An autistic child peers from between curtains at the Consulting Centre for Autism in Amman, March 30, 2010, one of the few places in the country that helps children with the condition. Reuters

While less than one percent of the general population born today will be diagnosed with autism spectrum disorder (ASD), a new study found that statistic jumps to 18.7 in children who have an autistic sibling: They have nearly a one in five chance of being diagnosed with the disorder.

The results confirmed expert suspicions that previous risk estimates -- which put the sibling of an autistic child's risk of being diagnosed at three to ten percent -- were lower than realistic.

"Those of us working in the field knew the rate was a lot higher than the previously published rate, but I don't think we expected it to be this high, " said lead study author Sally Ozonoff, professor and vice chair for research in the department of psychiatry and behavioral sciences at the University of California Davis Medical Center's MIND Institute.

The study, which is believed to be the largest of its kind to date, was conducted by the Baby Siblings Research Consortum, a large-scale network of researchers at several universities throughout the U.S., and was published in the August 15 online edition of the journal Pediatrics.

According to the results of the study, "Infant gender and the presence of >1 older affected sibling were significant predictors of ASD outcome." Males are at triple the risk for developing the disorder than females, and the risk increases twofold if there is an elder sibling with ASD.

While the results of the study will likely be considered by parents of autistic children who are concerned about the risks for additional children, the conclusions of the study "tell any family that their particular risk is 18.7 percent. That's an average," Ozonoff said. "Some families will probably have almost zero risk, and some will have much higher risk. We are not yet at the point where you can go to a genetic counselor, have blood drawn and look for certain genes that increase risk for an individual family."

Still, those at a higher risk can monitor siblings of children with ASD and be alert for signs of the disorder at an early stage, when treatment is most effective.

Autism is an enigmatic spectrum disorder the usually manifests with the first three years of life. It primarily affects social and communication skills. The disorder varies in severity, and some children are able to thrive despite the disorder, while others remain largely unresponsive to treatment.

Government statistics suggest the prevalence rate of autism is increasing 10-17 percent annually. There is no known cause or cure for autism.