Poor perinatal mental health is a common pregnancy-related morbidity with potentially serious impacts that extend beyond the individual to their family. A possible contributing factor to poor perinatal mental health is discontinuity in health insurance coverage, which is particularly important among low-income people. We examined impacts of Medicaid expansion on prepregnancy depression screening and self-reported depression and postpartum depressive symptoms and well-being among low-income people giving birth. Medicaid expansion was associated with a 16 percent decline in self-reported prepregnancy depression but was not associated with postpartum depressive symptoms or well-being. Associations between Medicaid expansion and prepregnancy mental health measures increased with time since expansion. Expanding health insurance coverage to low-income people before pregnancy may improve perinatal mental health.