Greater autism risk for siblings than believed, study says

The risk that parents who have a child with autism will have another with the disorder is higher than previously believed, according to new research published this week online in Pediatrics, the scientific journal of the American Academy of Pediatrics.

Researchers at the University of California Davis Mind institute say there's a 1 in 5 chance of having a second child with an autism spectrum disorder. Previously, that risk was believed to be between 3 and 10 percent.

The Centers for Disease Control and Prevention defines autism spectrum disorders (ASD) as a group of developmental disabilities that can "cause significant social, communication and behavioral challenges," including language delays, social and communication challenges and unusual behaviors and interests. Many have intellectual disability, but the designation covers a wide spectrum of symptoms and the degree to which one is impacted varies, as well. CDC believes about 1 in 110 children in the United States has an ASD.

The study authors wrote that the previous research "was limited by small sample sizes and biases related to ascertainment, reporting and stoppage factors." The new research relies on prospective methods to update the estimate of sibling recurrence. A prospective study "follows over time a group of similar individuals (cohorts) who differ with respect to certain factors under study, to determine how these factors affect rates of a certain outcome," Wikipedia notes.

Using a multi-site, international network, the Baby Siblings Research Consortium followed 664 babies who had an older sibling with autism spectrum disorder from early life to 3 years, at which point they were categorized by whether they met the classification requirements for autism spectrum disorder or not.

The researchers found that 18.7 percent of the infants did develop ASD — and the number was 32 percent if two or more siblings had ASD. The risk was nearly three times greater for males, 26 percent compared to the 9 percent for females. To that risk could be added "an additional twofold increase in risk" in families where an older sibling was affected with the disorder.

New York Times blogger Anahad O'Connor puts the numbers in perspective this way: "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 infant's age at enrollment, the gender and how well the older affected sibling functioned, birth order and the age of the parent were among factors that did not predict whether a child would be diagnosed with ASD.