No link found between prenatal exposure to tobacco smoke and autism, Swedish study finds

Researchers have considered a variety of chemical exposures in the environment during pregnancy and early life as possible contributing factors in the development of autism spectrum disorders. Many have considered prenatal exposure to tobacco smoke a possible cause due to known associations with behavioral disorders and obstetric complications. Past studies of maternal smoking and autism have had mixed results.

"We found no evidence that maternal smoking during pregnancy increases the risk of autism spectrum disorders," said Lee, an epidemiologist at Drexel's School of Public Health, who led the research in collaboration with researchers from Sweden's Karolinska Institute and the University of Bristol (Bristol, UK). "Past studies that showed an association were most likely influenced by social and demographic factors such as income and occupation that have associations with both the likelihood of smoking and with the rate of autism spectrum disorders."

In the new study, Lee and colleagues analyzed data from Swedish national and regional registries for a set of 3,958 children with autism spectrum disorders, along with a control set of 38,983 children born during the same period who did not receive an ASD diagnosis. Overall, 19.8 percent of the ASD cases were exposed to maternal smoking during pregnancy, compared to 18.4 percent of control cases. These rates showed an association between maternal smoking and the odds of an autism spectrum disorder, in unadjusted analyses. However, the association disappeared when the analysis was adjusted for sociodemographic factors such as the parents' income level, education, and occupation.

The report helps to reassure mothers who smoked during pregnancy that their behavior wasn't likely responsible for their child's autism, Lee said, and "crosses off another suspect on the list of possible environmental risk factors for ASD." He cautioned, however, that smoking during pregnancy is still unhealthy for mothers and has other known risks for their children.

Lee received his Ph.D. and M.H.S. degrees in Epidemiology from The Johns Hopkins University, and graduated Cum laude with an A.B. in Biological Anthropology from Harvard College. His research interests include the epidemiology of neurological development, maintenance and decline, including prenatal environmental exposures and autism risk; gene-environment interaction; and epidemiological methods including causal inference methodology, data mining and machine learning algorithms.

Lee was recently awarded a 3-year grant from Autism Speaks to study whether early immune system abnormalities are associated with the risk of autism spectrum disorders.

The maternal smoking risk study was funded by a grant from the Stockholm County Council.