Health reporter, BBC News
Women who drink a small amount of alcohol while pregnant do not increase their child's risk of behavioural problems, a study has suggested.
The University College London team classed "light" drinking as up to two drinks a week throughout pregnancy.
The study of 12,500 three-year-olds even found a lower risk of some problems in children of such drinkers.
But experts were divided over whether the study was reassuring or could lull women into a false sense of security.
Government advice says pregnant women, or those trying to conceive, should avoid drinking alcohol.
But if they do choose to drink, they should drink no more than one or two units of alcohol once or twice a week and should not get drunk.
The relationship between sustained heavy drinking in pregnancy and health problems for the child is well-established.
In the most severe cases, it can cause miscarriage or stillbirth, or permanent damage to the growing foetus.
A small number of babies in the most severe cases can be born with "foetal alcohol syndrome", with symptoms including physical and mental developmental problems.
The University College London team asked mothers about how much alcohol they had drunk during pregnancy when their babies were nine months old.
Light drinking was classed as ranging from one drink every so often to two drinks per week, while moderate drinking was between three and six units per week or three to five per one occasion.
Heavy drinking was regarded as seven or more units per week or at least six per occasion.
While 63% of the mothers had abstained from alcohol completely during pregnancy, 29% had been light drinkers, 6% moderate and 2% heavy.
The researchers then went back when the children were three to ask about their behaviour and understanding.
The study, published in the International Journal of Epidemiology, found boys born to light drinkers were 40% less likely to have conduct problems and 30% less likely to be hyperactive than those whose mothers had abstained.
They also scored more highly on vocabulary tests and on identifying colours, shapes, letters and numbers.
Girls born to light drinkers were 30% less likely to have emotional problems than those born to abstainers, although the researchers say this could be due to family and social backgrounds.
Light drinkers were more likely to be better educated and from higher income households and were less likely to have smoked during pregnancy than abstainers.
Dr Yvonne Kelly, the epidemiologist who led the study, said: "Our research has found that light drinking by pregnant mothers does not increase the risk of behavioural problems and cognitive defects.
"The reasons behind these findings might in part be because light drinkers tend to be more socially advantaged than abstainers, rather than being due to the physical benefits of low level alcohol consumption seen, for example, in heart disease.
"However, it may also be that light-drinking mothers tend to be more relaxed themselves and this contributes to better behavioural and cognitive outcomes in their children."
Dr Kelly added: "Our study's findings do raise questions as to whether the current push for policy to recommend complete abstinence during pregnancy is merited and suggest that further research needs to be done."
'Simplest is safest'
But Dr Vivienne Nathanson, head of science and ethics at the British Medical Association, said: "We are concerned that the findings from the UCL study may lull women into a false sense of security and give them the green light that there is no problem with drinking during pregnancy. This is not the case.
"So-called 'heavy' and 'moderate' drinking harm the unborn baby. Very light drinking may or may not.
"The BMA believes the simplest and safest advice is for women not to drink alcohol during pregnancy."
Patrick O'Brien, a spokesman for the Royal College of Obstetricians and Gynaecologists, said women should not set out to drink during pregnancy, but that they could be relaxed about the occasional drink.
"This is further evidence that pregnant women should not be worried about having a small amount of alcohol."