Monday, 17 October 2011

Birth weight and autism again

This is not the first time that the issue of birth weight in connection to autism has been discussed on this blog. Earlier this year a meta-analysis of various birth issues potentially linked to autism was published and blogged about which included birth weight, low birth weight, as a potential connector. The strength of that paper lay in the fact that it was a meta-analysis; so kinda the top of the tree level of inquiry.

The journal Pediatrics again publishes a study discussing the birth weight - autism connection. Jennifer Pinto-Martin and colleagues* report on a prospective population-based study that suggests that children born weighing less that 2kg (4.4lbs) may be five times more likely to present with an autism spectrum condition than those born heavier.

The details:
  • A regional cohort of children (n=1,105) followed as part of the Neonatal Brain Hemorrhage Study during 1984 and 1989 were included for analysis. Infants weighing between 501 and 2000g were prospectively enrolled and followed through childhood into adulthood. 
  • At 16 years old, just over half of the original group (n=623) were screened for autism based on either a previous diagnosis of autism or 'liberal' cutoff scores on the SCQ or other measure. Positive screens were detected in 117 of the 623 (18.8%). 
  • Author diagnostic testing of a proportion of positive-screeners and negative-screeners was carried out when participants were 21 years old using those gold-standards, ADOS and ADI revealing that of 70 positive screeners at 16 years old, 11 (14.3%) were confirmed as presenting with an autism spectrum condition and 3 out of 119 negative-screeners (2.5%) met criteria for an autism spectrum condition.
  • Balancing out positive and negative-screener results suggested an estimated prevalence rate of 5% for the entire cohort. This is compared with a 1% figure produced by the CDC back in 2006.
There are a few other details from this study which, whilst not significant at 5%, were nevertheless interesting. So the lower the birth weight, the higher the risk of autism and low birth weight was more often linked to autism in boys than girls. There are also some limitations set by the authors on the current dataset in view of the lack of normal birth weight control groups and the quite high loss at follow-up statistics.

I note that this study has again 'hit' the media headlines hard with places like the BBC reporting on it. With all due credit they also cite people like Prof. Dorothy Bishop who provides a good balance in that many children with autism are not born low birth weight and many low birth weight children do not have autism. Others have also discussed the important issue of low birth weight and learning disability as being perhaps a more primary relationship.

You can tell that this research, as with any longitudinal investigation, is a real labour of love for Pinto-Martin and colleagues. The results are interesting and due credit should be given to the possible 'risk' that comes with low birth weight without however getting too excited about the 'directness' of any relationship between autism and birth weight.

* Pinto-Martin JA. et al. Prevalence of autism spectrum disorder in adolescents born weighing <2000 grams. Pediatrics. October 2011.