PURPOSE: Mental health problems disproportionately affect women, particularly during the childbearing years. However, there is a paucity of research on the determinants of postpartum mental health problems using representative US populations. Taking a life course perspective, we determined the potential risk factors for postpartum mental health problems, with a particular focus on the role of mental health before and during pregnancy. METHODS: We examined data on 1,863 mothers from 11 panels of the 1996-2006 Medical Expenditure Panel Survey. Poor postpartum mental health was defined using self-reports of mental health conditions, symptoms of mental health conditions, or global mental health ratings of "fair" or "poor." RESULTS: Of the women included, 9.5% reported experiencing postpartum mental health problems, with over half of these women reporting a history of poor mental health. Poor prepregnancy mental health and poor antepartum mental health both independently increased the odds of having postpartum mental health problems. Staged multivariate analyses revealed that poor antepartum mental health attenuated the relationship between prepregnancy and postpartum mental health problems. Additionally, significant disparities exist in women's report of postpartum mental health status. CONCLUSION: Although poor antepartum mental health is the strongest predictor of postpartum mental health problems, prepregnancy mental health is also important. Accordingly, health care providers should identify, treat, and follow women with a history of poor mental health because they are particularly susceptible to postpartum mental health problems. This will ensure that women and their children are in the best possible health and mental health during the postpartum period and beyond.