Pre-natal smoking
Maternal smoking during pregnancy has often been cited as
a potential environmental risk factor for ADHD in offspring,
with estimated pooled odds ratio of 2.39 (Langley et al. 2005).
However, emerging evidence is beginning to suggest that this
association may be the result of confounding genetic or environmental
factors rather than the detrimental effects of maternal
smoking on foetal brain development. For example, Langley
and colleagues (2012) reported associations between both
maternal and paternal smoking during pregnancy, with no statistical
difference between the magnitudes of the associations.
The association between paternal smoking and child ADHD
also remained in families where mothers did not smoke during
pregnancy. In addition, genetically sensitive designs of children
conceived via assisted contraception allow comparison of
mother–child dyads who are genetically unrelated with those
genetically related (Thapar et al. 2013). Such study designs have
revealed that maternal smoking during pregnancy was only
related to ADHD symptoms in offspring who were genetically
related to their mothers (Thapar et al. 2009). Despite this, the
relatively small amount of mothers who smoke during pregnancy
(or report smoking during pregnancy) has resulted in
small sample sizes to date and the potential environmental risk
of smoking during pregnancy cannot yet be categorically ruled
out (Thapar et al. 2009).