MONDAY, Aug. 15 (HealthDay News) -- Younger siblings of a child with an autism spectrum disorder have a nearly one in five chance of being diagnosed with autism, much higher than previous estimates, a new study finds.

The researchers found that 18.7 percent of children who had a sibling with autism also went on to receive an autism diagnosis by their third birthday, and the risk was significantly higher for boys and those with more than one autistic sibling.

"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, a 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.

Previous research has estimated that about 3 percent to 10 percent of younger siblings of kids with an autism spectrum disorder (ASD) will also be diagnosed with the condition.

The study, said to be the largest study of its kind to date, is published online Aug. 15 and in the September print issue of Pediatrics.

About 1 in 110, or less than 1 percent, of the general population of children born today has an autism spectrum disorder, which is characterized by problems with social interaction, verbal and nonverbal communication and restricted interests and behaviors. The overwhelming majority are boys, and genetics are known to play a strong role in autism's development.

For the study, a team from the Baby Siblings Research Consortium, an international network studying the earliest signs of ASD in infants with an affected sibling, tracked 664 younger siblings of autistic kids from an average of 8 months old until age 3. The children were examined and assessed several times during the study period.

They found that 132 children -- including 103 boys and 29 girls -- met the criteria for an ASD diagnosis at age 3. About 41 percent were diagnosed with autism, while 59.1 percent were diagnosed with "pervasive developmental disorder not otherwise specified," a milder form of autism.

In terms of gender, about 26 percent of male siblings of autistic kids had autism versus nearly 10 percent of female siblings, the researchers said.

Roughly 32 percent of children with multiple older autistic siblings also received an autism diagnosis, compared to 13.5 percent of those with only one affected older sibling.

Many parents ask about their odds of having another autistic child, Ozonoff said. Though this research gives them more information, the statistics won't necessarily help families determine their individual risk.

The risk estimates are averages across all 664 kids. In order to tell parents what their personal risk is, researchers would need to know their specific genetic or environmental risk factors, something that remains a mystery for the vast majority of autism cases.

The findings don't "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."

Nor can researchers tell how severe the condition might be if younger siblings are affected, she added.

"We are also not good at predicting severity. One can be very mildly affected or severely affected. One child could have tiny little tinges of autism and could be a very high functioning, charming child that does really well, and another could be a child who has severe limitations," Ozonoff said.

The estimates should, however, remind pediatricians to watch younger siblings carefully, said Alycia Halladay, director of research for environmental science at Autism Speaks, which provided partial funding for the new study.

"This can be used for parents with a child on the spectrum to alert their pediatricians if they have a concern, and for pediatricians to monitor the sibling cohort that is at risk closely," Halladay said.

The sooner a diagnosis is made, the sooner kids can receive interventions to help with language and social skills, she added.

More information

The U.S. National Institute of Neurological Disorders and Stroke has more on autism.