One month before William and Carissa Hawn's second son was born, their first son, Logan, then 3, was diagnosed with autism.
"Logan was our first experience with autism," said William Hawn, 34, of Modesto, Calif.
But when their second son, Aaron, was born, his pediatrician told the Hawns there was a 10 percent chance that he too would develop autism.
Earlier studies are consistent with Aaron's pediatrician. Infants who have siblings with autism have a 3 to 10 percent increased risk for autism, a higher chance than the 1 percent risk among the general population.
But a new study published Monday in the journal Pediatrics now suggests the risk is higher than previously thought. The study, considered the largest autism study to follow infants for sibling recurrence, found that infants with an older autistic sibling have a near 19 percent risk that they too will develop the disorder.
"We were surprised and distressed to see how high the recurrence risk is," said Sally Ozonoff, professor of psychiatry and behavioral science at the MIND Institute at University of California Davis.
Researchers from 12 different sites across the U.S. and Canada followed 664 infants with at least one older sibling diagnosed with autism. Within three years, nearly 19 percent of the infants were diagnosed with autism. Thirty-two percent of those infants who had more than one sibling with autism were also diagnosed with the disorder. And the risk of autism nearly doubled for male infants, the study found.
Logan, now 4, has undergone almost a year and a half of applied behavior analysis.
"The progress he's made has been undescribable," said Hawn.
Hawn said Logan would have probably progressed further by this point if he had been diagnosed earlier.
"With Logan we were late in the game in some ways," said Hawn.
His brother, Aaron, who was enrolled in Ozonoff's study, was diagnosed at 18 months and has begun early behavioral intervention.
"In this study we did not find that the gender of the existing child in the family helps predict recurrence," said Ozonoff.
Since there are other risk factors for autism that could include genetic markers, an individual family's risk differs, Ozonoff said.
In fact, many parents overestimate the recurrence risk. Ozonoff said in her clinic, many parents predict as high as 50 percent likelihood that their subsequent child will have autism.
"For parents, [these number offer] awareness and a more accurate estimate," said Ozonoff.
Hawn and his wife Carissa grieved after receiving Logan's diagnosis. Although they were told that Aaron was also at risk for autism, Hawn said they redirected their focus on helping Logan, and tried to manage any environmental causes they thought might lead to autism.
"We tried to ignore the possibility," said. "We chose to prioritize the pain that we were dealing with and ignored it."
Aaron was already born by the time the Hawns were informed about their risk. But Hawn said, had they known about the increased before Aaron was conceived, they would have reconsidered.
"If we were given anything above 15 percent, we would have reconsidered," said Hawn. "I wouldn't have thought about having a second child if I knew, and we would've thought about adopting."
These findings could help parents who may be considering another child understand their overall quantifiable risk of autism recurrence. But these findings do not mean that every family is at the higher spectrum of risk, Ozonoff said.