Image copyright Getty Images Image caption The Duchess of Cambridge with her daughter, Princess Charlotte

In her past two pregnancies, the Duchess of Cambridge suffered from extreme morning sickness, known as hyperemesis gravidarum (HG).

It appears that she is having a tough time with it again, now she is expecting her third child.

HG is a condition thought to affect around one in every 100 women in pregnancy and is much more severe than "normal" morning sickness.

Some mums-to-be who have it report being sick up to 50 times a day.

Unlike regular morning sickness, HG may not get better after the first few months of pregnancy.

Why do only some women get it?

The cause is thought to be pregnancy hormones but it is unclear why some women suffer worse than others and why it can persist. If you are suffering from it, there is nothing that you are doing or have done that has brought on the symptoms. It is not your fault that you have it.

HG is more likely if you have had it before or if you are expecting twins or triplets. HG does appear to run in some families.

How bad is it?

A woman with HG can experience:

Persistent vomiting

Dehydration

Tiredness

Dizziness

It can be very unpleasant and have a huge impact on daily life.

Women who have experienced HG say it made them feel isolated, lonely, scared and frustrated.

Can it harm the baby?

There is no evidence that it is directly harmful. It is not a sign that the pregnancy is unhealthy.

Doctors will want to make sure that the mother is not losing too much weight during the pregnancy or getting dehydrated because this can be harmful.

What helps?

Seek help from your doctor and midwifery team. Doctors can prescribe drugs to help control the nausea.

Some women say keeping a diary of their symptoms helps them become more aware of what times of day they might feel well enough to eat and drink.

Keeping well hydrated by drinking enough fluid is vital. Water is great and small, frequent sips might be more manageable.

Some pregnant women say certain smells, including cooked food, make them feel even more nauseous. If that is the case, try to avoid them if you can.

Meals that are high in carbohydrate and low in fat, such as potato, rice and pasta, are sometimes easier to tolerate. Try plain biscuits or crackers. Some women find eating or drinking ginger products helps.

The charity Pregnancy Sickness Support has a support forum where people can share their experiences.

Some women find it helpful to carry a "sick kit" - a pack of wipes, some disposable bags, a bottle of water and some mints or chewing gum.

Have regular check-ups and stay in touch with your doctor. HG can be extremely serious. If you are unable to drink fluids you can become dangerously dehydrated. You may need to be admitted to hospital to receive fluids intravenously via a drip.