Children who have an older brother or sister with autism are significantly more likely to be diagnosed with the developmental disorder than previously thought, new research suggests.

In a study of over 600 infants, all of whom had at least one older sibling with autism, nearly 1 in 5 were also eventually diagnosed with the disorder.

The number was even higher — rising to about 32 percent — when a child had more than one older sibling with autism, according to research published Monday in the journal Pediatrics.

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The findings, which come from the largest study on sibling recurrence to date, are well above previous risk estimates of between 3 and 10 percent.

“The recurrence rate is a lot higher than previous studies had indicated,” said Gregory S. Young, an assistant professional research scientist at the M.I.N.D. Institute at the University of California, Davis and one of the study authors. “Families who already have a child with autism or multiple children with autism need to be incredibly vigilant about monitoring new infants.”

For the study, Young and his colleagues at 12 research sites across the U.S. and Canada followed 664 infants who had at least one older brother or sister with autism until they reached age 3. At that point, each child was assessed by an expert for an autism diagnosis.

Ultimately, 132 children, or 18.7 percent, were diagnosed with an autism spectrum disorder.

Gender did appear to influence a younger child’s odds of autism, with boys three times more likely than girls to be diagnosed. However, other factors such as the age of the parents, birth order, gender of the affected sibling and the functional level of the older sibling did not seem to play a role in the likelihood of the younger child having autism.

The findings show the importance of early screening for the developmental disorder, researchers said, and could have implications for genetic counseling and decisions about family planning.

Further research is needed to corroborate the findings, said Young, who worked on the study. But he emphasized that the study’s large size, its focus on families that had a child after learning that an older child had autism and the use of clinical diagnosis rather than parent reporting to determine if a child had autism make his team’s results more reliable than previous recurrence estimates.