Autism Risk for Siblings Higher Than Expected

Parents who have a child with autism have about a 1 in 5 chance of having a second child with autism, a far greater risk than previously believed, new research shows.

The findings, published in the journal Pediatrics, are significant because they come from the largest study to date focusing on the risks of autism among siblings.

Researchers at the University of California Davis Mind Institute tracked 664 infants who had an older sibling with an autism spectrum disorder. The babies joined the study at 8 months of age and were followed until they reached 36 months. The vast majority of the children did not go on to develop autism, but about 19 percent of the children were diagnosed with the disorder.

Based on the study population, that suggests that roughly 1 in 5 younger siblings of children with autism go on to develop the disorder. In the past, it was thought that about 1 in 10 younger siblings of children with autism go on to develop the condition, an estimate that was derived from studies of much smaller groups.

By comparison, about 1 in 110 American children born today, or fewer than 1 percent, will be given a diagnosis of autism spectrum disorder. That means the younger sibling of an autistic child has nearly 20 times greater risk of being diagnosed with autism than a child in the general population. The study found that for families with two or more autistic children, the risk is even higher. About a third of children with two or more autistic older siblings also will develop the condition.

The findings prompted child development researchers to urge greater awareness for families who have a child with autism and may be considering more children. Although 1 in 5 is a greater risk than previously expected, it still means that 4 out of 5 children with an autism spectrum disorder will not develop the condition, said Alycia Halladay, the director of environmental research for Autism Speaks, an autism research group that supported the study along with the National Institutes of Health.

“Family history is a very strong risk factor, but there are other risk factors as well,” she said. “There are strong environmental risk factors. We don’t know what those are yet, but this is not the only factor that goes into an autism diagnosis.”

But autism specialists say families at risk should adopt a more careful approach to monitoring younger siblings, looking for potential signs of autism in infancy. Early signs of autism can be subtle, but symptoms that could warrant further investigation include a lack of smiling by six months, and the absence of babbling or gestures like pointing and waving by 12 months. The group lists on its Web site a number of milestones that parents can look for at various ages to make sure their child is developing normally, starting at about 3 to 4 months.

If a younger sibling does exhibit potential signs of the disorder, the parent should consult a pediatrician or specialist who can provide a more comprehensive assessment, Dr. Halladay said. Earlier diagnosis can lead to earlier intervention, which can have a larger effect on a child’s language and social development the sooner it is started.

“Parents who have a child with autism and then have another child shouldn’t assume that the younger child is going to have a diagnosis,” said Dr. Halladay, “but they should be alerted to that possibility, and they should be very attuned to the child’s behavior and development.”