Familial confounding of the association between maternal smoking in pregnancy and autism spectrum disorder in offspring.
After families are held constant, maternal smoking does not increase autism risk.
01Research in Context
What this study did
The team looked at 1.3 million Swedish kids born 1983-2013. They asked: Does mom’s smoking while pregnant raise autism risk?
They compared regular cousins and full siblings. This trick keeps family genes and homes the same. If the link stays, smoking is guilty. If it vanishes, family stuff is the real cause.
What they found
At first, kids of smokers had 17 % higher autism odds. When they compared siblings, the extra risk dropped to zero.
Result: smoking itself does not cause autism. The small bump seen in older studies came from other family factors like genes or income.
How this fits with other research
Yuan et al. (2014) hunted for big maternal gene effects on autism and found none. Granieri et al. (2020) now adds: the smoking signal also disappears once family genes are held still.
Jenny et al. (2017) saw that prenatal stress worsens autism traits. Their finding stayed even within families. This contrast shows stress may have a true biological punch, while smoking does not.
Lyall et al. (2011) linked mom’s early menarche and high teen BMI to autism. Like smoking, those traits could be proxies for shared family risk rather than direct causes.
Why it matters
You can stop blaming mom’s smoking when explaining a child’s diagnosis. Focus your parent training on evidence-based targets like language or social skills. If a family feels guilt about past smoking, show them this data to ease shame and keep them engaged in therapy.
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Share the sibling-comparison finding with any parent still worried their smoking caused autism, then pivot to current skill-building goals.
02At a glance
03Original abstract
Evidence supports no link between maternal smoking in pregnancy and autism spectrum disorder (autism) overall. To address remaining questions about the unexplained heterogeneity between study results and the possibility of risk for specific autism sub-phenotypes, we conducted a whole-population cohort study in Denmark. We followed births 1991-2011 (1,294,906 persons, including 993,301 siblings in 728,271 families), from 1 year of age until an autism diagnosis (13,547), death, emigration, or December 31, 2012. Autism, with and without attention deficit hyperactivity disorder (ADHD) and with and without intellectual disability (ID) were based on ICD-8 and ICD-10 codes from Danish national health registers, including 3,319 autism + ADHD, 10,228 autism - no ADHD, 2,205 autism + ID, and 11,342 autism - no ID. We estimated hazard ratios (HRs) and 95% confidence intervals (95% CIs) between any maternal smoking (from birth records) and autism (or sub-phenotypes) using survival models with robust standard errors, stratifying by birth year and adjusting for child sex, parity, and parental age, education, income, and psychiatric history. To additionally address confounding using family designs, we constructed a maternal cluster model (adjusting for the smoking proportion within the family), and a stratified sibling model. Associations with maternal smoking and autism were elevated in conventional adjusted analyses (HR of 1.17 [1.13-1.22]) but attenuated in the maternal cluster (0.98 [0.88-1.09]) and sibling (0.86 [0.64-1.15]) models. Similarly, risks of autism sub-phenotypes with maternal smoking were attenuated in the family-based models. Together these results support that smoking in pregnancy is not linked with autism or select autism comorbid sub-phenotypes after accounting for familial confounding. Autism Res 2020, 13: 134-144. © 2019 International Society for Autism Research, Wiley Periodicals, Inc. LAY SUMMARY: Smoking during pregnancy has many harmful impacts, which may include harming the baby's developing brain. However, in a study of thousands of families in Denmark, it does not appear that smoking in pregnancy leads to autism or autism in combination with intellectual problems or attention deficits, once you account for the way smoking patterns and developmental disabilities run in families.
Autism research : official journal of the International Society for Autism Research, 2020 · doi:10.1542/peds.2014-0213