This paper examines regional heterogeneity both from statistical and cartographic perspectives, using factor analysis of non-demographic data, models of fertility determinants and district-wise mapping to test out the presence of regional clustering. Regional analysis reveals that economic, social and health indicators display spatial patterns as strong as fertility rates. All recent models of fertility incorporate a significant geographical component (using dummy regional variables or autocorrelation measurements). The map of fertility decline spreading along culturally and spatially contiguous regions also suggests that diffusion mechanisms may play an independent role in the spread of new reproductive behaviour (small family norm). Though diffusion per se, no real explanation for the fast decline of fertility in Southern or Coastal India (what would then be the cause of diffusion in the first place?), it would definitely be important to understand how these mechanisms are facilitated by social and cultural homogeneity or by regional policies. JEL Classification: J 11, J 13, I12, I 28 Key Words : Fertility, South India, diffusion, heterogeneity, family planning, reproductive behaviour, coastal India, infant mortality.