(AP, ABC7) - Windra Bowman has two sons. Both are under five years old and both have autism. 4-and-a-half-year-old Nickolas and 2-and-a-half-year-old Anthony.
"The only reason why I caught some of these really subtle signs was because of my older son," Bowman says about noticing Anthony had the disorder, too.
Anthony's diagnosis came at just one year old, after Bowman enrolled in a study that examined whether siblings are more prone to autism. The findings of the study released today in pediatrics confirm what she suspected.
The study suggests nearly one in five children with an autistic older sibling will develop the disorder too - a rate much higher than previously thought.
Researchers followed 664 infants who had at least one older brother or sister with autism. Overall, 132 infants or about 19 percent ended up with an autism diagnosis, too, by their third birthdays. Previous smaller or less diverse studies reported a prevalence of between 3 percent and 14 percent.
"We were all a bit surprised and taken aback about how high it is," said lead author Sally Ozonoff, a psychiatry and behavioral sciences professor with the Mind Institute at the University of California at Davis.
The highest rates were in infants who had at least two older siblings with autism - 32 percent of them also developed autism. Also, among boys with autistic siblings - 26 percent developed autism versus 9 percent of girls. Autism is already known to be more common in boys.
The study involved 12 U.S. and Canadian sites and was published online Monday in Pediatrics. Earlier studies were more local or involved fewer sites.
Ozonoff said parents of autistic children often ask her, "How likely am I to have another child" with autism? She said her study provides a more up-to-date answer.
However, Ozonoff noted that 80 percent of siblings studied did not develop autism, and that the prevalence rate was an average. It may be different for each family, depending on other risk factors they may face.
Autism has no known cause but experts believe that genetics and external influences are involved. Research is examining whether these could include infections, pollution and other non-inherited problems. Ozonoff noted that siblings often are exposed to similar outside influences, which could partly explain the study results.
Infants in the study were enrolled before they showed any signs of autism, such as poor eye contact and little social interaction.
The study is an important addition to autism research and "has critical implications for families who are deciding whether they'll have another child," said Catherine Lord, director of the Institute for Brain Development at New York-Presbyterian/Weill Cornell Medical Center. Lord was not involved in the study.
"That kind of proves that there is a genetic component, there could be an environmental component on top of it triggering but I think that there's a genetic component that's at the base of this," said Bowman.
"A lot of parents may say, you know what, let's either stop or let's wait or maybe look into adoption because it is a lot," Bowman said.
"I can't do things a normal mom can do. I can't take both of my children to the park at the same time, I can't go to the grocery store with both children by myself," she said.
Ozonoff said the study should prompt families and their children's doctors to be vigilant with infants whose older siblings have autism. Early diagnosis is important because experts say behavioral treatment has the best chance of working if started early.
"Pediatricians need to listen and make a very focused plan for how to monitor those things, rather than taking a wait-and-see attitude" toward children with autistic siblings, Ozonoff said.
Alycia Halladay, a research director at the advocacy group Autism Speaks, said the study provides a more robust, accurate prevalence estimate than previous studies, and strengthens the idea that family history is a risk factor.
Her group, the National Institutes of Health and the Canadian Institute for Health Research are among those who paid for the study.