TAMPA, United States (AFP) — The autism rate in the United States has climbed to one in 59 children, up from prior estimates of one in 68, a US government report said on Thursday.

Calling the neurodevelopmental disorder an “urgent public health concern,” the Centres for Disease Control and Prevention (CDC) said the change represents an uptick from 1.5 per cent to 1.7 per cent of children.

The latest figures are “broad and startling,” said Walter Zahorodny, an associate professor of paediatrics at Rutgers New Jersey Medical School who led the New Jersey portion of the study.

The prevalence of autism in the United States was just one in 5,000 in 1975, and has been mounting steadily in recent years, rising 150 per cent since 2000, the CDC said.

Researchers are still unclear what causes autism, or why it appears to be on the rise.

In 2014, the CDC said one in 68 US children had autism, a rate the agency said appeared to remain steady in 2016.

“It is now clear that what we saw in 2016 was just a pause along the way,” said Zahorodny. The CDC report said some of the change in prevalence in recent years “could be due to improved autism identification in minority populations, although autism is still more likely to be identified in white children than in black or Hispanic children.”

Autism is also about four times more common in boys than girls.

Certain risk factors are linked to autism, such as being born to parents over age 30, maternal illness during pregnancy, genetic mutations, birth before 37 weeks gestation and multiple births. “These are true influences that are exerting an effect, but they are not enough to explain the high rate of autism prevalence,” said Zahorodny.

“There are still undefined environmental risks which contribute to this significant increase, factors that could affect a child in its development in utero or related to birth complications or to the newborn period. We need more research into non-genetic triggers for autism.”

People with autism may display a range of symptoms including repetitive movements, difficulty communicating, and discomfort in social situations. Some excel at non-verbal skills such as mathematics.

States included in the research were Arizona, Arkansas, Colorado, Georgia, New Jersey, Maryland, Minnesota, Missouri, North Carolina, Tennessee and Wisconsin.

New Jersey had almost twice the nationwide rate, at 2.9 per cent, or one in 34 kids. The lowest rate was in Arkansas, with one in 76 kids affected.

Data for Thursday’s report came from health and special education records of 325,483 children who were eight years old in 2014.

While the figures, which come from the Autism and Developmental Disabilities Monitoring (ADDM) Network, are not representative of the entire nation, health experts widely consider them a benchmark of the autism rate in the United States.