Prior studies have shown that neighborhood disadvantage and disorder are associated with birth outcomes. This study examined preconception diet and physical activity level, as well as body mass index, as mediators of the association between neighborhood conditions and birthweight. Secondary data analyses were conducted using data from the National Longitudinal Study of Adolescent Health (Add Health). The final analytic sample consisted of 523 adolescent and young adult mothers giving singleton live births between 1997 and 1998. In contrast to previous research, we found that neighborhood characteristics were unrelated to birthweight. Consistent with prior studies, compared to those who were White, on average, Blacks had birthweights that were 163.25 grams lighter. In addition, compared to mothers who were married or living with a partner, mothers who did not have a partner at the time of birth, on average, had offspring that were 127.20 grams lighter. No evidence was found for the mediation hypotheses as there were no associations between neighborhood characteristics and preconception diet or physical activity or between these behavioral variables and birthweight. To the authors' knowledge, this is the first study examining diet and physical activity as possible behavioral pathways between the neighborhood context and birthweight.