The risk that a child with an older sibling with autism will also develop the condition is higher than previously thought, a US study has found.

Previous estimates of the risks faced by a younger sibling in developing autism ranged from 3-10%, but research led by the UC Davis MIND Institute in California found it is up to 18.7%. Researchers, led by Sally Ozonoff, professor of psychiatry and behavioral sciences at the MIND Institute, also found an even more elevated risk of recurrence of more than 26% for boys, and over 32% for infants with more than one older sibling with autism.

"This is the largest study of the siblings of children with autism ever conducted," Ozonoff said. "There is no previous study that identified a risk of recurrence that is this high." Ozonoff added that the study has significant family planning and genetic-counselling implications and for referral to early intervention for the infant siblings of children with autism if concerns arise about their development. "Parents often ask what their risk of having another child with ASD [autism spectrum disorder] is and, until now, we were really not sure of the answer." The international, multi-site study took in 664 subjects, infants whose average age at enrolment was 8 months, with two-thirds recruited prior to 6 months of age.

The researchers followed the participants' development until 36 months, when they were tested for autism. It is published online and will appear in print in the September issue of the journal Pediatrics. But Ozonoff was careful to point out that the results are estimates averaged across all of the families that participated. "So, for some families, the risk will be greater than 18.7%, and for other families it would be less than 18.7%. At the present time, unfortunately, we do not know how to estimate an individual family's actual risk," she said. "This study shows that the younger siblings of children with autism spectrum disorders need to be tracked very carefully, and this may require more than the normal surveillance that a pediatrician might typically do. This should include very explicitly and regularly checking in with parents on whether developmental milestones are being reached."