The four stages of the classical demographic transition model are pretransition, early transition, late transition and post-transition. They explain changes in population over time, based on birth rates and death rates.
The pretransition stage is characterized by high birth rates and high death rates. Famine, drought, lack of prevention measures and cure for diseases cause high death rates. Improved nutrition and health care in the early transition stage leads to a sharp decrease in death rates. However, the birth rate remains high, causing an exponential population growth.
Small family sizes and a change in traditional values characterize the late transition stage, causing a sharp decline in birth rates. Population growth declines, but the death rates continue to drop slowly. The low birth rates and death rates continue to the post-transition stage. Population growth becomes negligible and in some cases declines.