The present paper reports a study on the socio-economic determinants of completed fertility in Mexico. Special attention is given to how socio- economic factors such as religion and ethnic group affect the likelihood of transition from low to high order parities. This methodological approach allows the researcher to enquiry about the role that such socio-economic characteristics have played in the process of adoption and diffusion of a low fertility norm in Mexico. Hurdle Poisson and Negative Binomial count data models are used as main econometric tools. Among other models, an endogenous treatment (or sample selection) count specification is estimated. The findings indicate that Catholicism is associated to reductions on the likelihood of transiting from low to high order parities in Mexico and that broad ethnic group does not affect such a probability. Hence, empirical results suggest that ethnic background does not constitute an obstacle for the diffusion of a low fertility norm (contraception use) in Mexico.