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What the findings suggest is that although autism spectrum disorders share many of the same symptoms, subtypes of the disorder likely have many different genetic and environmental causes that vary from person to person and by type of autism, explained study author Amy Kalkbrenner, an assistant professor in the University of Wisconsin-Milwaukee Zilber School of Public Health.
"We know 'autism spectrum disorders' is an umbrella term. What we're showing is the response to a environmental toxin may differ by the subtype of autism a child has," Kalkbrenner said.
The study was published online in the journal Environmental Health Perspectives.
Kalkbrenner and her colleagues examined data on maternal smoking from birth certificates of nearly 634,000 U.S. children born in 11 states in 1992, 1994, 1996 and 1998. That data was compared with information on 3,315 children aged 8 and under diagnosed with an autism spectrum disorder from the U.S. Centers for Disease Control and Prevention's Autism and Developmental Disabilities Monitoring Network.
According to the study, kids born to moms who smoked during pregnancy had about a 25 percent increased risk of having high-functioning autism, such as Asperger's syndrome. However, the results did not reach statistical significance.
Nor was smoking a clear risk factor for autistic disorder (a more severe form of autism).
The researchers noted that the data used in the study may underestimate the true prevalence of autism spectrum disorders among mothers who smoke because lower-income kids are less likely to be identified as having autism, and lower-income mothers are also more likely to smoke during pregnancy.
When researchers did another statistical analysis that took into account a suspected undercounting of kids with autism, the analysis did suggest a statistically significant association between smoking and high-functioning autism in offspring.
Alycia Halladay, director for environmental research for Autism Speaks, said the research is consistent with prior studies that have found either no association or only a mild association between smoking during pregnancy and autism in children. What's interesting about this paper is that it included data on large numbers of kids, she added, and it hinted at differences in the contributing factors for various types of autism spectrum disorders.
"It really supports the idea that there are multiple causes of autism, both genetic and environmental. When we talk about autism being one group or disorder, we really need to ensure we have these groups as well-defined as possible," Halladay said. "This is a very heterogeneous disorder."
There are multiple reasons why tobacco might raise the risk of autism, Kalkbrenner noted. Tobacco can restrict oxygen flow to the baby, while the nicotine is known to interact with the nervous system and cross the placenta into the developing fetus. "There are many potential biological pathways for which tobacco can harm the developing baby," she said.
Autism is a neurodevelopmental disorder that's characterized by problems with social interaction, verbal and nonverbal communication and restricted interests and behaviors. An estimated one in 88 U.S. children has the disorder, according to the CDC.
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