Kate Middleton misses George's first day of school

Kate Middleton was absent at Prince George's first day of school because she's still suffering badly from hyperemesis gravidarum – severe morning sickness.

While the Duchess had wanted to be there on such a special "first", her pregnancy woes have continued past the week she and Prince William hoped they would.

Prince George appeared as nervous as any other four-year-old on his first day of school today at Thomas’s Battersea in London.

Wearing his tiny but dapper uniform, Prince George was accompanied by dad Prince William on the big day.

"The Duchess wanted more than anything to be there. It's a big day for her son. But she has been really very sick this week," a royal source told the Daily Mail.

On Tuesday – a day after it was announced they are expecting their third child – William told attendees at a mental-health conference that Kate was well, but admitted, "There's not much sleep going on at the moment."

"We need Catherine to get over this first bit and then we can start celebrating," he said.

"It's always a bit anxious to start with, but she's well."

His new school, where fees cost from GBP17,604 ($A28,820) a year, has been described by the Good Schools Guide as, "A big, busy, slightly chaotic school for cosmopolitan parents who want their children to have the best English education money can buy."

George and his classmates will be taught a range of subjects and activities from ballet and art to drama, French, music and physical education.

Kate has already said she is not sure her son "has any idea what's going to hit him" when he starts school.

The Duke and Duchess of Cambridge are overjoyed to be expecting their third child, but for Kate Middleton, the pregnancy will take a severe toll.

In a statement released by Kensington Palace announcing the news on Monday, while "delighted", the Duchess, "as with her previous two pregnancies... is suffering from Hyperemesis Gravidarum."

Hyperemesis gravidarum is acute morning sickness that develops during pregnancy. It causes women to suffer from severe nausea and vomiting, and sometimes requires the pregnancy to be managed with supplementary hydration, medication and nutrients.

The Duchess endured the condition during both pregnancies with Prince George and Princess Charlotte.

The condition is not common, affecting about one in 100 soon-to-be-mothers. Symptoms are usually at their worst between four to seven weeks but can last up to 15 weeks in some pregnancy cases.

Some women may even suffer with the condition up until the very end of their pregnancy.

In the past, Kate and Kensington Palace have been forced to make the announcement early in the Duchess' pregnancies due to the princess cancelling public engagements because of the debilitating illness.

This has also been the case in the latest pregnancy, with the statement confirming: "Her Royal Highness will no longer carry out her planned engagement at the Hornsey Road Children’s Centre in London today. The Duchess is being cared for at Kensington Palace."

Kate also previously kept a low profile up until around the 12 week pregnancy mark.

The Duchess was hospitalised when she was pregnant with George due to severe dehydration from hyperemesis gravidarum.

The cause of the condition is unknown although it is thought to be caused by hormonal changes in pregnancy.

In guidelines published by the UK Royal College of Obstetricians & Gynaecologists last year Dr Manjeet Shemar, lead author of the guidelines said: “Women suffering from nausea and vomiting and hyperemesis gravidarum can face a challenging time in early pregnancy. The more severe the condition, the more it can affect their day-to-day quality of life and mental health."

The Duke and Duchess wed in 2011 and are already parents to Prince George, four, and two-year-old Princess Charlotte.

UNDERSTANDING HYPEREMESIS GRAVIDARUM

WHAT IS IT?
A severe form of nausea and vomiting affecting pregnant women that can put the mum and baby at risk because the woman can't retain and utilise food and fluids.

SYMPTOMS
Constant nausea and severe vomiting that can lead to dehydration and cause an imbalance in electrolytes. Women can lose more than five per cent of their body weight as well as suffer headaches, fatigue, confusion, fainting and jaundice.

HOW COMMON IS IT?
It's estimated to affect about one per cent of pregnant women.

WHEN DOES IT START?
Usually between the first four to six weeks and symptoms don't usually improve until between 15 and 20 weeks but can last the entire pregnancy.

TREATMENT
Women with milder forms are advised to change diet, rest and take antacids. With more severe forms women are hospitalised so they can have their food and liquids closely monitored and usually require an intravenous drip.

CAUSES
Experts still don't fully understand the causes but experts describe it as a complex physiological disease with multiple causes. They say aggressive care early in pregnancy is vital to prevent life-threatening complications such as central pontine myolinolysis or Wernicke's encephalopathy.

(Source: HER Foundation, Mater Private Hospital, Brisbane)

Additional reporting by AAP.

Want more celebrity, entertainment and lifestyle news? Follow Be on Facebook, Twitter, Pinterest, Tumblr and Instagram