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Low‐to‐moderate alcohol consumption during pregnancy and child development – moving beyond observational studies

Heavy alcohol consumption during pregnancy has long been recognised as potentially harmful to the fetus. However, most women either stop drinking altogether once they know they are pregnant or, if they continue, consume only low-to-moderate amounts. Therefore, from a public health perspective, it is important to know the extent to which low-to-moderate consumption may be harmful (or not) to the developing fetus. In 2007, we published a systematic review in this journal on the effects on the fetus of low-to-moderate alcohol consumption during pregnancy. At that time, we considered that there was no convincing evidence of adverse effects of prenatal alcohol exposure at low-to-moderate levels of consumption. Nevertheless, we cautioned that weaknesses in the evidence precluded the conclusion that drinking at these levels during pregnancy is safe. Since that time, a number of well-conducted analyses using large prospective cohort studies have been published, including the study of Skogerbø et al., which appears in this edition of the journal. Skogerbø et al. followed up a sample of 1628 mothers and their children participating in the Danish National Birth Cohort Study when the children were aged between 60 and 64 months. Women were asked about their average weekly alcohol consumption and about the number of single occasions on which they consumed five or more drinks. The outcome measures used were scores of the child’s behaviour on parentand teacher-rated versions of a modified Strengths and Difficulties Questionnaire. No effects of either low-to-moderate alcohol consumption or of binge drinking were found. Similar findings on low-to-moderate drinking and child development have emerged from other observational studies (see, for example, Kelly et al., who followed up mothers and their children from the Millennium Cohort Study, and findings from the Western Australian Pregnancy Cohort Study). Curiously, however, findings from other high-quality observational studies suggest adverse effects of low-to-moderate drinking on child development (for example, Streissguth et al. and Willford et al.), and so it is important to ask why some studies show these effects, whereas others do not. It is of course possible to find methodological shortcomings in both positive and negative studies. To some extent, there is always the potential for bias created by errors in the measurement of exposure and by confounding in cohort studies. Nevertheless, one particular finding in studies of lowto-moderate drinking during pregnancy seems to be reported fairly consistently. In contrast with abstainers, women who drink low-to-moderate amounts during their pregnancies tend, on average, to be more affluent, better educated, better off financially, have better diets and are less likely to smoke. It is probably no surprise that they are more likely, on average, to have brighter, better behaved children who perform well at school. Thus, confounding caused by these social and lifestyle factors could go a long way to explain why any effects of low-to-moderate prenatal alcohol consumption are negated, or even why, in some studies, beneficial effects of alcohol are seen on child development. The standard way to deal with this confounding is to use statistical modelling to adjust for the effect. This is, unfortunately, sometimes referred to as ‘controlling’ for confounding, but, in reality, it almost always falls short of this ideal. Operationalising and measuring a complex multidimensional construct, such as social position or