The results from this comprehensive study using several analytic methods showed that there is not currently enough evidence to support a causal association between maternal smoking during pregnancy in utero and offspring with ASD or autistic-like behaviours. This still does not mean smoking whilst pregnant is not detrimental to the unborn child's health and wellbeing.
The study is a population-based birth cohort with prospectively collected data published in Translational Psychiatry by Doretta Caramaschi et al, and uses a number of analytic strategies to study the association between maternal smoking in pregnancy and autism-related outcomes.
It is known that Autism spectrum disorders (ASDs) are complex neurodevelopmental disorders which are characterised by persistent deficits within social communication, and social interactions whilst also having restricted, repetitive patterns of behaviours, interests or activities.
There has been increased evidence suggesting that environmental factors around birth may play a role in the aetiology of ASD. Maternal smoking during pregnancy has been implicated in the aetiology of autism. The harm to infants of in utero exposure to tobacco smoke from maternal smoking is well established, and a quarter of a century has passed since it was first shown in a clinical trial that stopping smoking during pregnancy could avoid the adverse effects of smoking on birth weight. There are also increased literary reports on adverse effects in neuro-behavioural outcomes of the infants as a consequence of smoking such as ADHD.
Several population-based studies have investigated the association between maternal smoking during pregnancy and offspring ASD. Positive results in some cases have been found outside of Europe and the America's, inconsistent results have been returned in studies done thus far, this may have been due to a variety of reasons such as;
studies were of smaller sample sizes, lower quality, with a case-control design, postnatal assessment of maternal smoking, lack of adjustment for potential confounders and parental reports of ASD diagnosis.
In this study evidence was collated from several different epidemiological approaches, the study strengthens its results by not only using self-reported smoking but also measured using an epigenetic biomarker. A score was then calculated from the maternal blood DNA methylation profiles, thus preventing cohort bias from claiming to smoke less than they actually were.
Further adjustments were made for maternal depression to ensure estimates of correlations between variables were also included. Women with depression being more likely to smoke during pregnancy (smoking rates are higher in persons suffering with stress and depression), and also an association of depression in pregnancy with ASD has been previously reported in other studies. They also investigated the effect of partner’s smoking as a negative control. Finally, performing an exploratory MR using a maternal genetic variant as proxy for smoking heaviness.
This study's results are in line with the meta-analysis based on 22 studies, the combination of approaches in this study show null association with smoking in utero with ASD. This being said it does not negate the seriousness of smoking effects on the unborn child. Smoking during pregnancy is linked with a range of poor birth outcomes including:
Low birth weight and preterm birth
Restricted head growth
Placental problems
Increased risk of still birth
Increased risk of miscarriage
Health and developmental consequences among children have also been linked to prenatal smoke exposure, including:
Poorer lung function, persistent wheezing, and asthma, possibly through DNA methylation Visual difficulties, such as strabismus, refractive errors, and retinopathy as reported by NIH.
Open access study available; https://www.nature.com/articles/s41398-018-0313-5