So what is hyperemesis gravidarum and how is it affecting Kate Middleton?
The severe form of "morning sickness" causes nausea and vomiting in pregnant women that can put the mother-to-be and baby at risk because the woman cannot retain and utilise food and fluids.
Symptoms of hyperemesis gravidarum include constant nausea and severe vomiting that can lead to dehydration and cause an imbalance in electrolytes.
Women can lose more than five per cent of their body weight as well as suffer headaches, fatigue, confusion, fainting and jaundice.
The illness is estimated to affect about one per cent of pregnant women and starts usually between the first four to six weeks, according to Brisbane Mater Private Hospital's HER Foundation.
Symptoms do not usually improve until between 15 and 20 weeks - but can last the entire pregnancy.
Women with milder forms are advised to change diet, rest and take antacids.
With more severe forms women are hospitalised so they can have their food and liquids closely monitored and usually require an intravenous drip.
As for the causes of hyperemesis gravidarum, experts still do not fully understand why the illness manifests but they describe it as a complex physiological disease with multiple causes.
They say aggressive care early in pregnancy is vital to prevent life-threatening complications such as central pontine myolinolysis or Wernicke's encephalopathy.
