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

In contrast, cereals were least likely to trigger sickness.

The University of Liverpool analysis of 56 previous studies in 21 countries appears in the Royal Society's Biological Journal.

Morning sickness occurs in the first three months of up to 80% of pregnancies.

Until recently, it was thought to be simply an unfortunate by-product of dramatic hormonal changes during early pregnancy.

However, research has suggested that morning sickness might have positive consequences - such as a reduced risk of miscarriage.

Toxins

The researchers believe it is possible 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.

Conversely, the low level of plant toxins in cereals may make them particularly unlikely to trigger nausea.

However, they say the body's rejection of sugars and oils is less easy to explain.

Lead researcher Dr Craig Roberts said: "While there may be no particular harm in eating, say, meat, now that we have refrigeration and best before dates, our bodies may be pre-programmed by evolution to avoid these particular foodstuffs in the first trimester.

"It may be that the nausea women feel towards certain foodstuffs could be helpful, and that although it is inconvenient and miserable, their nausea could be nature's way of avoiding problems in pregnancy for both mother and foetus."

Dr Maggie Blott, a consultant obstetrician at London's King's College Hospital, said: "I can understand why mother nature might do this.

"Morning sickness is always worst in the first three months, which is when the most important part of a foetus's development is happening."

Dr Blott said while it was sensible for a woman to eat healthily in pregnancy, it was more important to avoid alcohol, smoking or drugs.

It was also important to take folic acid supplements, and to avoid high levels of vitamin A.