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.

Premature babies have long been known to risk a host of health problems and cognitive delays, but the study in the journal Paediatrics 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 at birth.

Over time, five per cent of the low-birth weight babies were diagnosed with autism, compared to the one per cent 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 Centre for Autism and Developmental Disabilities Research and Epidemiology at the University of Pennsylvania School of Nursing.