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 env
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