That suggests autism may be an under-appreciated consequence of medical advances enabling the tiniest of premature babies to survive, said lead author Catherine Limperopoulos, a researcher at McGill University in Montreal and Children's Hospital in Boston.
She emphasized that the results don't mean extreme prematurity causes autism, but rather that it might be among contributing factors.
The risks associated with being born way too early have mostly been thought of as "neuromuscular, causing damage like cerebral palsy, and cognitive, like mental retardation," said Dr. Alan Fleischman, medical director at the March of Dimes.
"The study says there are also social and behavioral consequences which look like autism," Fleischman said. And he said it underscores a need for early autism screening among youngsters born very prematurely.
The American Academy of Pediatrics recommends autism screening for all children by age 2. Autism can't be cured but early behavior therapy can help lessen its severity.
Experts believe autism results from a combination of genes and outside influences. Some advocates believe those factors include childhood vaccines, but scientific studies have not shown that.
Previous research on autism and prematurity has generally looked back at groups of older children to see whether prematurity was more common among those already diagnosed with autism, and results have been inconsistent, said Craig Newschaffer, an autism researcher at Drexel University's School of Public Health.
Limperopoulos said her study design was more rigorous.
The study, released Wednesday and published in the April issue of the journal Pediatrics, involved 91 children aged 18 months to 2 years old. On average, they were born 10 weeks early weighing less than 4 pounds. Screening results found suspected autism in 23 children, or 25 percent.
The screening test is a 23-item checklist for parents, asking about behavior in very young children. The test is designed to screen youngsters before age 2, which is the more typical age of autism diagnosis. More comprehensive and definitive autism testing at around age 2 is recommended for those with positive screening results.
Dr. Edwin Cook, an autism researcher at the University of Illinois at Chicago, said using the preliminary screening test in preemies may be misleading because these children typically reach developmental milestones later than their peers but often catch up.
The researchers took developmental delays associated with prematurity into account, Limperopoulos said. She said the children in the study will be followed to see how many are subsequently diagnosed with autism.
Newschaffer said there's evidence that fewer than half of children the screening test identifies as at risk of autism are later diagnosed with it.