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No clear evidence that in utero exposure to maternal smoking increases risk of depression in offspring

1 November 2017

A recent study from the MRC IEU did not find clear evidence that offspring of mothers who smoke have a greater risk of developing depression due to changes within the uterus. Researchers concluded that the previously observed association between maternal smoking and offspring depression may be caused by shared environmental/genetic factors.

Smoking during pregnancy has many well-established risks including a higher risk of pre-term birth and low birth weight. However, less is known about effect of maternal smoking on mental health. Research in animals has indicated that exposure to nicotine or tobacco smoke in utero can disrupt the development of the brain and cause epigenetic changes to genes associated with areas of the brain which are often overactive in people with depression.

The study used three different methodologies to isolate whether the association between maternal smoking during pregnancy and offspring depression was likely to be causal. Firstly, the association of maternal smoking was compared to that of the mother’s partner’s smoking with offspring depression. If maternal smoking during pregnancy were a cause of offspring depression, the association of maternal smoking during pregnancy with offspring depression would be expected to be larger than the association between mother’s partner smoking during pregnancy and offspring depression. If the associations are of similar magnitude, it suggests that the association between maternal smoking and offspring depression may be due to confounding by other factors.

Secondly, researchers compared the association of maternal smoking during pregnancy with offspring depression across three different data sets from different countries (UK, Norway and Brazil). If maternal smoking during pregnancy were a cause of offspring depression, it is likely that this association would be seen across different countries with different patterns of confounding (e.g. lifestyle and demographic) factors.

Finally, researchers compared depression in siblings were the mother had smoked during the pregnancy of one sibling, but not the other. The sibling data was taken from a Swedish sibling study.

Researchers found that although a modest positive association was observed between maternal smoking during pregnancy and offspring depression in the UK, Brazilian and Norwegian studies, there was only weak evidence for a difference between the association of mother’s smoking and mother’s partner’s smoking with risk of depression in offspring. In addition, siblings exposed to maternal smoking in utero did not have a greater risk of developing depression compared to unexposed siblings. This indicates that the association between maternal smoking and depression may be caused by confounding factors which cause both maternal smoking during pregnancy and offspring depression. One possible example of this could be maternal mental health.  

The full study is available in scientific reports.

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