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Background Maternal anaemia remains a pressing global health challenge, with a notable burden in low- and middle-income countries. Existing studies in sub-Saharan Africa have largely relied on average associations, thereby concealing key variation among women and failing to account for heterogeneity.Objective To assess the association between completing at least eight antenatal care (ANC) contacts and maternal anaemia in Ghana and to explore heterogeneity in this association using causal machine learning.Design An institution-based cross-sectional study was conducted using a retrospective review of medical records and causal machine learning analysis.Setting Juaben Government Hospital.Participants Of 2326 women who delivered at the hospital, 2114 with complete data on the main exposure and outcome variables were included in the analysis.Main exposure Completion of at least eight ANC contacts. ANC contact was defined as the in-person visit to the clinic with a healthcare professional for routine ANC services and follow-up.Main outcome Maternal anaemia, defined as a haemoglobin level below 11 g/dL in the last ANC before delivery.Methods A causal forest model was used to estimate the association between completing at least eight ANC contacts and maternal anaemia. Conditional average treatment effects were used to explore individual-level variation in these associations, providing policy-relevant insights.Results Completing ≥8 ANC contacts was associated with a 6 percentage-point lower probability of maternal anaemia compared with having fewer visits (average treatment effect: -0.06, 95% CI −0.11 to −0.02). Predicted individual-level effects ranged from −0.21 to 0.09. Participants’ age, malaria prophylaxis, marital status, parity and educational level were the five most important contributors to the observed heterogeneity.Conclusions This study demonstrated that completing ≥8 ANC contacts is associated with a lower probability of maternal anaemia, with heterogeneity across subgroups. We recommend differentiated, context-specific ANC interventions that focus on high-impact subgroups while strengthening the effectiveness and quality of care delivered at each visit.