The risk of younger siblings developing an autism spectrum disorder is 14 times higher if an older sibling has the disorder.
Autism is a neuro developmental disorder defined by impairments in social interaction and communication, as well as restricted and repetitive patterns of behavior.
The findings showed that children with older siblings who had autism had the disease rate of 11.3 percent compared to 0.92 percent for those with unaffected older siblings.
The risk level was found consistent across gestational age at birth.
Younger siblings who were born at preterm (28-36 gestational weeks) and had an older sibling diagnosed with autism were at 10 times increased risk for developing the developmental disorder.
"Our study provides additional insights into how autism affects siblings," said Darios Getahun, Scientist at Kaiser Permanente - a US-based health care company.
Further, younger boys with Autism who had older brothers were much more likely to be affected by the disorder than younger girls with older sisters (15 percent vs. 7 percent, respectively.)
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"The study also contributes to a better understanding of the influence of factors such as gender on autism risk," Getahun added.
Previous studies have found that second-born children who are conceived sooner than two years or later than six years after the arrival of their older sibling have a significantly increased risk of autism.
In addition, children born at term (37-42 gestational weeks) who had an older sibling diagnosed with autism had more than 15 times the increased risk for autism diagnosis.
"It is possible that parents who have an older child with an autism diagnosis are more likely to have their younger siblings tested, too, resulting in a higher rate of diagnoses among younger siblings, compared with parents who do not have children diagnosed with an autism spectrum disorder," Getahun noted.
For the study, the team focused on 592 children diagnosed with autism spectrum disorder and with at least two siblings born to the same mother between 28 and 42 weeks of gestation from 2001 through 2010.
(With agency inputs)