The Duchess of Cambridge is beginning a second day in hospital for treatment for severe morning sickness.The duchess, who is thought to be under 12 weeks pregnant, is expected to remain at King Edward VII hospital in central London for several days.
The Duke of Cambridge spent a number of hours at the hospital on Monday.
Her admission prompted an unexpected announcement of the pregnancy, which has seen the couple receive congratulations from across the world.
The baby will be third in line to the throne after Prince Charles and Prince William.
Families 'delighted' BBC royal correspondent Peter Hunt says the duchess is likely to be receiving fluids intravenously to tackle the acute morning sickness.
Severe morning sicknessIt is not uncommon for pregnant women to experience morning sickness, particularly during the first few months of pregnancy.
But some women (around one in every 200) experience severe nausea and vomiting, which continues throughout pregnancy.
This condition is known as hyperemesis gravidarum or HG and needs specialist treatment and often requires a stay in hospital.
The main danger is dehydration - it can be difficult to keep enough fluid down orally.
Doctors can treat HG with medication to help ease the nausea. HG is unlikely to cause harm to your baby. However, if it causes weight loss during pregnancy there is an increased risk that your baby may be born with a low birth weight.
It is understood that Catherine is being cared for by gynaecologist Marcus Setchell, who delivered the Countess of Wessex's two children.Catherine and William, who are both 30, were married at Westminster Abbey in April 2011.
The duchess was last seen in public on Friday when she visited her old school, St Andrew's, in Pangbourne, Berkshire. During her visit she was involved in a brief game of hockey.
In a statement on Monday, St James's Palace said members of both the Royal Family and the Middleton family were "delighted with the news".
It said the duchess was suffering from hyperemesis gravidarum, which requires supplementary hydration and nutrients.
The conditions causes severe nausea and vomiting and the main danger is dehydration.
"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 statement added.
St James's Palace would not be drawn on when the royal couple had become aware of the pregnancy, only saying "recently", but it is understood the palace announcement was prompted by the duchess's medical condition.
|Prince William & Kate|
Prince William's brother, Prince Harry, who is serving with the Army in Afghanistan, is thought to have been told in an e-mail.
Prime Minister David Cameron wrote on Twitter that he was "delighted by the news...They will make wonderful parents."
Labour leader Ed Miliband also tweeted: "Fantastic news for Kate, William and the country. A royal baby is something the whole nation will celebrate."
Succession change Among those to offer congratulations from further afield were US President Barack Obama and his wife Michelle.
Commenting on the duchess's condition, Dr Peter Bowen-Simpkins, medical director at London Women's Clinic, told BBC 5 Live: "People who get it get intractable vomiting and may lose as much as 10% of their body weight and become very dehydrated.
"When this occurs the simple treatment is to get fluids into them - and usually they feel considerably better."
Dr Bowen-Simpkins also said the condition was "more common with a multiple pregnancy," adding the couple "would know by now whether there were twins there or not".
In October 2011, Commonwealth leaders agreed to change succession laws so that the daughter of any future UK monarch will have the same right to the throne as a son. Under current laws, if a girl was born first to the couple, she would lose her place in line to the throne if a boy was born later.
The law has not yet been altered but a Cabinet Office spokesman said formal consent was expected "very shortly" and ministers have indicated that the legislation will apply to any royal births from the date of the leaders' decision.
The monarch is also head of the armed forces, supreme governor of the Church of England, head of state of the UK and 15 other Commonwealth countries.
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