Alcohol in pregnancy and IQ of children
Some of the coverage of yesterday's story about drinking in pregnancy and IQ of children was not entirely accurate. The Times reported that 'women who drink even a couple of glasses of wine a week during pregnancy are risking a two-point drop in their child's IQ', and 'children whose mothers drank between 1 and 6 units a week - up to three large glasses of wine - had IQs about two points lower '(than mothers who did not drink).
But let's look at Table 3 of the paper, which is available here.
Here we see that children of mothers that drank had systematically higher IQs than those who didn't! But nothing can be inferred from this (except that the Times was not exactly correct).
This of course is the whole problem of carrying out these studies: there are many 'confounding' factors that are associated both with mothers' drinking habits and children's IQ, and this makes teasing out the underlying relationship very tricky.
This is where the ingenious idea of 'Mendelian Randomisation' comes in. Genes are assumed independent of confounding actors, so it is as if women have been randomly allocated to the genetic groups in the Table, and so they should be balanced for all other factors. The genes are seen to be associated with the IQ of offspring among women who drink, and the genes were selected as those that regulate uptake of alcohol, and babies who may be exposed to greater alcohol have on average lower IQs. But maybe the genes affect babies in other ways than in uptake of alcohol? But this is deemed implausible, as there is no relationship seen in non-drinkers.
This is a very clever and careful study, to be taken very seriously. But it does not allow an estimate of the effect of drinking, and the authors are careful not to give one (even if they appear to have been rather happy to declare public health implications, which seems to be somewhat overstepping their role as epidemiologists).
Very crudely, if we took the lowest group as similar to non-drinkers, then the effect of moderate drinking might be estimated as around 2 points, but nobody would want to put this in an academic publication.
In addition, an important issue is the alcohol quantities. The 'drinkers' include those reporting between 1 and 6 units a week - quite a range - and we can also assume that this is an understatement of true consumption. So what 'moderate' drinking actually means is open to some question.
As usual, the NHS Behind the Headlines site had an excellent discussion.
Added later: I have made a few edits in this blog to make it clearer that I am not questioning the study's basic conclusions, just pointing out that some of the coverage misunderstood the (somewhat subtle) findings