The Demographic Transition Model shows how the population of a country changes over time through five stages and particularly looks at birth rate, death rate and total population.
Stage 1 of the model is high fluctuating and consequently a country is this stage has high birth and death rate and the population remains stable but low. There are no countries in this stage of the model, but some tribes in the rainforests of Brazil are in this stage. The birth rate is high and so the pyramid structure is very wide based but narrows greatly due to a low life expectancy. Although birth rate is high, so is infant mortality, so there is a large drop between children aged 0-5 to 6-10 and the pyramid takes a large indent from a wide base. The pyramid peaks at a low age due to a low life expectancy.
Stage 2 of the DTM is early expanding. The falling death rate but remaining high population causes the structure to remain with a wide base. But the rapidly increasing total population as death rate falls to around 15/1000 causes the pyramid to grow taller and life expectancy increases. Countries in this stage are Nepal and Afghanistan. Life expectancy increases due to the improved health care, sanitation and diet and therefore the overall pyramid widens and gains height.
A country such as Egypt is in stage 3 of the DTM and so is in late expanding. Birth rate rapidly falls due to the use of birth control and family planning, and the continuing falling death rate causes the pyramid to take a more of a rocket shape as the base gets narrower with birth rate and death rate become more even again. But, the natural increase in population is still high and the pyramid base only narrows slightly and the top becomes wider as more people are living longer.
Stage 4 of the DTM is represented by developed countries like most Europe and the USA. Birth rate and death rate fluctuate at a low level and the population remains stable but high. Consequently, the pyramid takes a much more even...