Do children born too close together face autism risk?

There has been a fair bit of media attention (e.g. Telegraph and Daily Mail) on a study that has suggested that children born very close in time to their siblings face a increased risk of developing autism. In all of the media articles on this study, it has been reported that children conceived within 2 months of each other were 150% more at risk than children whose births were spaced further apart.  But what does this actually mean?

In our How to Research the Headlines series, we devoted a whole post to understanding the concept of risk as described within the media. Here we gave an example of imagining that eating too many cupcakes increased the chance of someone suffering a certain type of cancer by 50%. In this example it sounds like cupcake consumption is a major risk factor, without the context to judge that properly. If the risk of developing the cancer (without eating cupcakes) is only 0.2% for example, that means the increased risk is now 0.3% (a 50% increase on the no-cupcake 0.2% risk).

While we would still understand that there is an increased risk, stating the absolute value is generally much less sensational.  We pointed out that Ben Goldacre would recommend going one step further by not talking about percentages (which many readers and writers have trouble understanding properly) and instead use the “natural frequency”.  This means stating the increase in terms of the extra number of events per 100 or 1000 people.  So, if on average for every 1000 people, two will suffer from cancer, but for those who eat a cupcake every day there will instead be three, then it would be better for the journalist to say “Cupcakes can lead to an extra one case of cancer for every 1000 people.” As the prevalence of autism is estimated at about 1.1% a 150% increased risk leaves you at about 2.7% risk or an extra person per one hundred people – this obviously doesn’t sound as attention grabbing as a 150% increased risk.

On reading the autism study journal article it is interesting to see that the focus of the article is equally focused on long intervals between births (with increased risk here as well) as short 12 month gaps. Most of the media articles seemed to be focused on the short conception timed risk. Although the issue of risk wasn’t handled very well in the coverage of this study, other aspects of reporting were more favourable. The media coverage did well generally to refer to the original journal article and also to include the opinions of relevant experts. The Daily Mail  additionally included a box section taken from the National Autistic Society in which they described what autism is. This type of reporting is very good to see.

Cheslack-Postava, K. et al.(2014). Increased Risk of Autism Spectrum Disorders at Short and Long Interpregnancy Intervals in Finland. Journal of the American Academy of Child & Adolescent Psychiatry. DOI: 10.1016/j.jaac.2014.06.009