A difficult birth and a family history of mental illness may increase the risk of autism, say US researchers.
Babies born prematurely might be at increased risk
The Centers for Disease Control and Prevention team base their findings on nearly 700 Danish children with autism.
More than 500,000 people in the UK are thought to be affected by autism spectrum disorders, which limit their ability to develop friendships and understand other's emotional feelings.
The latest study findings appear in the American Journal of Epidemiology.
Experts have already noted that the condition has a strong genetic component.
However, the number of children with autism appears to be increasing more than expected for a genetic disorder.
Toxins, diet, viruses and other pathogens have been suggested, though there is no strong evidence for any of these.
Research has also linked the condition with a variety of conditions affecting brain development which occur before, during, or very soon after birth.
The CDC team, working with colleagues at the University of Aarhus in Denmark, found children with autism were more likely to have had difficult births than other children of the same age.
This included breech births, premature births and problems immediately after delivery.
Characteristics of autism
Difficulty with social relationships
Difficulty with verbal and non-verbal communication
Difficulty in the development of play and imagination
Source: The National Autistic Society
Parental psychiatric history was associated with the highest independent risk of autism.
But because none of these factors were present in the vast majority of the autistic children, other factors must also be important, said the researchers.
Diana Schendel from the CDC said: "At this point, we don't know for sure if these events are causes, but it certainly points us to look more closely at what happens during pregnancy as a possible opportunity for future prevention."
A spokeswoman from the National Autistic Society said: "The causes of autism are still being investigated.
"Many experts believe that the pattern of behaviour from which autism is diagnosed may not result from a single cause.
"There is strong evidence to suggest that autism can be caused by a variety of physical factors, all of which affect brain development - it is not due to emotional deprivation or the way a person has been brought up."