Population Momentum: The phenomenon whereby population continues to increase even after a fall in the birth rates, because the large existing youthful population expands the population’s base of potential parents.

Population momentum relates to the age structure of many developing countries’ populations. As share of population, young people greatly outnumber their parents.

When their generation reaches adulthood, the number of potential parents will inevitably be much larger than at present. It follows that even if these new parents have only enough children to replace themselves (two per couple, as compared with their parents, who may have had four or more children), the fact that the total number of couples having two children is much greater than the number of couples who previously had more children means that the total population will still increase substantially before levelling off.