[Photo/Xinhua]

Babies born very prematurely run a significantly higher risk of developing autism than those who are born after a full pregnancy, Swedish researchers said on Monday.

Babies who exit the womb 13 weeks or more ahead of schedule are more likely to have developed signs of autism spectrum disorder (ASD) by the time they reach six years of age, researchers at Karolinska Institutet in Stockholm said in a statement.

"We were surprised by how many - almost 30 per cent - of the extremely preterm-born children had developed ASD symptoms," Ulrika Aden, a researcher at Karolinska Institutet, said in a statement.

"Amongst children born after a full-term pregnancy, the corresponding figure is 1 percent," Aden added.

The study, published in the journal Cerebral Cortex, tracked the development of 100 babies who had been born prior to week 27 of their mothers' pregnancies.

Before they had been diagnosed with ASD, the affected children showed signs of diminished growth of the part of the brain that handles social interaction, empathy and language acquisition, the researchers said.

Aden said the results show that autism, which has been attributed to genetic factors, may stem from environmental factors surrounding one's birth.

"The brain grows best in the womb, and if the developmental environment changes too early to a life in the atmosphere, it can disrupt the organisation of cerebral networks," Aden said.