Nearly one in five children with an autistic older sibling may develop the developmental disorder as well, according to the findings from a new study, a rate considerably higher than previously thought.

More than 650 infants with at least one older sibling with autism took part in the study, which was published Monday in the journal Pediatrics.

In total, 19 per cent of that sample, or 132 of the infants, were diagnosed by age 3. Among infants who had more than one older sibling with autism, 32 per cent were diagnosed.

Older studies, by comparison, had placed the prevalence rate at between 3 and 14 per cent for children with at least one autistic brother or sister.

"We were all a bit surprised and taken aback about how high it is," said Sally Ozonoff, lead author of the study and professor of psychiatry and behavioural sciences at the University of California.

Boys were more than twice as likely to develop autism, the researchers found, with 26 per cent being diagnosed compared to 9 per cent of the girls.

The study was conducted at 12 sites across the United States and Canada. Earlier studies were conducted locally, or took place at fewer sites.

Alycia Halladay, with the advocacy group Autism Speaks, said the study's recurrence rates were "definitely much higher than what's been previously reported." She also noted that older studies had used smaller sample sizes.

The study's results could encourage parents to carefully screen siblings of autistic children and track their developmental milestones, so that they can be more easily diagnosed and receive early intervention services, Halladay said.

Studies have shown that early detection and treatment of autism "leads to better outcomes," she told CTV's Canada AM.

Ozonoff said that pediatricians "need to listen and make a very focused plan for how to monitor" children who have siblings with the disorder "rather than taking a wait-and-see attitude."

She said she is often asked by parents of autistic children, "How likely am I to have another child" with the disorder?

While 80 per cent of the siblings who took part in the study did not develop the disorder, the prevalence rate is an average and could vary depending on the presence or absence of other risk factors in each family.

New York resident Kathleen Lanese told The Associated Press that having one son with autism didn't make her reconsider trying to have another child.

"We were going to take whoever we got," said Lanese.

When her younger son was born, she said she "watched him like a hawk" for signs of autism. He was diagnosed at 16 months.

The cause of the disorder remains unclear, but research suggests a combination of genetics and external factors play a role. Experts are investigating whether factors such as infections, pollution or other environmental factors could be involved.

With files from The Associated Press