Following on from the news that low birth weight might put a person at greater risk of autism, there has been a flurry of activity on this topic. Lots of headlines about the 'increased risk' and lots of discussion about it on various social media and other platforms.
Lo and behold pretty much 24 hours after the 'hype' about the paper, details of another paper drop into my inbox, this time from Schieve and colleagues* published in the Annals of Epidemiology. After some more mathematical and statistical wizardry, Schieve and co. come to the conclusion that various birth factors might well be linked to some cases of autism but on the whole, ".. the contribution of many of these factors to the recently observed ASD increase is likely minimal".
I can't pretend to be able to offer a definitive overview of the mathematical model used in this recent study but they applied it to various pregnancy- and birth-related factors including: low birth weight (and very low birth weight), preterm births, multiple births, caeasarean section deliveries, breech presentation, and use of assisted reproductive technologies, all based on existing datasets. Their main message, that less than 1% of the observed increase in cases of autism is likely due to these factors.
Understanding that this paper does not rule out an increased risk of autism from various pregnancy and birth factors, it does suggest that the current data on such factors does not yet provide a satisfactory answer to questions of why so many more children (and adults) are being diagnosed with an autism spectrum condition. I may be looking at apples and oranges here (risk vs. reasons for an increase in numbers of cases) but ultimately increased risk is going to potentially tie into the wider argument on the numbers of cases.
Whilst not advocating that anyone place their universal trust in maths and statistics, particularly when applied to heterogeneous conditions like autism, I do wonder whether we are seeing another piece to the puzzle when it comes to investigating autism on the basis of phenotypes and clustering of symptoms and characteristics. My mind wanders back to the recent news from Prof. David Amaral on biological phenotypes and whether this is just one more factor to add to the array of others proposed. Such pregnancy and birth factors may be a major component of one person's 'route' to an autism spectrum condition; for others, an insignificant drop in an ocean of other risks.
* Schieve LA. et al. Have secular changes in perinatal risk factors contributed to the recent autism prevalence increase? Development and application of a mathematical assessment model. Annals of Epidemiology. 2011.