An analysis of millions of health records found that among all age groups, diagnoses rose 175 percent between 2011 and 2022, from a rate of 2.3 to 6.3 per 1,000 people.

The biggest increase was among young adults ages 26 to 34, with a 450 percent jump.

But children ages five to eight are still the most likely group to be diagnosed - at a rate of 30 per 1,000.

The researchers say better screening, raised awareness and a broader diagnostic criteria are driving the surge.

But recent studies have also pointed to environmental factors like pollution, and aging mothers also playing a role.

Researchers also noted that the autism 'gender gap' is closing as more girls and women are diagnosed with the condition, which primarily affects school-aged boys.

But they cautioned that actual rates are likely even higher than predicted, as not all autistic children have access to screening and therapy.

They wrote: 'Rates reported here may underestimate the true prevalence of ASD in adults, especially older female adults, as many would not have been screened in childhood and remain undiagnosed.

'Nevertheless, our findings indicate that the population of autistic adults in the US will continue to grow, underscoring a need for expanded health care services.'

However, other nations like the UK are facing even higher surge. A recent report from the NHS, for example, found that 200,000 Brits are waiting on an autism assessment, up 30,000 from last year.

About one in 36 US children and one in 50 adults have autism, according to the CDC.

Generally, most with the disorder are diagnosed by age five, though some can be tested as young as age two.

While experts still don't know what causes the condition, recent research suggests that environmental factors like pollution, illnesses during pregnancy, and older parental age could be to blame.

The newest study, published Wednesday in JAMA Network Open, looked at 12.2million Americans enrolled at 12 sites in the Mental Health Research Network, a group of US mental health centers.

Of those, 77,683 were diagnosed with autism.

The majority of autistic participants were ages zero to four, followed closely by ages five to eight.

Participants were then broken up into age groups ranging from infancy to 65 years and older.

Diagnoses decreased with age, with the smallest percentage (1 in 200) group being over age 65.

Boys and men outnumbered girls and women three to one, which experts have suggested could be due to genetic mutations and hormones more common in boys.

The researchers believe the increases in diagnoses are due to 'changes to developmental screening practices' for children and changes in 'diagnosis definitions, policies, and environmental factors.'

Dr Luke Grosvenor, lead study author and postdoctoral research fellow at Kaiser Permanente Northern California Division of Research, told USA Today that the growing autism rates in adults show that 'we need to improve transition services for autistic individuals and their families.'

He noted while autistic children often get access to services like speech therapy in school, they may lose access as adults. He said it's 'critically important' adults get access to these resources.

Other studies have suggested that environmental factors could be behind the surge.

An Australian study published earlier this year, for example, found that boys exposed to endocrine-disrupting chemical Bisphenol A (BPA) in the womb were six times more likely to be diagnosed with autism before age 11 than those without exposure.

That team suggested BPA was associated with neurological and behavioral changes associated with autism.

Additionally, a 2021 review found that 'the odds of an autism diagnosis were 3.3 times higher in individuals born preterm than in the general population,' as premature babies are vulnerable to birth complications and inflammation associated with autism.

This means the increase in autism could be due to more preterm babies surviving birth, as the survival rate has increased from 76 percent in 2008 to 2012 to 78.3 percent in 2013 to 2018, according to research from the National Institutes of Health (NIH).

The new study had several limitations, such as diagnoses likely being underestimated since the team relied only on those documented in health records. The study also excludes patients without access to testing and physicians, such as those without insurance.

The team wrote: 'Future work should characterize prevalence trends by these and other factors to represent individuals with varying levels of service needs and presentations of ASD.'

This article originally appeared on DailyMail.com