CURLED up on the bathroom floor, fighting day-long nausea and resisting the urge to vomit every time a smell wafts past you. That's right - morning sickness.

Certainly not one of the more glamorous sides of pregnancy but a common symptom for many mums-to-be in early pregnancy.

But how do you know if what you are suffering is normal morning sickness or something much more serious?

When the Duchess of Cambridge was admitted to hospital with severe morning sickness, clinically named hyperemesis gravidarum (HG), many asked "Why is she in hospital when women go through that every day?".

But they actually don't.

While morning sickness is experienced by most pregnant women, Warwick Hospital director of nursing said HG is much more rare and dangerous.

"Nausea and vomiting are common symptoms of early pregnancy and although many people call it morning sickness, symptoms can occur at any time," she said.

"HG is vomiting of frequency and severity enough to compromise nutrition and food balance and requires admission to hospital."

Pregnant women who suffer from this condition literally vomit all day and became at risk of dehydration, oesophageal damage and urinary tract problems.

Ms O'Shannessy said the Warwick Hospital deals with about two cases of HG each year, which requires the pregnant woman to be admitted to hospital and placed on an IV drip to replenish fluids.

Within a couple of days, the mum-to-be is usually feeling better and is released from hospital.

But it is usually not for long.

"Hospital stays are usually for two to three days, the patient is released and we usually see them and they are readmitted again a week or so later," she said.

There is no cure for HG, which is treated with fluids and rest, and is more common in women having multiple births and girls.