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The findings confirm those of other studies and support the idea that much of autism has a genetic base, researchers say.
"A child with breech is twice as likely as a child who did not present as breech to develop autism," said lead researcher, Dr. Deborah Bilder, an assistant professor of psychiatry at the University of Utah School of Medicine.
A breech presentation occurs when the infant presents with the legs and buttocks first instead of the head. These children are usually delivered by cesarian section.
For mothers 35 and older, the risk that their child will develop ASD is 1.6 times greater than for children of younger women, and for women having their first child, the risk is 1.8 times higher than if she has delivered before, Bilder said.
The absolute risk for autism in any child is still relatively small, Bilder stressed. "I don't think there is good reason right now to instill worry in moms over 34 who are already concerned with other genetic issues," she said. "But I do think this is a finding to pursue further, because this suggests a potential genetic cause for autism."
There is no doubt that ASD runs in families, Bilder said. "I see a lot of families with autistic children, particularly in Utah where families are larger," she said.
But there may be other causes of autism, Bilder said. "We are also looking at environmental issues -- prenatally," she said.
The report is published in the May issue of Pediatrics.
For the study, Bilder's team collected data on eight-year-olds born in Utah in 1994 who were diagnosed with ASD in 2002 via the Autism and Developmental Disabilities Monitoring Network. The researchers found birth records for 132 of these children.
Bilder's group then compared those children's birth records with the records of 13,200 boys and girls who did not have ASD.
The researchers found that ASD occurred more often among children of mothers 35 and older, mothers who were having their first child and children born in a breech delivery.
Bilder does not think that any of these factors are causes of ASD. "The implication is that there is a prenatal factor involved, which lends itself to breech presentations, that is also lending itself later to the development of autism, such as an impairment in neuromuscular development," she said. The "prenatal factor" might involve a genetic or environmental component, the researchers said.
Dr. Eugene R. Hershorin, an associate professor of clinical pediatrics and medical director of the Behavioral Pediatrics Clinic at the University of Miami, said the findings raise some questions.
"The problem is, you don't know what to make of these findings -- what's the cause-and-effect?" Hershorin said. "Is it if you are going to be autistic, you are going to have a breech presentation? Or is it that breech presentation has some influence on whether or not you are going to be autistic?"
Hershorin noted that the risk of abnormalities in children of older mothers is well known and perhaps a more important consideration for parents than a breech presentation.
SOURCES: Deborah Bilder, M.D., assistant professor, psychiatry, University of Utah Medical School, Salt Lake City; Eugene R. Hershorin, M.D., associate professor, clinical pediatrics, medical director, Behavioral Pediatrics Clinic, University of Miami; May 2009, Pediatrics
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