To review the evidence for the presence of prenatal and perinatal factors that affect the risk of autism and autism spectrum disorders.
Relevant articles were identified by searching MEDLINE, screening reference lists of original studies, and searching major journals likely to publish epidemiological studies on the topic.
For inclusion in this review, studies required (1) a well-defined sample of cases drawn from population-based registers or cohorts; (2) standardized, prospectively collected obstetric information from birth records or registers; (3) comparison subjects drawn from the general population with information on obstetric complications collected from the same source; and (4) a standardized format for presentation of data, allowing for comparisons among studies.
Parental characteristics and obstetric complications.
Main Outcome Measures
Rates of autism and autism spectrum disorders.
Seven epidemiological studies were identified that fulfilled inclusion criteria. The parental characteristics associated with an increased risk of autism and autism spectrum disorders included advanced maternal age, advanced paternal age, and maternal place of birth outside Europe or North America. The obstetric conditions that emerged as significant fell into 2 categories: (1) birth weight and duration of gestation and (2) intrapartum hypoxia.
Evidence to suggest that parental age and obstetric conditions are associated with an increased risk of autism and autism spectrum disorders is accumulating. Although not proven as independent risk factors for autism, these variables should be examined in future studies that use large, population-based birth cohorts with precise assessments of exposures and potential confounders.