OBJECTIVE: CenteringPregnancy group prenatal care has been demonstrated to improve pregnancy outcomes. However, there is likely variation in how the model is implemented in clinical practice, which may be associated with efficacy, and therefore variation, in outcomes. We examined the association of fidelity to process and content of the CenteringPregnancy group prenatal care model with outcomes previously shown to be affected in a clinical trial: preterm birth, adequacy of prenatal care, and breast-feeding initiation. STUDY DESIGN: Participants were 519 women who received CenteringPregnancy group prenatal care. Process fidelity reflected how facilitative leaders were and how involved participants were in each session. Content fidelity reflected whether recommended content was discussed in each session. Fidelity was rated at each session by a trained researcher. Preterm birth and adequacy of care were abstracted from medical records. Participants self-reported breast-feeding initiation at 6 months postpartum. RESULTS: Controlling for important clinical predictors, greater process fidelity was associated with significantly lower odds of both preterm birth (B = -0.43, Wald ?(2) = 8.65, P = .001) and intensive utilization of care (B = -0.29, Wald ?(2) = 3.91, P = .05). Greater content fidelity was associated with lower odds of intensive utilization of care (B = -0.03, Wald ?(2) = 9.31, P = .001). CONCLUSION: Maintaining fidelity to facilitative group processes in CenteringPregnancy was associated with significant reductions in preterm birth and intensive utilization of care. Content fidelity also was associated with reductions in intensive utilization of care. Clinicians learning to facilitate group care should receive training in facilitative leadership, emphasizing the critical role that creating a participatory atmosphere can play in improving outcomes.