Babies born small or prematurely go on to develop autism at higher rates, although the risk is still small, according to a new study from Finland.

The research is part of a global push to identify the culprits behind the developmental disorder and the recent uptick in its occurrence, which has had scientists scratching their heads for years.

“Previous reports of how birth weight or gestational age is associated with autism have not been consistent,” Dr. Andre Sourander, a psychiatrist at Turku University, told Reuters Health by email.

“Because autism spectrum disorders are one of the major challenges in child mental health it is extremely important to get more understanding of its causes,” Sourander said.

Autism spectrum disorders, which range from mild Asperger syndrome to severe mental retardation and social disability in childhood autism, are diagnosed in about one in 88 children in the U.S., according to the Centers for Disease Control and Prevention.

The new results, published in the Journal of Pediatrics, are based on almost two decades’ worth of data from more than one million births in Finland.

As of 2005, the rate of autism in the Northern European country was 9 per 10,000 children in Finland, whereas Asperger was diagnosed in 14.5 children out of 10,000.

After accounting for the mother’s age, smoking, number of previous births and other factors, Sourander’s team found an increased risk of autism, but not Asperger syndrome, in preemies and babies that were very small at birth.

For instance, those who weighed less than 1,500 grams, or 3.3 pounds, at birth had three times the odds of developing autism.

However, because autism is relatively rare, most children who are born very small don’t end up with autism, said Sourander.

No one is certain why some children develop autism spectrum disorders, but scientists assume it’s caused by an interplay between genes and environment, such as infections or other medical problems during pregnancy.

SOURCE: Journal of Pediatrics, online June 7, 2012

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