A U.S. study looking at children born more than three months prematurely provided fresh evidence on Thursday linking pre-term birth and autism.

These children were about two to three times as likely to show signs of autism at age 2 as measured in a standard screening tool compared to other children, the researchers wrote in the Journal of Pediatrics.

Autism refers to a group of developmental problems known as autism spectrum disorders that appear in early childhood and harm one's ability to communicate and interact with others.

Its causes remain unclear, and experts have pointed to possible genetic and environmental factors.

Dr. Karl Kuban of Boston University School of Medicine and Boston Medical Center, who led the study, said the increased risk for autism indicated in these children may not be directly caused by being born prematurely.

Instead, it is possible that whatever causes a child to have autism also may force an early birth, Kuban said in a telephone interview.

Following earlier research suggesting a link to pre-term birth, the researchers followed 988 U.S. children born very prematurely, at least three months before their due date.

At age 2, the children were evaluated using a screening method in which they are rated on a checklist of 23 behaviors for signs of autism. This tool flags children who may have autism but is not considered a definitive diagnosis.

Typically, a formal diagnosis of autism does not occur until around age 3, Kuban said.

While less than 6 percent of children born full-term screen positive for possible autism using this tool, 21 percent of these children scored positive, the researchers said.

Because pre-term babies may have certain developmental problems unrelated to autism that could trigger a positive score, the researchers then excluded children with motor, vision and hearing impairments. Even after doing that, 16 percent of the children scored positive for possible autism.

After also excluding children with cognitive impairment on the premise that it may not be autism related, about 10 percent of the pre-term children still had a positive screening score.

Pre-term birth is associated with a long list of health risks for a baby such as mental retardation, cerebral palsy, lung problems and vision and hearing loss.

About one in 150 U.S. children has an autism spectrum disorder, according to U.S. government figures.