Proteins of the maternal immune system during pregnancy may be able to affect the development of the fetal brain, as these proteins can cross the placenta and enter fetal tissues. Preliminary results have provided evidence that elevated levels of certain immune system proteins in the blood of pregnant women may be associated with an increased risk of autism in their children. These researchers will extend their preliminary data by conducting a large controlled study on the association between levels of autoantibodies and cytokines in maternal blood samples during mid-pregnancy with the associated risks of autism and mental retardation in the offspring. 1200 mother-child pairs will be included in this study, as well as 200 siblings of autistic and mentally retarded children. These data will determine whether inappropriate activation of the immune system during pregnancy is associated with an increased risk of neurodevelopmental problems. The results from this study should contribute to the understanding of the impact of the maternal environment on fetal development and its contribution to autism. It may also provide new, early tests for an increased risk of autism.