Investigators caution that the association must be confirmed, and that the increase in risk observed in a study was quite small. The findings are reported in the February issue of the Archives of Pediatrics and Adolescent Medicine.
"This is a modest effect, and it is the first time, to my knowledge, that it has been reported," lead researcher Lisa A. Croen, PhD, of the Kaiser Foundation Research Institute tells WebMD. "These findings certainly need to be replicated. But this gives us some interesting clues to pursue."
Second Trimester Riskiest
The researchers also investigated, but failed to find, a link between autism and a family history of autoimmune diseases like lupus, type 1 diabetes, and rheumatoid arthritis. Earlier studies had suggested such a link, but most relied on self-reported data. Autoimmune disorders occur when the body mistakenly attacks normal tissues as if they were foreign.
Croen and colleagues examined the medical records of more than 88,000 children born between 1995 and 1999 in Northern California. Of these children, 420 were later identified as having autism or a related disorder. These cases were compared to 2100 nonautistic children of similar sex, age, and birth hospital.
The researchers found that expectant mothers with asthma diagnosed in the second trimester were twice as likely to have a child that developed autism. The researchers found no risk among women found to have asthma during the third trimester.
The risk was smaller for women with allergies. The only significant effect was for women diagnosed with allergies in the second trimester. These women were 2.5 times as likely to have a child that developed autism.
Clues About Where to Look Next?
Although the causes of autism are not well understood, genetic predisposition is considered to play a major role. A few environmental triggers have also been identified, including prenatal exposure to certain chemicals.
Andy Shih, PhD, tells WebMD that what we now lump together as autism is likely to end up being several related, but distinct disorders. Shih is chief science officer for the National Alliance for Autism Research.
He says the newly published study is the first to look at the issue of autoimmune disorders and autism in an objective way. He agreed, however, that the findings are far from conclusive.
"This study does provide a tantalizing clue about a potential mechanism for autism," he says. "But the cautionary tone of the paper needs to be emphasized. We don't really know if these findings will hold up to further scientific scrutiny."
Shih says a larger study with a more comprehensive methodological approach is needed to better understand the potential link between allergies, asthma and autism.
"I think this does provide a suggestion about where we should look next," he says.