Prince William spent hours at Kate Middleton’s bedside this evening after driving her to hospital with acute morning sickness.

Duchess of Cambridge, 30, is suffering from hyperemesis gravidarum – a rare and debilitating condition that hits women in the earliest stages of pregnancy, causing severe vomiting.

Prince William drove Kate Middleton to hospital as she began to feel very unwell over the weekend at her parents’ home in Bucklebury, Berkshire.

He was later seen leaving the hospital at around 8:25 p.m. before getting into a waiting Land Rover Discovery.

Kate wife has been given a drip to restore her nutrient levels and fluids and will be kept in for a few days as a precautionary measure, royal sources said.

The Royal couple are said to be “delighted” about the pregnancy, but appear to have been forced to announce it early because she was rushed to the King Edward VII Hospital in central London.

“The condition – although unpleasant -poses no harm to mother and baby as long as it is treated, as your baby will take the nutrients it needs from your body – it’s the mum that suffers more,” Jules Robertson, a midwife for the baby charity Tommy’s said.

Senior Royal sources say there was no way they would have announced the pregnancy at this stage had she not fallen ill.

They are happy but are very nervous because it is so early in the pregnancy, the source added.

Kate Middleton’s slim figure is not a reason why she has become poorly.

But it can be a hereditary condition, meaning it is fairly likely that a close relative like her mother or grandmother could have suffered from the illness and she may get it again in future.

Professor Tim Draycott, of the Royal College of Obstetricians and Gynaecologists, said: “It definitely does seem to run in families, and in consecutive pregnancies. It is possible someone in her family has suffered with it in the past.

“We know it’s not a psychological illness – it is not a sign of mental weakness.

“It does seem to be slightly more common in women expecting twins.

“It affects one per cent of women. That rises to two to three per of women expecting twins.

“She’s clearly very slim, but it affects women bigger than her and smaller than her.

“It is a bit like feeling permanently hungover every single day and never getting better.

“The symptoms would suggest she is less than 12 weeks pregnant.”

Kate Middleton is suffering from hyperemesis gravidarum, a rare and debilitating condition that hits women in the earliest stages of pregnancy, causing severe vomiting

The condition, which could mean that Kate Middleton cannot eat or drink anything without being ill, usually occurs in the first 6 to 8 weeks of pregnancy and can last for up to two months.

In the most serious cases symptoms can persist for the length of the pregnancy and can be life-threatening for mother and baby if not treated early as the woman may vomit up to 30 times a day.

Symptoms also include severe nausea, low blood pressure and fast heart rate, headaches, lethargy or confusion.

Kate Middleton’s involvement in Royal engagements may now be limited as a result.

“If it is not treated properly it can lead to severe side effects and problems for mother and baby,” Dr. Lucy MacKillop, obstetric physician at the Oxford University Hospitals trust.

“The danger for mother is greater if not treated. There have been reported cases of liver failure, ruptured tissue and cells from so much vomiting and neurological problems.

“It is to do with the pregnancy hormone, and there is more pregnancy hormone in mothers expecting twins.”

Spelling out the treatment Kate Middleton is likely to be getting Dr. Lucy MacKillop added: “We would give the mother medication which will be safe for both her and baby, and which will allow her to stop vomiting.

“Initially she will be on a drip so we can replace fluids and nutrients and then we can start to give her tablets, which she can take home with her and continue there.

“There is no special diet, but she needs to ensure she is eating little and often.

“We would not expect her to be putting on weight, but the important thing is that she maintains her weight.

“Women with this condition can lose five per cent of their body weight, so she will need to have weekly weight measurements.

“The most important thing is to get mother to stop being sick and get home.”

The condition is thought to be caused by elevated levels of the “pregnancy hormone” hCG. The body begins to produce human chorionic gonadotrophin (hCG) after conception.

Mothers who have also suffered from hyperemesis gravidarum said that Kate Middleton will be having a tough time.

Caitlin Dean suffered from the condition and said: “Imagine having a stomach bug that lasts for days and days, months and months – it is just relentless.

“Any movement, any sound, any smell just makes you vomit.

“I vomited 20 to 30 times a day for the first few months, in the latter part it was just once or twice a day but it’s still unpleasant.

“One of the big issues with it is isolation because is causes many women to be bed-bound. There is a real lack of understanding about the condition.”

But despite being a debilitating illness, it is not usually dangerous, and is often a sign that the pregnancy is going to plan, experts say.

Dr. Peter Bowen-Simpkins, consultant obstetrician and medical director of the London Women’s Clinic, has said: “It is almost always a positive sign that the pregnancy is progressing well.

“The sickness is thought to be due to a rise in hormone levels. It normally occurs during weeks six and eight of pregnancy, when the placenta takes over production of hormones from the ovaries.

“It generally continues until around 12 or 14 weeks, but if it stops before, it can – although not always – be a sign that all is not well.”

Jules Robertson, a midwife for the baby charity Tommy’s, said: “If pregnant women are vomiting several times during the day and unable to keep food or drink down, then they will need to be treated to help them cope with the symptoms, and stay nourished and hydrated.

“We advise that anyone experiencing this should see their GP or midwife to be checked. It is more common in early pregnancy, and by around 14-16 weeks, the sickness should have gone – most of the time it is resolved by 20 weeks, and very occasionally it can continue throughout pregnancy.”

Kate Middleton and Prince William’s baby will be born third in line to the throne. The Queen, Prince Philip and other members of the royal family are also “delighted”.

British Prime Minister David Cameron said on Twitter: “I’m delighted by the news that the Duke and Duchess of Cambridge are expecting a baby.”

In a statement on the couple’s website, they 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 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.”