There is a demographic transition theory to understand the multiple population changes worldwide, that outlines changes in mortality, fertility and growth rates according to each stage or shifts in the population over time.
There are four stages in the demographic transition model:
Stage 1: Pre-transition; Stage 2: Early transition; Stage 3: Late transition; Stage 4: Post-transition;
and during the 2 & 3 transition stages, one can identify two big breaks:
