Background: CenteringPregnancy (CP) is a group antenatal care (G-ANC) model that has proven beneficial for mothers and their newborns. We conducted a feasibility study beginning in 2016 as part of the Mexican effort to implement G-ANC locally. This study reports on fidelity to the essential elements of CP during its implementation in Mexico. Methods: We collected prospective data using a standardized checklist at four primary-care centers that implemented our adapted G-ANC model. We performed a descriptive analysis of fidelity to 28 processes per G-ANC session (71 sessions made up of 10 groups and 129 women across 4 health centers). We calculated fidelity to each process as a proportion with 95% confidence intervals. We present overall results and stratified by health center and by facilitation team. Results: Overall fidelity to the G-ANC intervention was 82%, with variability by health center (78-88%). The elements with the highest fidelity were having space for activities such as checking vital signs, conversation in a circle, and medical check-ups (100% each) and the element with the lowest fidelity was using music to enhance privacy (27.3%). Fidelity was not significantly different by center. Conclusions: Our study suggests good model fidelity during the implementation of G-ANC in Mexico. Our findings also contribute useful information about where to focus efforts in the future to maintain and improve G-ANC model fidelity.