The purpose of this project has been to investigate the association between maternal plasma concentrations of perfluoroalkyl substances (PFAS) and duration of breastfeeding. PFAS are used in industrial and consumer products as surfactants, and are suspected endocrine disruptors. Animal studies show that PFAS can influence mammary gland differentiation and prolactin levels, which suggest that exposure could adversely impact breast milk supply. Two previous studies have tested this hypothesis and examined the association between maternal plasma PFAS concentrations in early pregnancy in relation to duration of breastfeeding, a proxy for breast milk production. We will improve on these analyses by examining the relationship within a large population of primiparas, by utilizing a population that is unlikely to stop breastfeeding prior to 6 months for work-related reasons, and that has measurements on additional PFAS beyond perfluorooctanesulfonate (PFOS) and perfluorooctanoate (PFOA). This analysis will efficiently utilize preexisting measures of PFAS from MoBa participants who were selected as part of a case-control study of preeclampsia, as well as breastfeeding duration data that has been previously characterized within the study population.