Autism is five times more common in children born at low birthweight, researchers said Monday, a finding that could help explain increased rates of the developmental disorder.

In a study of over 600 children born in New Jersey between 1984 and 1987 who weighed less than 2,000 grams — or about 4 pounds, 6 ounces — researchers found an autism occurrence rate of 5 percent. That’s significantly higher than the 1 percent prevalence rate that the U.S. Centers for Disease Control and Prevention reports for all children.

While previous studies have suggested that low birthweight children were flagged more often in autism screenings, the study published Monday in the journal Pediatrics is the first to assess whether or not such children are actually diagnosed at higher rates.

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Researchers followed children in the study through age 21, assessing them periodically. An autism screening conducted at age 16 suggested nearly 19 percent could have the developmental disorder.

When the study participants reached age 21, researchers conducted formal diagnostics on all those who screened positive for autism the first time around as well as a sampling of those who screened negative. Ultimately, 14 individuals were diagnosed with the developmental disorder, resulting in a 5 percent prevalence rate for premature children, researchers said.

“I was quite surprised to see that it was five times as high,” said Jennifer Pinto-Martin, a professor at the University of Pennsylvania and the study’s lead author. “Parents of preemies need to be especially aware of the potential for this and really pay attention to what’s happening with their children.”

Moreover, Pinto-Martin said the high rate of autism among premature children may be one reason behind the increased occurrence of the developmental disorder in recent decades. The estimated autism rate has grown from 1 in 2,000 in the 1980s to 1 in 110 today.

“When we started this study in 1984 it was pretty miraculous to keep a 1 pound baby alive. We now do that routinely,” said Pinto-Martin. “Clearly this is a risk factor that’s going up at the same time as the autism prevalence increase.”