WOMEN who experience difficult births or deliver their babies by caesarean section are more likely to have an autistic child than women who had normal births, according to the largest study of its kind ever undertaken.

The West Australian research will be presented at an international autism congress in Melbourne next week.

The study of almost 4,000 children found autistic children are more likely to have experienced threatened abortions early in a pregnancy or gone into distress during labour.

Their mothers are more likely to be older, to have had epidural anaesthesia and to have had emergency or elective caesareans.

The study confirms the findings of smaller, or more limited, studies by Swedish and US researchers linking autism to obstetric difficulties. Emma Glasson, who conducted the study as a PhD project for the University of WA's School of Psychiatric and Clinical Neuroscience, said most children are not diagnosed with autism until about the age of three or four. "I looked at the before birth factor to see if there's anything unique to that group," she said.

"We found as a group the autism kids were different."

Ms Glasson used the WA Maternal and Child Health Research Database to compare the birth records of the 465 children diagnosed with autism between 1980 and 1995 with the records of 1,313 randomly selected non-autistic children.

She also looked at 481 siblings of the autistic group and 1,634 siblings of the non-autistic group.

"Compared to the (non-autistic group), the autism cases experienced more difficulties during pregnancy, labour, delivery and during the neonatal period," she said.

"The autism group was characterised by increased maternal age, being first born, a threatened abortion before 20 weeks gestation, foetal distress and an elective caesarean."

Interestingly, brothers and sisters of the autistic group - while not autistic themselves - also have more complications than siblings of the non-autistic group, Ms Glasson found.

She said this suggested autism is the cause, rather than the result of, birth complications.

"The best explanation we can have at the moment is that there's a big genetic component," she said.

"It's thought that autism is caused by a large number of genes, and some of those genes will be shared with siblings.

"It may be an interaction effect with some of those genes causing problems in utero."

She stressed that women who had experienced a difficult birth or resorted to a c-section should not feel guilty about contributing in any way to their child's autism.

"I didn't find any evidence that any of these factors were directly causing autism," she said.

"Autism is unlikely to be caused by a single obstetric factor and the increased prevalence of obstetric complications are most likely caused by the underlying genotype or an interaction of the genotype and the (uterine) environment."

The World Autism Congress 2002 runs from November 10-14.

AAP