MONDAY, June 2 (HealthDay News) -- Children who are born underweight or early have more than double the risk of developing autism, new research shows.

The risk was especially pronounced among low-birth-weight girls, said the authors of the study, which was published in the June issue of Pediatrics.

The study, by researchers at the U.S. Centers for Disease Control and Prevention, helps tease out the mysterious underpinnings of this disorder but is unlikely to translate into benefits for patients anytime soon.

"This gives us more clues [about autism], which we desperately need, but it's not anything clinicians can use right away," said Dr. Cindy Molloy, an autism researcher and clinical assistant professor of pediatrics at the Center for Epidemiology and Biostatistics at Cincinnati Children's Hospital.

The results do reinforce the importance of monitoring children who are born underweight or early for behavioral problems so they can be treated, said study author Diana Schendel, lead health scientist at the CDC's National Center on Birth Defects and Developmental Disabilities.

So-called "autism-spectrum disorders" are a group of developmental disorders characterized by social and communication problems. According to the National Institute for Neurological Disorders and Stroke, some three to six children out of every 1,000 will have autism, while males are four times more likely to develop the disorder than girls.

Previous studies have indicated that low birth weight and being born premature are important risk factors for developmental problems generally in children. But the association between these factors and autism is less clear.

A Canadian study published earlier this year did find that premature infants who were born at a very low birth weight -- about 3.3 pounds -- were more likely to screen positive on tests of autistic behaviors, but the findings were considered preliminary.

The investigators on the current study looked at 565 children with autism born in metropolitan Atlanta between 1986 and 1993, and compared them to a set of children without autism, as well as to children with other developmental disabilities, such as mental retardation, cerebral palsy, hearing loss or vision problems.

Overall, low birth weight was associated with a twofold increased risk for autism, but the risk was higher for girls than for boys.

For all low-birth-weight children, the risk for autism accompanied by other developmental problems, such as mental retardation, was higher than the risk of developing autism alone.

There was also double the risk for developing autism in babies born prematurely, although this was primarily due to a more than fivefold increased risk in girls born early.

"This was one of the first studies that had a large enough sample to look at girls," Molloy said. "They really were able to tease out what is different about boys and girls."

Even so, the elevated risk for autism seen in low-birth-weight and preterm babies was much lower than that linked with cerebral palsy, mental retardation, hearing loss or vision impairment.

"It's not yet clear why being small or being born too soon could lead to these problems but, Schendel said, "[these factors] could be a marker for an impaired fetus, one that has a neurological problem which is retarding its growth. On the other hand, being small or being born too soon may be related to factors that could harm the neurological development of the fetus such as infection during pregnancy."

The findings support the idea that there are different kinds of autism and different mechanisms underlying those cases, Molloy said.

More information

Visit the National Institute of Neurological Disorders and Stroke for more on autism.

SOURCES: Diana Schendel, Ph.D., lead health scientist, National Center on Birth Defects and Developmental Disabilities, U.S. Centers for Disease Control and Prevention, Atlanta; Cindy Molloy, M.D., assistant professor, pediatrics, Center for Epidemiology and Biostatistics, Cincinnati Children's Hospital; June 2008, Pediatrics

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