Background: The SMART Moms/Mamás LÍSTAS Project was a randomized control trial that tested the efficacy of a prenatal stress management program in reducing stress and cortisol levels among low-income women. The current study is a process evaluation of the stress management program (intervention arm of the original randomized controlled trial) and assessed whether implementation fidelity factors (i.e., intervention delivery, receipt, and enactment) were associated with lower stress (perceived stress, salivary cortisol), improved negative and positive mood states (Positive and Negative Affect Schedule), and increased confidence to use relaxation and coping skills. Method: Fifty-five low-income pregnant women (71% Latina, 76% annual income <$20,000) attended weekly group-based sessions over an 8-week period in which a clinically trained researcher taught relaxation and coping skills. Process evaluation measures were obtained via participant self-report and videotaped class sessions that were coded for delivery, receipt, and enactment of the intervention to determine which implementation factors were associated with changes in program outcomes (stress, mood, confidence) over the 8-week period. Results: Women in stress management showed a significant reduction in their stress and cortisol levels (p < 0.001), improvements in negative and positive mood states (p < 0.001) and were more confident in using relaxation and coping skills postintervention (74%). The implementation factors of delivery (i.e., instructor adherence to intervention content; p = 0.03) and enactment (i.e., participant use of intervention skills; p = 0.02) were most associated with improvements in program outcomes. Conclusion: These findings highlight that implementation factors should be considered when delivering stress management interventions in underserved communities.