In our present work, we utilize recently developed methods to extract and measure bis (1,3-dichloropropyl) phosphate (BDCPP) and diphenyl phosphate (DPP), the respective metabolites of TDCPP and TPP, in urine samples from a group of pregnant women (Cooper et al., 2011). We investigate both the short- and long-term variability in the levels of OPFR metabolites in urine throughout pregnancy. This information will help inform exposure assessment in epidemiological studies and aid in the interpretation of results using single exposure measures to capture information on exposure throughout pregnancy.