Skip navigation

Risk factors for autism


Clinical bottom line

Risk factors for autism include parental psychiatric history, early birth, low Apgar score, and breech presentation.


HJ Larsson et al. Risk factors for autism: perinatal factors, parental psychiatric history, and socioeconomic status. American Journal of Epidemiology 2005 161: 916-925.


The study was conducted in Denmark, where unique patient registration numbers and linked registries make it possible to perform high quality epidemiological studies. This study examined records of children born in Denmark between 1973 and 1999 and at risk of diagnosis of autism. All children with a diagnosis of autism were identified, and for each of them 25 control were randomly identified also.

For all, information about perinatal characteristics were available, together with information about parental psychiatric history. A parent was recorded as having a psychiatric history if a diagnosis had been made before the date that autism was diagnosed in the child. Sociodemographic factors were based on the gross income of each parent assessed at the child's birth.


There were 698 children with infantile or atypical autism, and 17,450 controls. About 76% of cases were boys, and about 75% of cases were diagnosed by age 10 years. The median age of diagnosis was six years.

A number of factors were associated with autism, and in a fully-adjusted analysis several factors were significantly associated (Table 1). These included parental psychiatric history, low birth weight, low Apgar score, and breech presentation. Not associated were maternal education or parental wealth.

Table 1: Risk factors for autism

Adjusted risk ratio
(95% CI)
Parental schizophrenia like psychosis
3.4 (1.5 to 8.0)
Parental affective disorder
2.9 (1.7 to 5.1)
Other parental psychiatric history
2.9 (2.2 to 3.7)
Gestational age <35 weeks
2.5 (1.6 to 3.9)
Apgar score <8 at 5 minutes
1.9 (1.1 to 3.3)
Breech presentation
1.6 (1.2 to 2.3)



These results strongly suggest that perinatal environmental factors, and parental psychiatric history, are associated with increased risk of autism. The study was large, and particularly useful because the Danish linkage of different data sets.