A US study published Wednesday in the Journal of Pediatrics reveals children born more than three months premature have double the expected rate of autism aged two compared to full-term children.

Overall, one in 10 extremely premature infants, without other health problems like cerebral palsy, mental impairment, vision or hearing problems, tested positive for autism aged two.

The study assessed the children via behavioural survey known as the Modified Checklist for Autism in Toddlers (M-CHAT), however, M-CHAT is not considered a definitive test.

Dr. Antonio Hardan, Director of the Autism and Developmental Disabilities Clinic at Stanford University 's Lucile Packard Children's Hospital, who did not participate in the study, says the findings are not surprising as: 'We have to be in the womb for nine months for a good reason. There are a lot of steps of brain development that should occur in the right environment, and the ideal environment is in the womb.'

Further, Dr. Hardan says, in addition to brain development, premature birth cuts short crucial nutritional and hormonal support for the baby. Pre-term birth is also associated with a long list of health risks for baby, such as, mental retardation, cerebral palsy, lung problems and vision and hearing loss.

Autism refers to a group of developmental problems, known as autism spectrum disorders, which appear in early childhood and harm one's ability to communicate and interact with others. Its causes remain unclear, though experts point to possible genetic and environmental factors.

Typically, a formal diagnosis of autism does not occur until around age three. According to government figures, about one in 150 children in USA have an autism spectrum disorder.