Kate Middleton is treated at Kensington Palace for acute morning sickness which can leave sufferers dangerously dehydrated and vomiting up to 30 times a day

The Duchess of Cambridge is once again being treated for extreme morning sickness, Kensington Palace officials confirmed today. The condition, known as hyperemesis gravidarum, led to her being hospitalised during her first pregnancy.

Sources have told MailOnline that Kate, 32, has yet to reach the crucial 12-week stage, but as before, her sickness led to the pregnancy being announced earlier than planned. An expert told MailOnline the condition is usually detected around the six-week mark, prompting speculation the Duchess is just over a month into her pregnancy.

Hyperemesis gravidarum is common, affecting around 15 per cent of all pregnancies. However in its extreme form - where a pregnant woman is  admitted to hospital or suffers the condition throughout her pregnancy - it is much more rare.

It is much more serious than the nausea commonly experienced by expectant mothers. The condition is thought to be caused by elevated levels of ‘pregnancy hormone’ HCG, or human chorionic gonadotropin, which increases after conception.