There's been no rise in the rate of children diagnosed with autism over the last three years in the US, a new study reveals, a sign that the situation might be stabilising after several years of steady increases.

Based on surveys of more than 30,000 parents, the number of children with autism spectrum disorder ( ASD) was 2.41 percent in the US between 2014 and 2016, with no statistically significant increase across that time.

However, the new estimate from the US National Institutes of Health (NIH) is a higher figure than the one cited in another recent study – according to the Autism and Developmental Disabilities Monitoring (ADDM) Network, the rate was 1.46 percent in 2012.

While the numbers are different, what both the NIH and ADDM seem to agree on is that the rate is levelling out. The ADDM had previously noted a rise from 0.67 percent to 1.47 percent between 2000 and 2010.

"Autism now is not something rare," says one of the researchers, epidemiologist Wei Bao from the University of Iowa.

"It's not as rare as 1 per 1,000, as it was in the 1970s and 1980s. With this data, now we can see it is already 1 per 41. The prevalence is much higher than previously thought."

So why the discrepancies? One reason could be that the NIH and ADDM surveys used different methods, with the former surveying parents and the latter based on reports from healthcare professionals.

In this case, parents were asked if their child had ever been diagnosed with autism, Asperger's disorder, pervasive developmental disorder or ASD by a doctor.

On top of that, the older ADDM research only covered a select number of sites in the US, while the NIH data was collected to form a nationally representative sample.

However, the researchers themselves are cautioning against drawing too concrete a conclusion from just three years of data.

Some experts have suggested that diagnosis rates are shifting because we keep changing the definition of what autism is. High-functioning children with Asperger syndrome are now included in ASD figures, for example.

In fact, the definition of autism has been shifting for decades as doctors try to understand the causes behind it. With a high inheritance rate, the condition does appear to be genetic to some extent.

Part of the problem in understanding how autism originates is that the signs of it – like difficulties in social interaction and communication – don't appear until the ages of 2 or 3.

Despite the complexities, any statistical insight we can get into ASD should help in understanding the condition and working out how it starts, and the new research did throw up some differences between various groups of kids.

For example, 3.54 percent of boys were reported to have ASD, compared to 1.22 percent of girls. Rates were 1.78 percent in Hispanic children, 2.36 percent in black children, and 2.71 percent in white children, though the researchers didn't go into why this might be.

At its most basic, this research means we know a little more about what we're dealing with, and that there doesn't seem to have been an increase in rates over the last three years.

The researchers involved state that they want to see these figures carefully monitored in the years ahead.

"After many years of seeing a slow but steady increase in the prevalence of autism spectrum disorders, it is encouraging that the most recent national data failed to find any increase during the most recent three-year period," says Andrew Adesman, from the Cohen Children's Medical Center of New York, who wasn't involved in the study.

"Although it is encouraging that the prevalence of autism spectrum disorders is not increasing further, we don't have a good understanding of why the prevalence increased in recent previous years, and it remains concerning that the prevalence is as high as it is."

The research has been published in the Journal of the American Medical Association.