Smoking in pregnancy is far less damaging to the unborn baby than commonly supposed, detailed analysis suggests. If women give up smoking by the fifth month of pregnancy, the effect on the baby is negligible, the study found. And even if they do not, the effect on birthweight is surprisingly small.

The study by Emma Tominey, a research assistant at the Centre for Economic Performance at the London School of Economics, throws new light on government efforts to stop women smoking when they become pregnant. While it does not suggest that such efforts are pointless, it shows that directing advice towards the newly pregnant is worthwhile.

It also shows that the worst effects are suffered by women from the poorest backgrounds, because in their case smoking is often combined with other unhealthy activities, such as poor diet and consumption of alcohol.

Middle-class women suffer almost no damaging effects, the analysis suggests, even if they continue to smoke throughout pregnancy.

The findings, published as a report by the centre, will not be welcomed by anti-smoking groups, whose message to young women is intended to make them feel guilty about damaging their babies.

In Ms Tominey’s view, the damage is real but relatively small, and even if all women gave up smoking, only about one in eight babies with a low birthweight would avoid being classified as such.

The report uses data from the UK National Child Development Study, which provides details of mothers and their children between 1973 and 2000 — a total of 3,368 women and 6,860 children.

The information includes the mothers’ smoking habits, information about their families, and the birthweight and gestation period of the children.

Analysis of the data shows that smoking throughout pregnancy reduces birthweight by 5.6 per cent, and the gestation period by just over a day. But when the results are corrected for other factors, such as diet, lifestyle and alcohol, the effect of smoking on birthweight drops to 1.8 per cent and the reduction in gestation becomes insignificant.

The study also finds that, contrary to the normal belief that damage is done early in pregnancy, it is the final third that matters most, because this is when babies gain the most weight.

Another surprising finding is the strong class effect. The damage is greatest among mothers with the lowest levels of education. Those who leave school at 16 cause twice the harm to their babies with each cigarette smoked. Ms Tominey concludes: “Other behaviours of the mother play a large role . . . over and above her smoking habits.”

Policies intended to help babies should aim to educate mothers generally, not simply try to persuade them to stop smoking, she said.

However, she does not conclude that smoking is harmless. “We find that up to 13 per cent of children classified as low-birthweight born to smoking mothers could have been classified as healthy, had their mothers not smoked.”

The policy implications, however, are that stopping smoking alone is not enough to deal with inequalities in child health, she concludes.

“Not only is it the low-socioeconomic-status mothers who choose to smoke, but they are also the mothers bearing the greatest burden from the smoking.”

She said: “Therefore, any potential solution must offer help to these mothers, to target those with the worst habits and poorest records of child health.”

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