The research, by scientists at Oxford University, found that babies of mothers from industrial countries weighed the same at birth regardless of the amount of food eaten by the mother, or any vitamin supplements used.

There is a direct correlation between birthweight, and the size of the placenta, and the future health of the child.

But a scientist from the British Nutrition Foundation insists that eating a well-balanced diet during pregnancy is still a must for mothers-to-be.

More than 1,000 pregnant women from the UK were invited to take part in the survey, which asked them about their eating habits.

Smoking and height taken into account

After making allowances for obvious factors like smoking, and the height of the woman - both known to influence baby weight - the researchers found no obvious difference between babies born to women with widely differing eating habits.

And when vitamin supplements were considered, only vitamin C appeared to have any beneficial effects - and even these were described as of "doubtful clinical significance".

The research concludes that while severe malnutrition of the mother, like that seen in developing countries, can have a detrimental effect on the child, among relatively well-nourished women, diet had only a "marginal" effect.

But Dr Sarah Schenker, a nutrition scientist at the British Nutrition Foundation, said there were good reasons to eat well during pregnancy.

The mother suffers, not the child

The most important, she said, was that while poor nutrition rarely affected the unborn child, the health of the mother was a completely different matter.

She said: "The baby will take what it needs from the mother, regardless of what she is eating."

A growing foetus needs calcium and iron, and if the woman does not take on enough supplies, will have to hand it over from her own metabolism.

This shortage of calcium could lead to brittle bone disease in later life.

And despite the fact that folic acid had no discernable effect on birthweight, Dr Schenker said that taking it in the months leading up to conception, and during pregnancy had been shown to reduce the number of congenital deformities.