A sizable body of evidence indicates that prenatal maternal stress (PNMS) has an adverse impact on birth outcomes, including birth weight and gestational age at delivery. The authors hypothesized that effects of PNMS are attributable in part to dispositions such as pessimism that lead women to view their lives as stressful and that effects of PNMS and disposition on birth outcome are mediated by prenatal health behaviors. Using structural equations modeling procedures, the authors examined prospective impact of PNMS and dispositional optimism on birth weight and gestational age in a medically high-risk sample (N = 129), controlling for effects of risk and ethnicity. After its strong inverse association with optimism was accounted for, PNMS had no impact on birth outcomes. Women who were least optimistic delivered infants who weighed significantly less, controlling for gestational age. Optimists were more likely to exercise, and exercise was associated with lower risk of preterm delivery. Results suggest that chronic stress in pregnancy may be a reflection of underlying dispositions that contribute to adverse birth outcomes.