Preterm birth represents the most significant problem in maternal-child health. The ongoing search to elucidate its underlying causes and pathophysiological mechanisms has identified maternal stress as a variable of interest. Based on emerging models of causation of complex common disorders, we suggest that the effects of maternal stress on risk of preterm birth may, for the most part, vary as a function of context. In this paper we focus on select key issues and questions that highlight the need to develop a better understanding of which particular subgroups of pregnant women, under what circumstances, and at which stage(s) of gestation, may be especially vulnerable to the potentially detrimental effects of maternal stress. Our discussion addresses issues related to the characterization and assessment of maternal stress and candidate biological (maternal-placental-fetal endocrine, immune, vascular and genetic) mechanisms. We propose the adoption of newer approaches (ecological momentary assessment) and a life-course perspective to further our understanding of the contribution of maternal stress to preterm birth.