Demographic change in Latin America has been driven by the behaviour of the middle and upper strata. Given that fertility and mortality in these groups are now relatively low, future changes will mainly come from the behaviour of less advanced sectors. This paper analyses the contribution of these less advanced groups to the decline in fertility, distinguishing between the "distribution effect" and the "rates effect". In less advanced sectors the desired number of children is lower than the actual number, with early marriage and limited use of modern contraceptives continuing to be the rule. Even so, these groups have entered the demographic transition. A number of countries have recently seen falls in their fertility rates due to the contribution of women with low levels of education: in the late transition countries behaviour is heterogeneous, while in the advanced transition countries the greatest contribution is being made by women with primary education.