WASHINGTON, Oct 17, 2011 (AFP) - Babies who are born early and small are five times as likely as normal infants to develop autism, according to a two-decade-long US study released Monday.

Premature babies have long been known to risk a host of health problems and cognitive delays, but the study in the journal Pediatrics is the first to establish a link between low birth weight and autism.

US researchers tracked 862 children from birth to young adulthood. Those in the study were born between 1984 and 1987 in three counties in New Jersey.

The children weighed between 500 and 2,000 grams (1.1 to 4.4 pounds) at birth.

Over time, five percent of the low-birth weight babies were diagnosed with autism, compared to the one percent prevalence in the general population.

"As survival of the smallest and most immature babies improves, impaired survivors represent an increasing public health challenge," said lead author Jennifer Pinto-Martin, director of the Center for Autism and Developmental Disabilities Research and Epidemiology at the University of Pennsylvania School

"Cognitive problems in these children may mask underlying autism," she added, urging parents to get their child tested if they suspect autism spectrum disorder.

"Early intervention improves long-term outcome and can help these children both at school and at home."

Autism is the term for an array of conditions ranging from poor social interaction to repetitive behaviors and entrenched silence. The condition is rare, predominantly affecting boys, and its causes are fiercely debated.