Polybrominated diphenyl ethers (PBDE) are man-made chemicals used as a flame retardant in common items in the home, such as clothing, furniture, and electronics. Use of PBDEs is being banned in the U.S. over time, but the presence of these chemicals persists in our environment, evidenced by existence of PBDEs in blood. There is concern that exposure to PBDEs during pregnancy may lead to neurodevelopmental problems in the baby. However, no studies have looked at PBDE exposure during pregnancy and the risk for autism spectrum disorders (ASD). This proposed study will use data from the Early Autism Risk Longitudinal Investigation (EARLI) Study to look at PBDEs and autism which collects biological data and other information on mothers during pregnancy and tracks the child outcome and also collects biological information on the child These biological samples will be used in this proposed study to see whether PBDE exposure during pregnancy is related to three things: a) the mother’s thyroid hormone level; b) epigenetic changes in the mother’s or baby’s DNA; and c) ASD in the baby. Lastly, this study will see if the mother’s thyroid hormone level or changes in the mother’s or baby’s DNA methylation has an impact on any relationship between prenatal PBDE exposure and ASD in the baby. This study, which will examine the relationship between prenatal exposure to PBDEs, is related to ASD and identify and describe the mechanism by which this may occur. These findings will inform further studies to identify steps that can be taken to reduce exposures in pregnancy mothers and possibly reduce the risk for ASD.