Aug. 15, 2011 -- The risk of autism in a child whose older sibling has been diagnosed with the disorder is higher than previously believed, according to new research.
Autism and autism spectrum disorders, a group of developmental disorders that affect the ability to think, communicate, and socially interact, affect one in 110 U.S. children, according to the CDC.
In the past, the risk of an autism spectrum disorder occurring in a younger brother or sister was estimated to be from 3% to 14%. However, researchers have now found it is close to 19%.
"The average risk for the whole sample was 18.7%,'' says researcher Sally Ozonoff, PhD, professor of psychiatry and behavioral sciences at the MIND Institute at the University of California, Davis.
Certain children had an even higher risk, she says. ''Families who had male infants had a higher recurrence of 26.2%," she tells WebMD. ''Families that had more than one child with autism prior to the birth of this infant [in the study] had almost a one in three risk, or 32%."
"Overall we found this almost 1 in 5 risk, but more like 1 in 4 if the new baby was a boy and 1 in 3 if the family has more than one child with autism."
The study is published in Pediatrics.
Autism Sibling Risk: Practical Matters
"This was very sad," Ozonoff says of the new estimate. "It brought tears to our eyes for many of us."
However, she says, the information is valuable. Parents can ask for close monitoring of a young sibling. That way, if autism is detected, early intervention can begin. The new information may also help parents in their family planning decisions.
Autism Sibling Risk: Study Details
The researchers evaluated infants who were enrolled in an international network known as the Baby Siblings Research Consortium. All 664 infants had an older sibling with an autism spectrum disorder.
The infants were followed from about 8 months of age until 36 months. The children were evaluated many times during the three years.
At the 36-month mark, the children were classified as having an autism spectrum disorder or not.
At the end of the follow-up, 132 children were diagnosed with an autism spectrum disorder. Of the 132, about 41% were diagnosed with autism and 59% were diagnosed with other conditions on the spectrum, such as Asperger's syndrome.
It is impossible, Ozonoff says, to estimate an individual family's risk.
"Remember that overall, over 80% of those babies didn't have autism," she says. "The general population risk is under 1%."
Autism Sibling Risk: Perspective
The new estimates are very sound, says Laura Schreibman, PhD, director of the Autism Intervention Research Program at the University of California, San Diego. She reviewed the study findings but was not involved in the research.
The design of the study, which followed the infants forward, ensures more accuracy than looking backward, she says. "The fact that they could look ahead meant they could be certain about how the diagnoses were obtained." The group included children from 12 locations and was ethnically diverse. Both of these factors reduce bias.
"Families need to know this is an estimate," she says. "It doesn't reflect what will happen to an individual family."
"This new study provides a more definitive estimate of the recurrence of autism in younger siblings," says Alycia Halladay, PhD, director for environmental research for Autism Speaks. Autism Speaks supports the Baby Siblings Research Consortium. Autism Speaks, the National Institutes of Health, and other organizations supported the study.
For parents who have an older child with autism, the new information should motivate them to be sure the younger child has close monitoring, she says. That should be done as early as six months, she tells WebMD.
Genetic counselors can use the information to help parents interpret the findings, says Karin Dent, president of the National Society of Genetic Counselors and assistant professor of pediatrics at the University of Utah, Salt Lake City.
"The updated data from this study ... should be incorporated in genetic counseling sessions with parents and families of affected individuals," she says.
"This will help families to have a number on something that is difficult to assess."