This paper contributes to the study of the relationship between religion and fertility. More speci cally, I investigate the impact of being Catholic on fertility in France. Fertility is measured either by the number of children ever born or by completed fertility. I show that women who are strong practicers have signi cantly more children than other women ; however, being a Catholic believer has no signi cant impact on fertility. I also construct two variables allowing me to detect that the particularized ideology mechanisms can partially explain why religion has an impact on fertility in my dataset. Nevertheless, I cannot exclude the social interactions hypothesis. The multivariate analysis I provide also validates the main mechanisms of the rational actor model.