Mental health problems disproportionately affect women, particularly during childbearing years. We sought to estimate the prevalence of antepartum mental health problems and determine potential risk factors in a representative USA population. We examined data on 3,051 pregnant women from 11 panels of the 1996-2006 Medical Expenditure Panel Survey. Poor antepartum mental health was defined by self report of mental health conditions or symptoms or a mental health rating of "fair" or "poor." Multivariate regression analyses modeled the odds of poor antepartum mental health; 7.8% of women reported poor antepartum mental health. A history of mental health problems increased the odds of poor antepartum mental health by a factor of 8.45 (95% CI, 6.01-11.88). Multivariate analyses were stratified by history of mental health problems. Significant factors among both groups included never being married and self-reported fair/poor health status. This study identifies key risk factors associated with antepartum mental health problems in a nationally representative sample of pregnant women. Women with low social support, in poor health, or with a history of poor mental health are at an increased risk of having antepartum mental health problems. Understanding these risk factors is critical to improve the long-term health of women and their children.