This article examines the role of components of adequate antenatal care (ANC) in disparities in birth weight between indigenous and non-indigenous women in Mexico. We estimate the potential for added weight gain among indigenous infants if their mothers received timely, frequent ( ≥4 visits) and complete ANC (≥75% of recommended processes of care). We used population-based survey data (2012; N = 6612 women 12–49). We applied quantile regression to examine heterogeneity of the association between adequate ANC, indigenous ethnicity and birth weight across quantiles of the birth weight distribution. A greater proportion of indigenous women reported a low-birth weight infant (<2.5 kg) at last delivery (14 vs 8% among non-indigenous women). Coverage of adequate ANC (timely, frequent and complete care) is lower among indigenous (59%, CI:53;65) than non-indigenous (68%, CI:66;70) women. Indigenous ethnicity is associated with a lower birth weight across quantiles of the observed birth weight distribution: between 300 g in the 0.05, 0.10 and 0.25 quantiles. Among indigenous women, greater newborn weight gains are achieved in the lowest quantiles if they have access to ≥75% of the content of ANC compared with those that did not have access: ~180 and 260 g are gained in both quantiles 0.05 and 0.10, respectively. This means that the smallest indigenous newborns could potentially reach 2.36 kg (from 1.86 kg), close to the normal weight threshold. The frequency of ANC was positively associated with birth weight for all women but complete ANC appears to differentially affect indigenous women at the bottom of the birth weight distribution. The marginal gains obtained among indigenous newborns that received complete ANC compared with indigenous/non-indigenous newborns did not receive it, is particularly important in low-birth weight quantiles. Delivering basic processes of ANC may therefore have the potential to impact the highest risk women and help them to overcome the low-birth weight threshold.