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Risk factors for autism remain elusive: study

By Amy Norton

NEW YORK (Reuters Health) - Studies have hinted at various factors around the time of birth that may raise a child's risk of autism -- but there is still too little evidence to point to specific culprits, a new research review concludes.

Looking at 40 previous studies, researchers found that a range factors around the time of birth have been linked to the risk of autism later in life.

Those include low birth weight, certain delivery complications like problems with the umbilical cord, fetal distress during labor and signs of "poor condition" in the newborn -- such as problems with breathing or heart rate.

But the studies often came to conflicting conclusions as to whether any single one of those factors was related to autism. On top of that, the researchers say, birth and newborn complications generally do not occur in isolation, but in combination.

And in fact, in a complex disorder like autism, it would be very unlikely that a single birth factor would stand out as the key culprit, explained Hannah Gardener, a researcher at the University of Miami School of Medicine who led the study.

In an interview, she stressed that parents of children affected by any one factor identified in this study -- low birth weight or umbilical-cord problems, for example -- should not be alarmed.

"There is no single strong cause of autism," said Gardener, who was at the Harvard School of Public Health at the time of the study.

"It's important that parents not worry about any particular one of these risk factors."

Moreover, autism is generally believed to involve a complex interaction between genes and environmental factors.

The current findings, Gardener said, underscore the importance of continuing to study which environmental factors -- whether before, during or after birth -- may act in concert with genetics to cause autism.

She and her colleagues report their findings in the journal Pediatrics.

It's estimated that about one in every 110 U.S. children has an "autism spectrum disorder."

The term refers to a group of developmental brain disorders that hinder a person's ability to communicate and interact socially -- ranging from the severe cases of "classic" autism to relatively mild forms like Asperger's syndrome.

Experts have long believed that genes play a key role in autism risk. That's based largely on twin studies showing that when one identical twin develops autism, the other has a high likelihood of being affected as well.

Most studies have found less similarity between fraternal twins. Unlike identical twins, who share all their genes, fraternal twins share only about half.

But a twin-based U.S. study published last week found that genes appeared to explain a much smaller portion of the risk than previous studies have suggested.

The researchers estimated that environmental factors common to twins accounted for about 55 percent of the risk.

But they could not weed out what those factors might be.

That study, Gardener said, highlights the need for further research into the role of environment in autism.

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