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The Duchess of Cambridge’s extreme morning sickness has given rise to speculation that she may be carrying twins.
The Duke and Duchess were forced to go public with the news much earlier than intended after the 30-year-old Duchess was admitted to hospital with hyperemesis gravidarum, a form of severe morning sickness that is also associated with multiple births.
Mothers-to-be who suffer from the condition are three times more likely to have a multiple birth than other women. The average likelihood of twins in the overall population is about 3 per cent, though your genes play a significant part.
Twins have never been born into prominent positions in the British line of succession before.
Were the duchess to bear more than one child next year, the first to be born would take a higher place in the line.
Last year, Crown Princess Mary of Denmark gave birth to twins, a boy and a girl. Before the birth, there was speculation that Princess Mary would have a caesarean section delivery, leading to reports that the presiding doctor would effectively be choosing the babies’ position in the line of succession.
The babies were eventually delivered without surgery. The boy, Prince Vincent, was delivered around 25 minutes before his sister, Princess Josephine, and therefore took a higher place in the Danish line of succession.
The Duchess, who found out last month that she was expecting, has not yet reached the 12-week stage of her pregnancy, when couples usually share their news.
She was taken ill while staying with her parents in Bucklebury, Berkshire, and drove with her husband to the King Edward VII Hospital in London, where she is expected to remain for several days.
Hyperemesis gravidarum causes excessive, persistent vomiting and nausea which can linger for an entire pregnancy. Left untreated, hyperemesis patients lose weight because they are unable to keep food or drink down. It drives some women to terminate and has even been blamed for the death of English novellist Charlotte Bronte at four months' pregnant.
Sydney GP and sufferer Dr Melinda Griffiths likens the feeling to a continuing bout of gastroenteritis. It's like being on the cusp of vomiting, without respite.
I felt well enough for five, 10 minutes and then it would come back.
"You can feel your mouth drooling, you feel really, really nauseous. You can't get comfortable. It feels like you're stuck at that point because even after a vomit you might get relief but if you do it will be short-lived. I felt well enough for five, 10 minutes and then it would come back. It just felt like being caught in that stage for week after week," Dr Griffiths says.
The Duke of Cambridge spent yesterday at her bedside. He was seen leaving the hospital at 8.20pm and is expected to be granted compassionate leave from his RAF Search and Rescue duties.
The Duchess's illness prompted a scramble to tell members of the Royal family about the pregnancy before news of her arrival at hospital leaked out.
Royal aides said the decision to go public "was very much driven by the Duke and Duchess", who were aware that it would be impossible to keep the news a secret in the age of Twitter. "It's very hard thing to go public at such an early stage, but they wanted to be open with people as much as possible," said an aide.
The Queen, the Prince of Wales and Prince Harry, who was contacted in Afghanistan, were all told the news yesterday, before St James's Palace made the formal announcement at 4.01pm.
A spokesman for St James's Palace said: "Their Royal Highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting a baby. The Queen, the Duke of Edinburgh, the Prince of Wales, the Duchess of Cornwall and Prince Harry and members of both families are delighted with the news.
"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."
The announcement was followed 10 minutes later by a tweet from Clarence House, the first time a royal pregnancy has been confirmed on Twitter.
The Duchess's illness, which affects fewer than four in every thousand pregnant women, causes severe vomiting and can lead to dehydration. It is treated by giving fluids intravenously and by anti-sickness tablets but can last for months.
The Duchess is reportedly being treated by Marcus Setchell, the former gynaecologist to the Queen, who delivered the Countess of Wessex's two children as well as Leo Blair, the son of the former prime minister.
David Cameron, who was handed a note to tell him the news during a 2pm policy meeting in Downing Street and who was among the first to tweet his congratulations, said: "It's absolutely wonderful news, and I'm delighted for them and I'm sure they will make absolutely brilliant parents."
He admitted that after he received the note "I found it quite difficult to keep it to myself".
President Barack Obama was among the world leaders to send their congratulations, while the Archbishop of Canterbury, who married the couple on April 29 last year, said: "The whole nation will want to join in celebrating this wonderful news."
Earl Spencer, the Duke's uncle, said: "It is wonderful news and I am thrilled for them both."
The baby will become the third in line to the throne, ahead of Prince Harry, regardless of whether it is a girl or boy. The ancient rule of royal primogeniture, which gave males precedence, was scrapped last year.
Elizabeth was installed as the bookies' favourite for a name if the baby is a girl, with Ladbrokes quoting 8/1, followed by Frances, John and Charles, each at 10/1, and Diana at 12/1.
The Duchess, who will be 31 in January, appeared to be suffering no ill-effects when she visited her old school in Pangbourne, Berkshire, on Friday, when she played hockey in high heels. Her immediate diary of engagements has been cancelled and it is thought unlikely that she will carry out any official duties before Christmas.
The timing of the pregnancy led to speculation about where and when the baby was conceived. The fact that the Duchess is not yet 12 weeks pregnant means the baby could have been conceived during the couple's tour of the Far East in September. The baby could be born near to the Duke's 31st birthday on June 21.
He fuelled speculation of a pregnancy in Cambridge last week when he was given a babygrow with the words "Daddy's little co-pilot" on it and said: "I'll keep that."