According to theory, populations go through predictable stages from high birth and death rates to a stable population with low birth and death rates.

The answer to this question is the Demographic transition theory. The Demographic transition theory is a theory being developed by Warren Thompson. The theory of demographic transition is the transition of the high birth rate and death rate into a lower death and birth rate of the country or region. This theory has four stages, where in the first stage it states that both the birth and death rate of people are balanced. In the second stage of demographic theory the dropping of death rate is happening quickly. The falling of birth rate because of factors of fertility, urbanization, and other factors happens during the third stage of the theory. While in the fourth stage, both birth and death rate are low.