This study did not show any overall benefits or “blessings” from drinking as a casual reader of the headline might think, and several design issues make it impossible to be confident in the finding that light drinking during pregnancy is not harmful.

Current NHS advice is to avoid drinking during pregnancy, but if women choose to drink they should have no more than one or two units once or twice a week. There are known risks associated with heavy drinking during pregnancy, and given that studies have yet to determine what amount of alcohol is safe, women could just as well avoiding alcohol during their pregnancy.

Where did the story come from? Dr Yvonne Kelly and colleagues from University College London, the University of Essex and Warwick Medical School carried out this study. It was published in the peer-reviewed medical journal, International Journal of Epidemiology.

What kind of scientific study was this?This publication was based on analysis of data from a cohort study – the Millennium Cohort Study (MCS) – where these researchers were particularly interested in exploring the link between mothers’ drinking during pregnancy and the results of cognitive testing in their children aged 3 years.

The Millennium Cohort study includes a sample of infants born in England and Wales between September 2000 and August 2001, and in Scotland and Northern Ireland between November 2000 and January 2002. Households were interviewed for the first time when babies were 9 months old and at this point interviewers asked how often they drank during pregnancy and also how much was drunk.

Using this information, drinking was categorised as ‘never’, ‘light, not more than 1 to 2 units per week or per occasion’, ‘moderate, not more than 3 to 6 units per week or 3 to 5 units per occasion’, ‘heavy/binge, 7 or more units per week or 6 or more units per occasion’. They also asked about other health-related behaviours, social and economic details, and details about household composition.

A second round of interviews occurred when the child was three years old and at this time the interviewers performed cognitive tests on the child and asked questions about behaviour, social and economic factors and the psychosocial environment of the family. Parents also completed a questionnaire called the Strengths and Difficulties Questionnaire which is often used to assess behavioural problems.

Scores from some parts of this questionnaire were added up to give an overall difficulties score. Caucasian, singleton infants (i.e. not one of twins) whose mothers had participated in the first two interviews of the MCS study were included in this analysis. This amounted to 12,495 children but some had missing data on different aspects of behavioural tests, so final analyses included about 9,000 children.

The analyses of the link between drinking during pregnancy and behavioural outcomes (total difficulties, conduct problems, hyperactivity, emotional symptoms, peer problems) at age three years took into account various other factors that may affect the link, such as gender, socioeconomics etc. Males and females were analysed separately.

What were the results of the study?The majority of women reported abstaining from alcohol during pregnancy (63%); 29% were ‘light’ drinkers while 6% and 2% were moderate and heavy/binge drinking. Compared with abstainers they also found that ‘light’ drinkers were more likely to be better educated, from higher income households and were less likely to have smoked during pregnancy.

Boys were more likely than girls to have high difficulty scores across behaviour. Mothers’ occupation and socioeconomic status were also related to their child’s behaviour.

The effects of light drinking compared to abstaining were statistically significant only in two behavioural measures with boys and none with girls. Boys born to light drinkers were 0.6 times less likely to have conduct problems and 0.7 times less likely to be hyperactive.

What interpretations did the researchers draw from these results? The researchers conclude that children born to mothers who drank up to 1 to 2 drinks per week or per occasion during pregnancy were not at increased risk of behavioural difficulties or cognitive deficits compared to mothers who didn’t drink anything during pregnancy. They say that while heaving drinking appears to be linked to behavioural problems at age three years, light drinking does not.

What does the NHS Knowledge Service make of this study?This study – which analysed data from a large, ongoing cohort study – has concluded that light drinking doesn’t appear to lead to poorer cognitive or behavioural outcomes for children compared with no drinking. The study must be interpreted in light of the limitations associated with its methods:

Importantly, data on miscarriages, stillbirths and neonatal deaths were not captured by the MCS. Given that there is some evidence that drinking during the first three months of pregnancy may increase the risk of miscarriage, this is an important point.There are a number of other factors that may be responsible for the effect on behaviour that the researchers were not able to adjust for.Mothers reported their own drinking, and given the possible stigma associated with drinking alcohol during pregnancy, some women may have chosen not to give full details of their drinking.Mothers reported their drinking during pregnancy when 9 months after birth.,women may not have remembered how much they drank up to 18 months earlier.This point is particularly important if women whose children had behavioural problems reported their drinking habits in a way that differed systematically from how other mothers did.There is a wide range of drinking habits included in the ‘light’ drinking category in this study – very occasionally during pregnancy up to two drinks a week for the whole pregnancy. This makes it very difficult to draw conclusions about what ‘light’ actually means and how much the women in this category were actually drinking.Very importantly – as acknowledged by the researchers  – ‘light’ drinking may be indicating particular social circumstances in which children are brought up. This is true in this study as ‘light drinkers’ were more likely to be from higher socioeconomic classes. It may be these social circumstances that lead to improved behavioural outcomes and not the drinking at all.The design of this study means it cannot rule out other factors that may be responsible for the behavioural differences between ‘light’ drinkers and abstainers.. Ultimately it is the woman’s choice whether to drink or not and those that choose to have also been officially advised to drink no more than one or two units once or twice a week.

Official advice regarding drinking during pregnancy should not ignored on the basis of this study. As there are known risks associated with heavy drinking during pregnancy and given that it is proving so difficult for studies to determine what lower limit is safe, women could just as well avoid alcohol during their pregnancy.

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