The aim of this study is to investigate whether diffusion contributed to the geography and the speed of the fertility transition. To this end, we put together a new and unique dataset from historical sources in Belgium containing yearly information on fertility at the municipality level and a range of structural and cultural indicators over 47 years (1887–1934). We use this dataset in a diffusion model based on multilevel event-history analysis. We find that diffusion between neighbouring places influenced the geography of the fertility transition only in its early stages; and diffusion accelerated the speed at which municipalities initiated fertility decline at the onset of the transition. We argue that, in the early stages of the transition, the bulk of people’s interactions was confined to their own communities and neighbouring places and, as such, new ideas, attitudes and information about fertility would spread among adjacent areas. Later on, since the turn of the twentieth century, the way people interacted in space was transformed by the growing urbanisation, the development of transportation infrastructure, labour migration, etc. In this new context, opportunities for social learning were less constrained by space.