It is much more serious than the nausea commonly experienced by expectant mothers.
The condition is thought to be caused by elevated levels of "pregnancy hormone" HCG, or human chorionic gonadotropin, which increases after conception.
Miss Leila Hanna, Consultant Gynaecologist and Obstetrician at Queen Mary's Hospital in London and BMI The Sloane Hospital, told MailOnline: "The condition is extremely common in the first 12 weeks of pregnancy.
"It is associated with the changes in hormonal levels in the body, where the pregnancy hormones are quite high and it is the effect of those on the expectant mother.
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"If anything it is associated with a normal, healthy pregnancy. Every so often, in extreme cases it is necessary to scan the mother to see if she could be expecting twins.
"In twins expectant mothers experience twice the hormones, and so often twice the sickness."
But Miss Hanna moved to dispel the idea the Royal couple could be preparing to welcome twins, saying it is "highly likely" Kate will have already been scanned.
She said the condition is usually diagnosed around the six week mark and in most cases carries on until 12 weeks.
"The majority of ladies feel much better after 12 weeks, but a small group do carry on suffering the symptoms well into their second trimester, up to 24 weeks," Miss Hanna told MailOnline.
Indeed the Duke of Cambridge indicated his wife may be over the worst of her morning sickness in a "few weeks' time" when he appeared in Oxford today.
Watch: Prince William: 'Thrilled' by second baby
William suggested that Kate may be able to resume royal duties in a fortnight as he opened Oxford University's new £21 million China Centre without his wife.
When a well-wisher sympathised with Kate's condition and said they wished she could have attended, the Duke replied: "I know, she wishes she could be here - a few weeks' time."
Hyperemesis gravidarum leads to severe dehydration and puts both mother and baby at risk of being deprived of essential nutrients.
Sufferers can be left vomiting up to 30 times a day, with exhausting and hazardous consequences.
They cannot eat or drink without retching and may lose up to 10 per cent of their body weight.
This can trigger a build-up of toxins in the blood or urine known as ketosis as the body tries to compensate for lack of food.
Hospital treatment for these women is essential, as without intravenous feeding and fluids they are at risk of becoming dangerously dehydrated.
In less severe cases the condition can be managed with supplementary hydration, medication and nurtrients.
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Miss Hanna said Kate is more susceptible to suffering hyperemesis gravidarum, having endured the acute sickness during her pregnancy with Prince George.
She said: "If you have suffered this condition before, it is more likely you will again.
"People react to things in different ways and if you have already reacted once to the raised hormone levels in the way she has, it is quite normal during subsequent pregnancies."
She added that an expectant mother would only be treated for hyperemesis gravidarum when she shows signs of the condition, so it is unlikely the Duchess's medical team would have pre-preemptively treated her.
She said in most cases pregnant women are given advice to avoid spicy foods and eat a diet of bland food including toast, bananas and mashed potato, little and often.
"Women suffering the condition are at risk of dehydration and so we often advise they keep their water levels up and avoid fizzy drinks," said Miss Hanna.
For the majority of women these simple measures will make them feel better, but in some more extreme cases we would prescribe anti-sickness medication, something like Stemetil or Maxolon (metoclopramide).
"If, despite everything, the expectant mother is not keeping anything down, and is suffering bloating, they are often taken into hospital where they can be closely monitored, given regular fluids and more regular anti-sickness drugs.
"In very extreme cases, steroids can be given."
Hyperemesis gravidarum can cause serious complications for mother and baby.
Those who are hospitalised, as the Duchess was with Prince George, before 12 weeks are around 20 per cent more likely to be at risk of pre-eclampsia, according to some studies.
But Miss Hanna, who has 35 years of experience and has been a consultant for the last 25 years, said the risk of any complications for Kate is reduced given the fact she did not experience any during her first pregnancy.
"She did very well during her first pregnancy, and as a result there is no reason to think anything will be different this time around.
"Second pregnancies are kinder and easier on the body, because the body has done it before.
"Kate will know what to expect and there is no reason she will face any complications."
Dr Annabel Bentley, medical director at Bupa health funding, added: "Most pregnant women feel sick and may vomit in early pregnancy.
"Rarely, women can experience severe and repeated vomiting, leading to dehydration, dizziness and weight loss.
"This is called hyperemesis gravidarum, which affects less than one in 100 women. Women who have had this previously are more likely to get it again in another pregnancy.
"The treatment is to get plenty of rest and try to drink if possible. Some women may need treatment with fluids through a drip in hospital.
"It usually gets better in three months or so, and should not affect an otherwise healthy pregnancy."
Consultant obstetrician Daghni Rajasingam, of the Royal College of Obstetricians and Gynaecologists, said: "In very simple terms hyperemesis means vomiting a lot and gravidarum means in pregnancy.
"The diagnosis is given when women cannot keep food or fluid down because she has severe vomiting.
"The women who are vomiting pretty much constantly, who cannot keep any nutrients down, they need to be admitted to hospital."
Dr David Williams, consultant obstetric physician at the Institute for Women's Health, part of University College London Hospital, said: 'It's a really miserable condition.
Catherine, Prince William, and Prince George meet a Bilby called George at Taronga Zoo. Photo / Getty Images
"Pregnancy should be a joyful happy time but these women feel dreadful. Most are affected in early pregnancy and the worst should be over by week 16, but for some it goes on for much longer."
Dr Williams said he tends to keep newly diagnosed women in hospital for several days to ensure their condition is stabilised, adding: "There is a risk of discharging them too soon and then the sickness comes straight back."
Treatment typically includes an injection of the drug heparin to protect against blood clots triggered by dehydration, as well as supplementation of vitamin B, one of the vitamins most depleted by the condition.
An intravenous infusion of saline for rehydration is standard practice. Anti-emetic, or anti-sickness, drugs that are commonly given include metoclopramide and Stemetil.
These may be used in hospital or when the woman is discharged to help prevent recurrent attacks.
Steroids are a useful drug, often in the form of oral prednisolone tablets, Dr Williams said.
Another drug that can be used where necessary is ondansetron, originally developed to combat sickness caused by cancer agents, he added.
Dr Williams continued: "Hyperemesis sufferers often suffer from acid reflux, where stomach acid keeps repeating up the throat. A drug called omeprazole, also known as Losec, can be used for this.
"Although none of these drugs have been licensed for pregnant women - drug companies are reluctant to do this - we have a solid body of experience because sickness in pregnancy is relatively common, albeit not usually at the extreme end of the spectrum."
Kensington Palace were once again forced to reveal details of the new Royal baby because of her condition.
The Duchess was due to accompany her husband at an engagement in Oxford today, but pulled out due to her condition.
Unlike when she was pregnant with Prince George, Kate is being treated by doctors at Kensington Palace and has not been admitted to hospital.
In a statement, Kensington Palace said: 'Their Royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their second child.
"The Queen and members of both families are delighted with the news. As with her first pregnancy, The Duchess is suffering from Hyperemesis Gravidarum.
"Her Royal Highness will no longer accompany The Duke of Cambridge on their planned engagement in Oxford today. The Duchess is being treated by doctors at Kensington Palace."
A senior royal source said: "The couple only found out very recently.
"The Duchess has not even reached the crucial 12-week stage but when it became apparent that she was not well enough to join the Duke on a joint engagement in Oxford today and that the number of forthcoming engagements could be affected, the Duke and Duchess have taken the difficult decision to be fully open about the pregnancy."
- Daily Mail
