Autism spectrum disorders are a group of pervasive developmental disorders characterized by impaired communication and social interaction as well as restricted and repetitive interests and behaviors. Included are autistic disorder (infantile autism), pervasive developmental disorder–not otherwise specified, and Asperger's disorder.

The term "autism" is often used to refer to autistic disorder, which affects approximately one to two children per 1,000 (1–5). The prevalence of autism spectrum disorders as a whole may range from two to six per 1,000 (2, 3, 5, 6).

The etiology of these disorders is not completely understood, but twin and family studies suggest that genetic factors play a substantial causal role; the concordance rate is found to be at least 36 percent for monozygotic twins and is expected to be about 3 percent for dizygotic twins (7). However, a concordance rate for monozygotic twins of less than 100 percent indicates that nongenetic factors also play a causal role.

Prenatal exposure to valproic acid, thalidomide, rubella, and alcohol has been associated with an increased risk of autism spectrum disorders (8–11).

Several studies have focused on perinatal risk factors for autism (12–21). However, the sample sizes have been relatively small, studies have used different methodologies, and results among studies are conflicting. Some studies (12–15), but not all (16, 17), found an increased risk of autism associated with the presence of one or more unfavorable obstetric events.

Studies focusing on single perinatal risk factors have reported a positive association for low birth weight (<2,500 g) (15, 16), low Apgar score (<6 or <7) at 5 minutes (15, 18), being small for gestational age (18), gestational age at birth of less than 37 weeks (15, 18), cesarean section (18), and congenital malformations (18). A number of studies examined several or all of the factors mentioned without identifying any significant risk factors (19–21).

A gender stratification in one study indicated an increased risk of autism among boys, but not girls, of low birth weight (<2,500 g) (21).

Compared with parents of children with other handicaps, parents of autistic children more frequently have schizoid personality traits (22), and women with schizophrenia are at increased risk of an adverse pregnancy outcome (23, 24). A family record of psychiatric history may therefore confound the association between perinatal factors and autism.

A familial propensity to disorder has been taken into account somewhat in some studies of perinatal risk and autism by using siblings as controls (14, 21), but no studies are known to have examined whether the association between perinatal factors and autism disappears when adjusting for parental psychiatric history.

Most studies on perinatal factors and autism have not been able to adjust for socioeconomic status. Socioeconomic status may be associated with adverse pregnancy outcomes as well as autism (2, 24).

source: http://aje.oxfordjournals.org/cgi/content/abstract/161/10/916