A fertility index has been estimated which takes into account the space–time variations of mortality rates of children and women and the different fertility rates of women across age groups. The basic data are the recorded frequencies at district level for four decennial census dates (1961 to 1991) of children aged 0–4 years and women aged 15–49 years. On that basis, we have 326, 349, 400 and 454 data points on the map for each decennial census, respectively. To construct fertility surfaces on the map, the kriging technique of spatial interpolation was applied. To highlight on these surfaces the areas of regional homogeneity and zones of major change, on a subcontinental scale, a technique of image analysis and the wombling procedure of detection of the zones of abrupt change were used. The fertility maps and the variogram surfaces show that the transition took place by an expansion of heterogeneity starting from the southwestern zones. This expansion reduced the extent of the pre-transition area of homogeneity in such a way that its original transverse SW–NE direction turned into a longitudinal direction by 1991 and its size became appreciably smaller. In 1961 a long zone of abrupt change detected by wombling was observed crossing the Indian subcontinent transversely. From west to east, it extended longitudinally for a length of nearly 2500 km, including Bangladesh. This zone of abrupt fertility change divided the subcontinent into two regions, one of high fertility in the north and the other of lower fertility in the south. This latter zone is separated partially by State boundaries and geographical landmarks. A procedure of regression trees (classic AID tree) applied to the co-variables shows that this zone also corresponds to a major regional change in the female literacy rate. From 1961 to 1991, with a general fall in fertility rates all over the country, this zone of abrupt change has progressively disappeared. Copyright © 2001 John Wiley & Sons, Ltd.