Routine visits to the pediatrician are often far too short to accurately identify children at risk for autism, a new study suggests.

Researchers say even trained autism experts missed 39 percent of children on the spectrum when they were asked to screen kids by observing them in 10-minute videos in a study published online Monday in the journal Pediatrics.

The finding is significant, researchers say, since pediatrician visits often last just 10 to 20 minutes.

For the study, researchers videotaped 42 children ages 15 to 33 months while they were being assessed for autism. The kids in the study were evenly divided between those with the developmental disorder, individuals who were typically developing and children with speech delays but no autism.

Two licensed psychologists with autism expertise were asked to evaluate the children’s behaviors in the videos and determine whether further evaluation was warranted for each child.

In nearly 4 out of 10 cases, the specialists did not recommend further assessment for children with autism, the study found.

That’s because children on the spectrum tended to display more typical than symptomatic behaviors during the course of the 10-minute observations, researchers said, making it difficult to flag kids who are at risk.

The findings could help explain why most children with autism are not diagnosed until after age 4 even though the American Academy of Pediatrics recommends that all children be screened at 18 and 24 months, researchers said. Identifying children at young ages is seen as critical since early intervention is known to have the greatest impact.

“Certainly, some young children with autism are clearly impaired and easy to recognize,” said Judith Miller of the University of Utah, an author of the study. “However … we found that many children’s impairments are not immediately obvious. For those children, formalized screening instruments and more time with a specialist may be critical.”

Accordingly, researchers are urging parents to learn the signs of autism and to act if they have concerns.

“Parents see their children at their very best and very worst,” said Terisa Gabrielsen of Brigham Young University who led the study. “They’re the experts for their children. They can be educated about signs and symptoms, and need to help their care providers by speaking up if there’s a problem and being involved in referral decisions.”