Kate Middleton is being treated in hospital for a very severe form of morning sickness called hyperemesis gravidarum.

Hyperemesis gravidarum (HG) is a rare condition believed to affect up to 2% of women in pregnancy and is a severe form of morning sickness.

It tends to be more common in young mothers, women who are in their first pregnancy, and those with multiple pregnancies.

Women with HG often lose weight – usually over 10% of their body weight – and feel tired and dizzy.

They may also find they are passing water less often than usual, and the main risk is dehydration which can lead to headache, palpitations and confusion.

There is also a risk of nutritional deficiencies.

Then symptoms are severe, admission to hospital may be needed for observation and to treat dehydration with intravenous fluids.

This usually only means a few days in hospital.

Kate Middleton is being treated in hospital for a very severe form of morning sickness called hyperemesis gravidarum

Unlike most morning sickness, HG usually persists past the first trimester of pregnancy, and typically subsides by week 21 of pregnancy – although it can last much longer.

However, leading doctors say that along with more ordinary morning sickness, it is a sign the pregnancy is progressing.

Dr. Peter Bowen-Simpkins, consultant obstetrician and medical director of the London Women’s Clinic, says: “It is almost always a positive sign that the pregnancy is progressing well.

“The sickness is thought to be due to a rise in hormone levels. It normally occurs during weeks six and eight of pregnancy, when the placenta takes over production of hormones from the ovaries.

“It generally continues until around 12 or 14 weeks, but if it stops before, it can – although not always – be a sign that all is not well.”