Revised 24 May 2019

In a transition from a growth to a ZPG population, there would be a period of continued momentum of growth in the population called population momentum. The age composition of the population will change as the crude birth rate declines to match and equal the crude death rate. During the decline in crude birth rates, the crude death rate of low-income countries would also decline with better nutrition and medical care thus delaying crude birth rates from matching crude death rates. If crude births rates were to equal crude death rates and maintained, then the population would continue to grow due to a bulge of females moving up the age composition stack of cohorts into the child-bearing rank of cohorts between the ages of 15 to 44. The growth rate would slowly decline towards zero as females from the last and final growth cohort move up through the age composition stack of cohorts from the 0 to 4 cohort to the 45 to 49 cohort beyond the child-bearing age rank of cohorts. During this period the population would continue to grow, albeit progressively more slowly. 

In 1971, demographer Nathan Keyfitz calculated that if the less developed countries with high birth rates were to achieve a replacement fertility rate overnight, then their populations would continue to increase until they stabilised at about 1.6 times their then present size. If a replacement fertility rate took 30 years to achieve, then their final populations would be 2.5 times their then present size (Keyfitz, 1971).