The water exposure assessment subcommittee of the National Birth Defects Prevention Study (NBDPS) developed a detailed protocol and approach to assign disinfection byproducts (DBP) exposures in drinking water systems to residences in ten states. This project will enable the NBDPS, in conjunction with data on individual drinking water usage, to evaluate relationships between DBPs (trihalomethanes and haloacetic acids) in drinking water and select birth defects. The exposure assessment protocol includes linking the geocoded maternal address to the appropriate drinking water utility, obtaining all relevant DBP water quality data from that utility for that subject, and modeling water quality data to account for spatial and temporal variability. Centralized geocoding of the residences of NBDPS participant was conducted by ATSDR’s Geospatial Research, Analysis, and Services Program Office. CHEEC is linking geocoded residential addresses to water utilities, collecting DBP data for those utilities, and linking the DBP data to the residences where the mother resided 1 month prior to conception through the first trimester of pregnancy. The University of North Carolina, in cooperation with US EPA, is developing algorithms for models to account for spatial and temporal variability of DBPs.