund-raising session on a brokers' trading floor, but they have all been cancelled, said St James's Palace.
The Queen's former gynaecologist Marcus Setchell, who delivered the Countess of Wessex's two children, is treating her.
It is understood that the pregnancy has not passed the 12-week point and yesterday's announcement was prompted by the Duchess's medical condition.
For women with hyperemesis gravidarum, vomiting can be so severe that they cannot keep food or liquid down.
The condition usually continues past the first three months of pregnancy and can pass by week 21, but may also last longer.
But while there is concern for the royal couple, there is excitement across Britain and beyond.
British Prime Minister David Cameron led the congratulations when he said: "It's absolutely wonderful news and I'm delighted for them. I'm sure they will make absolutely brilliant parents, and I'm sure everyone around the country will be celebrating with them tonight."
A statement released yesterday read: "The Duchess was admitted this afternoon to King Edward VII Hospital in central London with hyperemesis gravidarum. As the pregnancy is in its very early stages, Her Royal Highness is expected to stay in hospital for several days and will require a period of rest thereafter."
St James's Palace would not say when the royal couple became aware of the pregnancy, only saying "recently".
Congratulations for the Duke and Duchess poured in from around the world.
Julia Gillard, prime minister of Australia, called it "delightful news" and said it would "bring joy to many around the world", while John Key, prime minister of New Zealand, said the nation's women would sympathise with Kate's condition and wished her a speedy recovery.
Jay Carney, press secretary to US president Barack Obama, said: "On behalf of everyone here in the White House, beginning with the President and the First Lady, we extend our congratulations to the Duke and Duchess of Cambridge on the welcome news this morning out of London that they are expecting their first child."
Asked if Mr Obama had any advice as parents, he said: "I haven't had that conversation with them, but 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."
Babies were the topic of conversation when William and Kate visited Cambridge last week for the first time as a married couple.
The Duke, an RAF search and rescue pilot, was given a tiny baby romper suit made by well-wisher Samantha Hill, 27, from Sudbury, Suffolk, which was decorated with a picture of a helicopter and the words "Daddy's little co-pilot".
The baby theme continued when Kate met Tessa Davies, 35, from Cambridge, who had given her son five-month-old James the middle name William in honour of the Duke and had brought him to meet the royal couple.
The Duke and Duchess have made no secret of their desire to start a family and in September during a Diamond Jubilee visit to Singapore the Duke revealed he would like to have two children.
Their first-born will be third in line to the throne, regardless of whether it is a boy or a girl.
"We wish the Duchess the best of health and happiness in the months ahead."
William's uncle Earl Spencer also welcomed the announcement, saying in a statement: "It is wonderful news and I am thrilled for them both."
The baby would have been a first grandchild for the Duke's late mother and the Earl's sister, Diana, Princess of Wales.
Harry, who is serving in Afghanistan as an Apache helicopter co-pilot gunner, was told the good news in an email.
Kate's visit to her old school on Friday afternoon was her last public engagement.
She appeared well and had lunch at St Andrew's school in Pangbourne, Berkshire, with pupils and staff during the two-hour visit.
A spokeswoman for St Andrew's - where Kate showed off her hockey skills in an Alexander McQueen frockcoat and three-inch-high calf-length boots - said they had "no inkling at all" that she was expecting.
Tony Little, headmaster at Eton College, where the Duke went to school, added his congratulations, saying: "We are delighted with the news and wish the Duke and Duchess of Cambridge every happiness."
The sex of the baby is not known but whatever name the royal couple choose for their first child it is likely to set a trend.