The question of the contribution of maternal stress to preterm birth is a challenging issue. Concepts in evolutionary biology and developmental plasticity support a rationale for considering a role for maternal stress in preterm birth. Evidence from population-based epidemiological and clinical studies suggests that after accounting for the effects of other established socio-demographic, obstetric and behavioral risk factors, women reporting higher levels of psychological stress during pregnancy are at significantly increased risk of preterm birth. However, at the individual level, the specificity and sensitivity of maternal stress as a predictor of preterm birth risk is, at best, modest. In order to translate population-level findings to public health and clinical practice applications it is critical to identify which subgroups of women, in what circumstances, and at which stage(s) of gestation, may be particularly susceptible to the potentially detrimental effects of high prenatal stress.