Demographic momentum is the phenomenon of continued population increase despite reduced reproductive rates. (see also Population momentum.)
Even in the face of extreme measures aimed at lowering reproductive rates, the population will continue to grow due to a large proportion of its population entering its reproductive years.
For example, when China first introduced its one-child policy, population growth continued regardless. Even though the number of children born reduced dramatically, the sheer number of maturing youth was significant. In 1979 when the one-child policy entered into force, the number of people becoming adults was based on the number of births around 1950s, not 1979. As a result, the Chinese population maintained the same momentum of increase as for the past 20 years. It is only now that the Chinese population has reached a somewhat stabilized population growth.