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ROBERT SIEGEL, host: Researchers have some important new information for families who have a child with autism. They now have a better idea of the risk of autism for that child's younger siblings. And as NPR's Richard Knox reports, it is higher than experts had thought.

RICHARD KNOX: It's one of the first questions parents ask after a child's been diagnosed with autism. What's the chance this could happen again? Earlier, smaller studies suggested it was between 3 and 14 percent, but the new research shows it's at least 19 percent. And Sally Ozonoff, who led the study, says for some children, the risk is even higher.

Dr. SALLY OZONOFF: If a family has a child with autism and now, for example, has a newborn boy, that baby's risk of having autism are one in four, essentially. And that's a really high rate compared to the general population, one in 110.

KNOX: Ozonoff says if there are two older siblings with autism, the risk jumps to one in three. She's a researcher at the University of California, Davis. But she says there are two ways of looking at this.

OZONOFF: No matter how alarming these statistics are - and truly, they are for us as researchers and clinicians and parents - I also tell families that even in the highest risk groups in this study, the likelihood of that new baby not having autism is always higher than the likelihood of them having autism.

KNOX: The new study appears in the journal Pediatrics. It has important implications for families wondering if a younger sibling of an autistic child may be affected, too.

Alycia Halladay works for Autism Speaks, an advocacy group that helped fund the new study.

Dr. ALYCIA HALLADAY: It tells parents who may show a concern, don't just wait and see. Take your child in consistently, and monitor them carefully. And it also is a message to health-care providers that they should be listening to the concerns of parents.

JUDITH URSITTI: OK, what do you want me to do?

JACK: Just spin me.

URSITTI: Spin you one more time?

JACK: Yeah.

URSITTI: OK. Ready?

JACK: Yeah, go.

URSITTI: Go.

KNOX: Judith Ursitti is twirling her son, Jack, in a swing at their home in suburban Boston. Jack, who's 7, is severely autistic. His 11-year-old sister, Amy, was diagnosed two years ago with Asperger's syndrome, a milder form of autism.

URSITTI: I missed those red flags, and my children weren't diagnosed as early as they could have been. So I do live in the world of shoulda-coulda-woulda, because they both had signs much earlier than they were diagnosed.

KNOX: Autism specialists say there's good evidence now that early diagnosis and treatment can offset many of the symptoms of autism: lack of interest in other people, reluctance to speak, learning problems, difficulty in making wants and needs known.

Ursitti worries that Jack's missed some of those chances.

URSITTI: And I don't know for sure but I really - I think it could have made a huge difference in his life had we known.

KNOX: Still, she says, the attention he's been getting the past three years is making a difference.

URSITTI: Between school and then treatment outside of school, medical treatment, he really started to learn to utter syllables, learned to nod his head yes or no, and then it just developed from there, you know. Now he can say, I want to put on my shoes - you know - or, I want to go swing.

KNOX: Researcher Sally Ozonoff says there's another important implication in the latest research. It has to do with the contribution of genes versus environment in autism. The new study clearly shows genes are important. But another study, published just last month, shows that there's a higher risk of fraternal twins both getting autism - roughly 30 percent. Ozonoff points out that genetically, fraternal twins are just like any two siblings who are not twins.

OZONOFF: That suggests that the shared intrauterine environment may also contribute to autism.

KNOX: So researchers are looking for things that pregnant women are exposed to that may help unravel the puzzle of just what causes autism.

Richard Knox, NPR News.

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