A new study has found children with autism are more likely to have severe social symptoms if their mothers had chronic asthma or allergies while pregnant.

Australian researchers have found a link between chronic asthma or allergies during pregnancy and the severity of autism symptoms in children.

A retrospective study of 220 children published in journal Molecular Psychiatry explored the relationship between a mother's immune history and autism spectrum disorder (ASD), led by researchers at the University of Sydney in partnership with the Telethon Kids Institute.

Results showed a positive immune history - in this case pregnant mothers with allergies or asthma - was associated with increased severity of social symptoms in child.

"While we were not able to determine a causative relationship, this study suggests that children with ASD who are born to mothers with an immune activation history present with more severe social deficits than those born to mothers without an immune history," the authors wrote.

ASD is a set of lifelong neurodevelopmental disorders, characterised by difficulties in social interaction, impaired communication, restricted and repetitive interests and behaviours and sensory sensitivities.

Previous animals studies have shown that immune activation during pregnancy results in offspring displaying autism like symptoms, such as abnormal communication, decreased sociability and repetitive/restricted behaviours.

The authors believe prenatal exposure to certain proteins excreted by the immune system - known as cytokines - and antibodies may interfere with the development and regulation of the unborn baby's central nervous system.

"Furthermore, immune activation in the mother may have been associated with immune system dysregulation in the child, leading heightened inflammation to increase ASD symptom severity," they write.

Children recruited to the study were administered the Autism Diagnostic Observation Schedule-Generic (ADOS-G) that uses simple activities and questions designed to prompt and observe communication, social and stereotyped behaviours relevant to the diagnosis of ASD.

A primary caregiver also completed the Social Responsiveness Scale (SRS), a 65-item rating scale measuring social interaction, language and repetitive/restricted behaviours and interests in the child. The SRS provides a total score and individual scores on five subscales: awareness, cognition, communication, motivation and mannerisms.

A primary caregiver completed a family history questionnaire, which included a medical history including any diagnosed illnesses or chronic conditions.

According to the results, children of mothers who reported a history of immune activation had significantly higher Social Responsiveness Scale total scores. They specifically had higher scores on cognition and mannerisms, suggesting they had more difficulty understanding social situations and displayed more restricted behaviours or unusual interests.

The researchers note that this link may be one of many possible biological pathways that could lead to autism in children and larger studies are needed.

However they do say the findings provide possible avenues for novel targets for immune-modulating pharmaceutical therapies.