Why won't the population stop growing immediately when ZPG is reached?

Zero Population Growth (ZPG) occurs when the number of births equals the number of deaths, stabilizing the population size. However, the population won't stop growing immediately because of existing demographic momentum; a large proportion of the population may still be in their reproductive years, leading to continued births despite the Birth Rate aligning with the Death Rate. Additionally, factors such as improved healthcare and lower mortality rates can sustain population growth for a period even after ZPG is reached.