When it was announced that the Duchess of Cambridge was pregnant, but was admitted to hospital for Hyperemesis Gravidarum, that might leave some Royal fans – and pregnant mothers – wondering what exactly that is.

Hyperemesis Gravidarum is an acute form of morning sickness which causes severe and potentially dangerous nausea and vomiting, with some pregnant mothers being unable to keep any food or liquid down.

Treatment requires supplementary hydration, through intravenous and nutrients.

In general, morning sickness is not serious. However, in Kate Middleton’s case, untreated Hyperemesis Gravidarum is as life-threatening as a result of severe malnutrition, dehydration and significant weight loss.

It only affects one per cent of expectant mothers.

Meanwhile, research published in the American Journal of Obstetrics and Gynecology shows women who are underweight when they become pregnant have an increased risk of Hyperemesis Gravidarum. At the same time, obese women who become pregnant have a 10 per cent decreased risk of experiencing this condition.

Middleton, 30, is expected to stay in hospital for several days, as she receives treatment.