By DANIEL MARTIN

Last updated at 22:28 14 February 2008

Middle-class women who smoke in early pregnancy do almost no harm to their unborn baby, researchers claimed last night.

Only women from poorer backgrounds damage their babies by smoking, because they tend to combine it with alcohol and a poor diet.

The study by the London School of Economics also casts doubt on the traditional view that smoking during early pregnancy does the most harm to the baby.

If women stop smoking by the fifth month the impact on the baby is negligible - and even if they carry on it has surprisingly little impact on birthweight.

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The study may come as a relief to mothers such as Kerry Katona and Kate Garraway, who have come under attack for smoking while pregnant.

But it was criticised last night by charities, which said earlier research clearly linked smoking with an increased risk of miscarriage, premature birth and cot death, as well as breathing problems.

The Government advises pregnant women not to smoke but 20 per cent do.

The babies of mothers who smoke weigh an average 7oz less than those born to mothers who do not. Children with low birthweight are more likely to die early or have developmental problems.

But the author of the LSE study, Emma Tominey, says smoking in early pregnancy makes little difference and most damage is done in the final third of pregnancy, when babies gain the most weight.

She claims that if all pregnant women quit smoking, the number of low birthweight babies would fall only by an eighth.

Miss Tominey, research assistant at LSE's Centre for Economic Performance, said: "Other behaviours of the mother play a large role, over and above her smoking.

"Not only is it the low socio-economic status mothers who choose to smoke but they are also the mothers bearing the greatest burden from the smoking.

"Any solution must offer help to these mothers, to target those with the worst habits and poorest records of child health."

She said that rather than just telling mothers not to smoke, the Government should highlight the importance of other lifestyle factors. And they should target "stop smoking" services towards the lowest social groups.

Miss Tominey used the National Child Development Study, which recorded smoking habits and family information for 3,368 women and the birthweight and gestation period for their 6,860 children from 1973 to 2000.

She said smoking throughout pregnancy shortens the gestation period by little more than a day and reduces birthweight by 5.6 per cent.

However, other factors make a significant difference. Among women with a healthier lifestyle, diet and alcohol consumption, the effect of smoking on birthweight falls to 1.8 per cent, while the reduction in gestation becomes negligible.

Professor Robert West, of Cancer Research UK, said: "The vast body of research shows that smoking is harmful throughout pregnancy and stopping at any stage will be good for your baby.

"This study is focused on birthweight but many other studies have found smoking in pregnancy is linked to an increased risk of miscarriage, premature birth and cot death, as well as breathing problems.

"Pregnancy is the perfect time to stop smoking. Giving up is the best way to reduce the risk of cancer and many women who give up during pregnancy remain smoke free after birth."