In the long term, zero population growth can be achieved when the birth rate of a population equals the death rate, i.e. fertility is at replacement level and birth and death rates are stable, a condition also called demographic equilibrium. Unstable rates can lead to drastic changes in population levels. This analysis is valid for the planet as a whole (assuming that interplanetary travel remains at zero or negligible levels), but not necessarily for a region or country as it ignores migration. A population that has been growing in the past will have a higher proportion of young people. As it is younger people who have children, there is large time lag between the point at which the fertility rate (mean total number of children each woman has during her childbearing years) falls to the replacement level (the fertility rate which would result in equal birth and death rates for a population at equilibrium) and the point at which the population stops rising. The reason for this is that even though the fertility rate has dropped to replacement level, people already continue to live for some time within a population. Therefore, equilibrium, with a static population, will not be reached until the first "replacement level" birth cohorts reach old age and die. The related calculations are complex because the population's overall death rate can vary over time, and mortality also varies with age (being highest among the old).