TORONTO -- An estimated one in every 66 Canadian children and youth aged five to 17 has autism spectrum disorder, says the inaugural report on the prevalence of the neurodevelopmental condition in this country.

Autism spectrum disorder, or ASD, is typically detected in early childhood and causes impairments in communication skills and social interactions, often combined with repetitive behaviours and narrowly focused interests or activities.

Thursday's report by the Public Health Agency of Canada includes 2015 data from six provinces and one territory. It found that prevalence -- the number of affected people within a specific population -- ranged from a high of one in 57 children and youth in Newfoundland and Labrador to one in 126 in Yukon.

The overall one in 66 prevalence rate "was not unexpected," said Judy Snider, PHAC's acting chief of maternal, child and youth health and a co-author of the report. "We had been tracking the rates in the U.S. and it's a very similar rate ... So we're not surprised at all."

Esther Rhee, national program director for Autism Speaks Canada, welcomed the report, saying that having up-to-date numbers empowers advocacy organizations like hers to help drive progress for the autism community.

"We know that Canada is doing a lot, but there is still so much more to do in terms of effectively serving all individuals with autism and their families across the country," she said.

"So we can use a report like this to highlight the need here, and not be using data that we've been pulling and borrowing from other countries, to speak to what is really happening in our own community and then utilize that to make positive change."

In the U.S., which has been collecting ASD prevalence information for decades, one in 68 children was estimated to have the disorder in 2016, when the latest data was released.

However, there is a difference between the two countries' data collection: the U.S. gathers prevalence statistics only on eight-year-olds in 11 states, representing less than 10 per cent of that age group.

PHAC compiled numbers that represent about 40 per cent of the population, but which captured 88 per cent of all five- to 17-year-olds in the seven participating jurisdictions, which also included B.C., Quebec, New Brunswick, Nova Scotia and P.E.I.

This means they used information collected from "a much richer source," Snider said.

"We count all children in that age group, in the jurisdictions that are currently participating, and we have great confidence in the numbers that we have."

PHAC hopes to have the remaining territories and provinces join the National Autism Spectrum Disorder Surveillance System -- including Ontario, the country's most populous province.

"We actually don't know the impact of not having Ontario involved or the other provinces," Snider said, but added that the agency is confident the data it has collected is "representative of Canada."

As expected, based on the experience of other countries, the report found that boys in Canada are four to five times more likely than girls to develop ASD.

"When we look at the prevalence of autism by sex, we found that one in 42 males were diagnosed with autism spectrum disorder, whereas we see it in one in 165 females," said Snider.

Why the disorder affects so many more boys than girls remains a mystery. While research is looking at all different aspects of autism, "we don't have a good understanding of that."

She said the national surveillance report provides a benchmark that will allow researchers and policy makers to track whether the proportion of Canadians affected by autism continues to escalate in the future.

P.E.I., Quebec and Newfoundland and Labrador have seen increases in prevalence since 2003. And in the last two decades of collecting data, the U.S. has also seen rates steadily rise.

"We've seen a 100 per cent increase," said Rhee, pointing to U.S. report for 2000, when the prevalence rate for ASD among eight-year-olds was one in 166; by 2014, it had soared to one in 68, remaining at that level in 2016. The 2018 report is yet to be released.

So why are cases of autism going up?

That's the "magic question," said Rhee. "We know that genetics play a role, we know that environmental factors play a role, we know that we've gotten significantly better at awareness and screening and diagnosing and that those components play roles in there being a higher prevalence.

"But it doesn't explain the whole picture ... there's still so much we have to understand."

Canada does not have a national autism strategy, though this year's federal budget earmarked $20 million over five years towards ASD. The money would include funding for a network to connect people with ASD and their families to information, resources and employment opportunities, and community-based projects to strengthen health, social and educational programs.

Snider said the report will help inform planning for ASD programs, services and research "that will impact Canadians living with autism spectrum disorder and their families and caregivers."