Low birth weight and autism: rise of the population attributable risk

RIP Cheggers.

"LBW [low birth weight] accounted for 6.0% of all ASD [autism spectrum disorder] cases, 2.4% of BCD [behaviour and conduct disorder], and 6.8% of LD [learning disability] among the study population."

Those were the observations made by Sandie Ha and colleagues [1] and with it, another example of the use of the population attributable risk/fraction in the context of autism (see here for another occasion). Published in 2014 but only recently appearing on PubMed, Ha et al report results based on data from the 2011 (US) National Survey of Children’s Health, (NSCH) - a "random-digit-dial phone survey conducted between February 2011 and June 2012" - where data on birth weight and receipt or not of a diagnosis of "attention deficit/hyperactivity disorder (ADHD), autism spectrum disorder (ASD), behavior and conduct disorder (BCD) and learning disability (LD)" were available. As an aside, I've talked about the other studies arising from the NSCH program before on this blog (see here and see here for examples).

Including data pertinent to around 81,000 children aged between 2 and 17 years of age, researchers reported that around 9% of the cohort were "born with a LBW as reported by their parent" in response to the question: "What was [sampling child’s] birth weight?" There were some interesting correlates alongside those responses regarding LBW status: "children who were female, non-Hispanic black, had single mothers, had less educated mothers, were poorer, lacked insurance, were exposed to in-home smoking, or born prematurely were more likely to have LBW compared to those with normal BW."

Insofar as 'neurobehavioural disorders' (ND) also asked about: "The weighted prevalence of parent-reported ND among children ages 2 to 17 was approximately 9.9% for ADHD, 2.3% for ASD, 4.1% for BCD, and 10.6% for LD." Yes, this was a telephone-based survey where "both exposure and outcome are based on parental reporting, and thus the information may not represent actual diagnoses" but with the size of the participant numbers included, these prevalence/frequency figures still make for important reading.

Then to the main event - the population-attributable risk percentage (PAR%) and the finding headlining this post: "LBW [low birth weight] accounted for 6.0% of all ASD [autism spectrum disorder] cases, 2.4% of BCD [behaviour and conduct disorder], and 6.8% of LD [learning disability] among the study population." The authors caution that "maternal age at delivery, gestational age, and pregnancy complications could be important confounders" and were not taken into account in their analyses and could be "potential reasons for LBW" alongside undetected "congenital anomalies or genetic disorders." Caution is required.

It's not new news that birth weight might impact on something like autism risk (see here and see here). One also has to bear in mind that something like LBW may not necessarily appear in isolation to other pregnancy or birth events (see here) so a wider research agenda perhaps needs to be followed. But the size of the PAR% talked about by Ha and colleagues is not easily ignored. Taking into account that LBW for some may very well have some 'genetic' influences, one is left asking whether those more 'social' variables linked to LBW might be to some degree 'influenced' with a corresponding effect on neurodevelopmental 'consequences' reported. I say this in the context that poverty as a variable, has already been linked to some diagnoses included in the Ha study (see here)...

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[1] Ha SU. et al. Population attributable risks of neurobehavioral disorders due to low birth weight in US children. Adv Pediatr Res. 2014;1. pii: 2.

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