Researchers at Sydney University's Brain and Mind Centre have published a study that shows a relationship between a mother's active immune response during pregnancy to allergies and asthma and "severe social impairment symptoms" in children with autism.

"Children of mothers who reported a history of immune activation had significantly higher 'Social Responsiveness Scale' total scores, suggesting they had more severe caregiver-reported deficits," said co-author Professor Ian Hickie.

"They had higher scores on cognition and mannerisms subscales, suggesting they had more difficulty understanding social situations and displayed more restricted behaviours or unusual interests."

The retrospective study, involving 220 Australian children with autism, is in the latest Molecular Psychiatry. The researchers did not determine a causative relationship.

The study required the primary caregiver to complete the Social Responsiveness Scale, a 65-item rating scale measuring social interaction, language and repetitive/restricted behaviours and interests in the child.

They also completed a family history questionnaire, which included any diagnosed illnesses or chronic conditions.

While there is a lot of mystery as to why "maternal immune activation" is linked to the increased severity of symptoms, the authors say it may involve prenatal exposure to special proteins called cytokines and antibodies that may interfere with the development of a foetus.

Lead author Professor Adam Guastella said they had identified an immune mediated sub-type in autism spectrum disorder, which would enable more streamlined diagnosis and treatments and improve outcomes.

"Our next step is to show clear markers for this profile, and after that to develop treatments for this subgroup, because we know that autism has many causes," said Professor Guastella.

He said mothers should not feel concerned by the findings, but focus on looking after their health and taking preventative steps.

"At the moment, the evidence is based on an infection, mixed with a genetic profile, mixed with a range of other factors, which may increase adverse outcomes down the track, but there are a number of factors," he said.

"We know that there are many things women can do in pregnancies to improve outcomes such as reducing stress and exercising, looking after their health."

Natalie Pollard, whose family participated in the study, said trying to understand why her eldest son, Ethan, 7, has autism was a "long journey", but the findings were a positive step.

The academic from Dural doesn't suffer from asthma and allergies, and her two younger sons do not have autism.

"I knew something wasn't quite right early on, and his development was slower and he would scream for hours," she said.

"As a mum, I think the findings are great, because we need more information out there and it could potentially help solve the puzzle of autism, which is multi-factorial."

The researchers divided the 220 children into two categories - those with mothers who reported a history of chronic immune activation (caused by asthma and allergies) and those with mothers who reported a diagnosis of an autoimmune condition, such as diabetes and thyroid problems.

They found a history of autoimmune conditions in the mother was not associated with increased symptom severity. However the autoimmune group was much smaller.

Autism spectrum disorder is a set of neuro-developmental disorders, characterised by impaired reciprocal interaction and communication skills, and restricted and repetitive behaviours and interests. It occurs in one in every 68 people around the world.

Experts do not yet know exactly what causes autism.

The Brain and Mind Centre led the study together with researchers from the Children's Hospital Westmead, Macquarie University and the Telethon for Kids Institute.