It’s always difficult in human studies to disentangle the effects of lifestyle factors. Alcohol is a case in point, and in particular the vexed question of whether any alcohol is safe during pregnancy. A new study, however, has avoided the complication of co-occurring lifestyle and environment factors by looking directly at genetic variants.
This study, believed to be the first substantial one of its kind, used genetic variation to investigate the effects of moderate (<6 units of alcohol per week) drinking during pregnancy among a large group of women and their children. Since the individual variations that people have in their DNA are not connected to lifestyle and social factors, the approach removes that potential complication.
The study, involving 4,167 children, found that four genetic variants in alcohol-metabolizing genes were strongly related to lower IQ at age eight. But this effect was only seen among the children of women who were moderate drinkers (heavy drinkers were not included in the study), pointing to the effect requiring exposure to alcohol in the womb.
Ten SNPs from four genes previously implicated in alcohol metabolism, intake, or dependency, were analyzed. Four SNPs (particular variants) were related to children’s scores on the cognitive test (WISC), of which three are rare and one quite common. There was an additive effect, with carriers of multiple ‘bad’ alleles being more affected.
There was some evidence that only drinking one or two drinks a week was not harmful to the fetus, but because the numbers of women were relatively small, and individual variability was high, this can’t be assessed with any great certainty.
The critical factor appears to be metabolism of alcohol, with mothers who are ‘fast' metabolizers being safer for their fetus than mothers who metabolize alcohol more slowly.
Mothers' alcohol intake was based on questionnaires completed when they were 18 weeks and 32 weeks pregnant. ‘Moderate’ was defined as between one and six drinks a week. All participants were of white-European origin.
 Fetal Alcohol Exposure and IQ at Age 8: Evidence from a Population-Based Birth-Cohort Study. PLoS ONE [Internet]. 2012 ;7(11). Available from: http://dx.doi.org/10.1371/journal.pone.0049407
Full text available at http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0049407