It’s long been thought that autism severity remained largely static over a person’s lifetime, but new research suggests otherwise.

A study looking at 125 kids on the spectrum finds that nearly half displayed a significant change in the level of their symptoms between the ages of 3 and 6.

“We found that nearly 30 percent of young children have less severe autism symptoms at age 6 than they did at age 3. In some cases, children lost their autism diagnoses entirely,” said David Amaral, a professor of psychiatry and behavioral sciences at the University of California, Davis and senior author of the study published recently in the Journal of Autism and Developmental Disorders. “It is also true that some children appear to get worse.”

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Researchers measured the children’s autism severity using the 10-point ADOS Calibrated Severity Score. The kids — all of whom received community-based autism intervention — were assessed at ages 3 and 6. A difference of two points or more over the course of the three-year period indicated a significant change in severity, researchers said.

About 29 percent of the children saw their severity decrease, with seven of the kids recording scores below the threshold for an autism diagnosis when they were reevaluated at age 6, the study found.

Meanwhile, nearly 17 percent of the children experienced an increase in severity and roughly 55 percent exhibited stable symptoms, according to the findings.

The researchers said they were surprised to see that children whose symptoms increased over the time period studied displayed lower severity levels initially. They also observed different patterns for girls versus boys.

“We found that girls with autism decrease in severity more than boys and increase in severity less than boys during early childhood,” said Einat Waizbard-Bartov, a graduate researcher at the University of California, Davis MIND Institute and an author of the study. “The fact that more of the girls appear to have decreased in autism severity may be due to an increasing number of girls compared to boys who, with age, have learned how to mask their symptoms.”

In addition, children with higher IQs were more likely to see their symptoms decline, the study found.

More research is needed to further understand what’s causing autism severity to change in some children, those behind the study said.

“Unfortunately, it is not currently possible to predict who will do well and who will develop more severe autism symptoms and need different interventions,” said Amaral.