While most pregnant women experience morning sickness in some way or another, the affliction that has put the Duchess of Cambridge in hospital is significantly more rare and serious.
In a statement today, palace officials said she was hospitalised with hyperemesis gravidarum - a potentially dangerous type of morning sickness where vomiting is so severe no food or liquid can be kept down.
It affects between one to 10 out of every 100 women, although the causes for it are not well known.
It is believed to be due to rising levels of pregnancy hormones such as HCG and oestrogen, which some women are more sensitive to than others.
Women with a twin pregnancy a generally worse affected by it because their hormone levels are doubled.
Nausea and vomiting are two of the most common symptoms of pregnancy, with morning sickness in some form affecting up to 70-85 per cent of women.
Vomiting and nausea are generally considered signs of a healthier pregnancy and some research has shown can lead to a lower miscarriage rate.
Women with hyperemesis however, are generally admitted to hospital to manage dehydration.
It can be difficult to stop the vomiting and nausea and women the condition often end up being admitted more than once.
Dehydration is treated with an intravenous line.
There can be significance risk to the health of both mother and baby if not treated.
Hyperemesis can lead to excessive weight loss, renal failure and liver damage, although that was rare as long as it was being treated.
Palace officials said the duchess was expected to remain hospitalised for several days and would require a period of rest afterwards.
The condition is thought to be more common in young women, women who are pregnant for the first time, those expecting multiple babies and in non-smokers.