The number of children with autism who are being hospitalized — often due to mental health concerns — is on the rise, particularly among teens with the developmental disorder, new research suggests.

In a study looking at data on over 2 million hospitalizations of California children ages 1 to 18, researchers at the Stanford University School of Medicine found that between 1999 and 2009, inpatient treatment related to autism nearly tripled.

The rise came as the number of hospitalizations related to other developmental disabilities including cerebral palsy, Down syndrome and intellectual disability remained largely steady, according to findings published in the May issue of the Journal of Autism and Developmental Disorders.

While the risk of hospitalization rose for kids with autism across all age groups, researchers found that the increase was most pronounced for those older than 11. Once children reached the ages of 15 to 18, individuals with autism were three-and-a-half times more likely than their peers without the developmental disorder to be hospitalized, the study found.

In all but the youngest children studied, mental health diagnoses were the most common reason those with autism were treated.

It’s unclear what precisely is driving the growth in hospitalizations, but researchers said that it could be due to a lack of outpatient and community resources as the number of people with autism has grown.

“Overwhelmed parents, schools and community providers of mental health resources may have been unable to meet the needs of these patients and this failure to treat adequately in the outpatient sector may have led to a direct increase in hospitalizations,” the study authors wrote.