But what exactly is hyperemesis gravidarum?
Hyperemesis gravidarum, put simply, is extreme morning sickness and is highly unusual.
Symptoms can include prolonged and severe nausea and vomiting, dehydration, weight loss and hypotension (low blood pressure when standing up).
Women can also suffer from ketosis, a condition caused by an increased number of ketones (poisonous acidic chemical produced when your body breaks down fat, rather than glucose, for energy) in your blood.
Video: The Duchess of Cambridge is pregnant again
Often the vomiting and nausea are so severe pregnant women are unable to keep any food or liquid down and require specialist treatment, including – in some cases – intravenous fluids given through a drip to prevent vomiting and treat the ketosis.
Fortunately, although symptoms can make the expectant mother miserable, hyperemesis gravidarum is unlikely to affect the unborn child.
Although most women only experience hyperemesis gravidarum during their first pregnancy, the Duchess of Cambridge is clearly among the unlucky few who experience it during later pregnancies as well.
Although the cause of hyperemesis gravidarum is unknown, research suggests it may be due to hormonal changes.
The condition is thought to be hereditary.