In spite of this transition the total population of the country in stage IV is found to be much higher. It will be interesting to understand each stages of growth i.e. stage I through stage IV for understanding how the population eventually stabilizes (World Population: Growth & Trends).
This is the stage when birth rates and death rates are high and even then there is no population growth. High birth rate is due to high fertility rate or no use of contraceptives. Similarly, high death rate during the stage is attributable to the deaths occurring from the deadly diseases such as plague, cholera, typhoid as medical science was not developed enough to provide required protection. This is a preindustrial stage when living conditions are not favorable.
Infant mortality goes down due to improved health conditions that in turn, bring down the death rate; however, the birth rates still remains high keeping the population growth rate at high levels. This is also known as the transitional stage and most of the developing world is found in this stage.
The country develops economically and the people start using contraceptives that controls the birth rates significantly. Population growth rate in this stage falls down significantly. Most of the developed world and a few developing countries can be found here.
From the equation above, it is amply clear that if the death rate equals birth rate, there will be no growth in population of the country. In other words, if the death rate declines, the birth rate must decline by the same rate for the population to achieve stabilization. With the advances in medical sciences; better nutrition, enhanced sanitation and clean water, the death rates have declined substantially in last several decades; however, the birth rates have not kept pace with the death rates resulting into positive population