Serotonin, a neurotransmitter found throughout the brain and body, has long been 
of interest in autism. Repeated findings of elevated platelet serotonin levels 
in approximately one third of children with autism has led some to believe that 
dysfunctional serotonin signaling may be a causal mechanism for the disorder. 
Because serotonin is critical to fetal brain development, concerns have arisen 
regarding prenatal exposure to substances that manipulate serotonin levels, such 
as selective serotonin reuptake inhibitors (SSRIs). This review examines 
evidence regarding the serotonin system and autism spectrum disorders (ASD), as 
well as what the literature has reported thus far on developmental effects of 
prenatal exposure to SSRIs. Possible mechanisms by which SSRIs could affect the 
fetus during pregnancy and clinical implications are also discussed. Though the 
majority of studies conducted in infants and children suggest prenatal exposure 
to SSRIs does not affect neurodevelopment, interpretation must be tempered given 
small sample sizes. The only published study that focused on prenatal SSRI 
exposure and ASD found an increased risk with exposure to SSRIs, especially 
during the first trimester. Obstacles that will be faced in future research are 
isolating medication effects from maternal depression and, given the infrequent 
occurrence of exposure and outcome, obtaining an adequate sample size. Whether 
serotonin is an etiologic factor in ASD, and what it points to as a marker for 
subgrouping, remains unclear. Understanding how the development of ASD might be 
affected by prenatal factors that influence serotonin levels, such as SSRIs, 
could identify modifiable targets for prevention.