It wasn't just when pregnant with Prince George and Princess Charlotte that Kate suffered severe morning sickness - Kate's suffering again with her third pregnancy.

[Read more: In pics: Prince George starts school]

A Kensington Palace statement said: "Their Royal Highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting their third child.

"As with her previous two pregnancies, the Duchess is suffering from Hyperemesis Gravidarum. The Duchess is being cared for at Kensington Palace."

But rather than severe morning sickness being a rarity, Kate isn't alone.

Around 2% can suffer from hyperemesis gravidarum, a rare but serious disorder that triggers severe vomiting during pregnancy and which needs medical treatment.

[Read more: Prince George: How does he compare to William and Harry at the same age?]

Pregnant women often feel under pressure to continue to work no matter how bad they're feeling with sickness or nausea, points out Justine Roberts, co-founder and chief executive of Mumsnet.

"Sickness during pregnancy is often brushed off as being 'normal', but when you're suffering it, sometimes at all times of the day, that's not much comfort," she says

"There aren't many other situations in which a person suffering from severe nausea and vomiting would be expected to continue as though nothing was wrong - and exhaustion and stress can exacerbate pregnancy sickness."

[Read more: Travel sickness: Why you get it, and how to avoid it]

How to beat morning sickness

Annabel Karmel, best-selling author of books on nutrition and cooking for babies, children and families, gives her advice on morning sickness in her book Eating For Two.

- Start the day with a light snack before you get out of bed. Keep a box of crackers by the bed, or get your partner to bring you tea and dry toast.

- Eat a small high-carbohydrate snack every hour or so as an empty stomach makes nausea worse in most people.

- Minimise cooking smells by opening windows and using extractor fans. Choose cool or cold foods as these have less of an odour.

- Some people find that food or drink containing ginger help to relieve nausea.

- Avoid eating greasy foods because they'll sit in your stomach for longer.

- Make meals that are quick to prepare - or try to get someone else to do it.

- Suck on a slice of lemon or a sour sweet.

- Have a sip of fizzy drink every few minutes when feeling nauseous.

- Eat smaller meals but include more snacks to compensate. Eat more at the times of the day when you feel better.

- Try to avoid the temptation to live on biscuits. They are a convenient snack but not rich in the nutrients necessary for pregnancy. Try dried fruit and nuts, a small bowl of breakfast cereal, or wholegrain crackers and cheese instead.

- Don't drink much with meals (but compensate by sipping drinks regularly between meals). This will help prevent you feeling as full at mealtimes and help with nausea.

[Read more: Pregnant Kate’s £16.50 skincare secret and baby-proofed beauty routine]

When to seek medical advice

If you are vomiting and can't keep any food or drink down, there is a chance that you could become dehydrated or malnourished, according to guidance on NHS Choices.

Contact your GP or midwife immediately if you:

- are unable to keep any food or fluids down for 24 hours, are losing weight or feeling dizzy

- have very dark-coloured urine or do not pass urine for more than eight hours

- have repeated episodes of vomiting

- have abdominal (tummy) pain

- have a high temperature (fever) of 38C (100.4F) or above

- have a racing heart rate or vomit blood