While it's true that autism prevalence has increased in the U.S. during the past two decades, that doesn't necessarily mean that the developmental disorder is growing worse. However, President Donald Trump seems to think it is.

Trump's thoughts on the issue were revealed on Tuesday when he appeared at a White House event with parents and teachers.

When combined with his past comments suggesting that he believes there may be links between autism and vaccines, Trump's comments represent another troubling instance of the U.S president rejecting mainstream scientific evidence on a range of critical issues.

At the education roundtable with newly-installed Education Secretary Betsy DeVos on Tuesday, Trump had an exchange about autism rates with a principal of a school that educates students with intellectual disabilities.

The exchange, as seen in a video released by the White House, revealed that the president believes autism rates are rising in the U.S.

“So what’s going on with autism? When you look at the tremendous increase, it’s really, it’s such an incredible — it’s really a horrible thing to watch, the tremendous amount of increase," Trump said to Jane Quenneville, a principal at a school specializing in special education in Virginia during the roundtable.

He asks Quenneville what she knows about the reason behind the increase, and she cited statistics showing 1-in-66 to 1-in-68 children are being diagnosed with autism now, which is consistent with figures released by the Centers for Disease Control and Prevention (CDC).

Trump then, inaccurately stated. “And now it’s coming even lower than that."

Quenneville is the only parent or teacher in the roundtable that Trump seemed to take interest in immediately, asking her multiple questions as people were still introducing themselves.

An increase in diagnosis

Trump is right that autism rates have increased over the long-term, but this increase may be misleading on its surface, doctors warn.

Research has shown an increase in the diagnosis of autism spectrum disorder and a nearly three-fold increase in autism diagnoses in special education programs in the U.S. This may be why Quenneville reported the increase that she did.

However, the increase in autism cases may partly be explained by the reclassification of individuals that would previously have been diagnosed with other intellectual disabilities, according to a study published in the American Journal of Medical Genetics in 2015.

"For quite some time, researchers have been struggling to sort disorders into categories based on observable clinical features, but it gets complicated with autism because every individual can show a different combination of features" said Santhosh Girirajan, an assistant professor of biochemistry and molecular biology and of anthropology at Penn State who was the lead author of that study, in a press release.

"The tricky part is how to deal with individuals who have multiple diagnoses because, the set of features that define autism is commonly found in individuals with other cognitive or neurological deficits."

The CDC published a report in 2016 that found that the prevalence of autism spectrum disorder was consistent with the rate from two years prior, at 1-in-68 children. Boys are more likely than girls to be identified with the condition, with 1-in-42 boys being diagnosed, compared with just 1-in-189 girls.

The CDC's first report on autism prevalence, based on 2000 and 2002 data, showed that one in 150 children were diagnosed with the disorder in the United States.

Parents are now being told to screen their children for autism in order to potentially diagnose the disorder and start treatment earlier.

Vaccines don't cause autism

Researchers are still trying to figure out what exactly accounts for the rise in autism cases that is not an artifact of changes in reporting methods and screening. Trump, however, has a record of lending credence to the most specious and dangerous of these explanation, which centers on childhood vaccines.

So far, it's unclear what Trump wants to do about autism, but he has considerable leeway in directing federal agencies to study links between vaccines and autism, create a White House task force on the topic or propose regulatory changes through the Education Department. Any such moves would be controversial.

In January, Trump floated the idea of forming an advisory committee on vaccines and autism, and met with prominent anti-vaxxer Robert F. Kennedy Jr. That committee has not yet moved forward after initially attracting criticism regarding the lack of scientific evidence showing any link between vaccines and autism.

Vaccines do not cause autism, according to research that has debunked links between inoculations and the disorder.

As a candidate for president, Trump said that he was in favor of vaccines, but expressed concerns about how they're administered.

"I am totally in favor of vaccines," Trump said during a Sept. 16 debate. "But I want smaller doses over a longer period of time. Because you take a baby in — and I've seen it — and I've seen it, and I had my children taken care of over a long period of time, over a two or three year period of time."

The idea that vaccines should be spaced out over years would actually render many of life-saving vaccinations ineffective, scientists have said.