The Duchess of Cambridge's pregnancy has turned a little known medical condition into a much talked about topic. Channel 4 News answers the key questions about hyperemesis gravidarum.

Kate Middleton has Hyperemesis gravidarum - severe morning sickness (Getty)

What is hyperemesis gravidarum (HG)?

In simple terms, hyperemesis means vomiting a lot and gravidarum means in pregnancy.

Some pregnant women, like the Duchess of Cambridge, experience severe nausea and vomiting which continues throughout pregnancy.

Hyperemesis gravidarum is not common but it can be severe, sometimes requiring hospital treatment. It affects around one in 200 women, though estimates vary depending on how the condition is defined.

What are the symptoms of hyperemesis gravidarum?

Symptoms include prolonged and severe nausea and vomiting (around 20 times a day), as well as dehydration. It can also lead to ketosis which is a serious condition that is caused by a raised number of ketones - poisonous acidic chemicals - in the blood.

The inability to keep any food or water down can result in marked weight loss (greater than 5 per cent) and low blood pressure when standing up.

As well as physical symptoms, the condition can have a significant effect on a woman's life and may lead to further complications, such as depression.

Read more: Pregnant Kate in hospital overnight

What causes hyperemesis gravidarum?

After conception, the body begins to produce a hormone called human chorionic gonadotrophin (hCG). It is thought that a rise in the level of hCG may cause nausea and vomiting during pregnancy.

How is hyperemesis gravidarum treated?

HG needs specialist treatment, and some women, such as Kate Middleton, may need to be admitted to hospital if they are losing too much fluid. This is so that doctors can assess the condition and give appropriate treatment, such as increasing fluid levels through a drip, and treating the ketosis.

Consultant obstretrician Daghni Rajasingham, from the Royal College of Obstetricians and Gynaecologists, said the length of a woman's hospital stay would depend on how well she was able to keep down fluids.

Can hyperemesis gravidarum harm the unborn baby?

HG is unlikely to cause harm to the baby. However, if it causes weight loss during pregnancy there is an increased risk that the child may be born with a low birth weight.