Although it is more common in the first three months some women are nauseous throughout their pregnancy.

The name "morning sickness" is misleading because nausea can occur at any time of day and in the most serious cases it can become hyperemesis gravidarum, or excess vomiting, which can be fatal.

In research by Liverpool University, where an analysis of 56 previous studies in 21 countries was carried out, the prevalence of nausea and sickness in pregnant women was examined.

Researchers Dr. Gillian Pepper and Dr. Craig Roberts linked these figures to the typical diet in each country and suggest there is a link between nausea and diet.

They say morning sickness might have evolved to ensure pregnant women do not digest too much unhealthy food.

They found evidence that nausea and vomiting in pregnancy is associated with high intake of sugar, alcohol, oils and meat.

For some women even the common smells, particularly fat and fried food, and the sight of food cooking is enough to bring on a wave of nausea.

Another study found that 50% of women developed an aversion to alcohol in their first three months and there appears to be mounting evidence that morning sickness is the body protecting itself against harmful substances in food.

It seems countries with a high intake of sugars, sweeteners, stimulants such as caffeine, vegetables, meats, milk and eggs had more sick pregnant women, and those with high intake of cereals and pulses had lower levels.

The researchers believe that the pregnant human body may have evolved an aversion to foods containing high levels of toxins, and that this may have carried over into modern living.

They suggest that the body might reject meat because of the relatively high risk that it might harbour disease-causing agents while the low level of plant toxins in cereals may make them particularly unlikely to trigger nausea.

But the body's rejection of sugars and oils is less easy to explain.

Lead researcher Dr. Roberts says womens' bodies may be pre-programmed by evolution to avoid particular foodstuffs in the first trimester and the nausea could be nature's way of avoiding problems in pregnancy for both mother and foetus.

Experts say the theory makes sense as morning sickness is always worst in the first three months, which is when the most important part of a foetus's development is happening.

They say it is sensible for a woman to eat healthily in pregnancy, but more important she avoid alcohol, smoking or drugs.