'Replacement level fertility' is a technical term which seems almost self-explanatory. However there are some important qualifications which make it a more difficult concept than might be supposed. Also, the relationship between replacement level fertility and zero population growth is complicated. The article explains why this is so and thus why, although the United Kingdom's current level of fertility is below replacement level, population is projected to grow for the next thirty years.
PIP: Replacement level fertility is the level of fertility at which a population exactly replaces itself from one generation to the next. In developed countries, replacement level fertility can be taken as requiring an average of 2.1 children per woman. In countries with high infant and child mortality rates, however, the average number of births may need to be much higher. Replacement level fertility is not associated with an unique set of age-specific birth rates. When a country reaches replacement level fertility, other conditions must be met for zero population growth to also be attained. Replacement level fertility will lead to zero population growth only if mortality rates remain constant and migration has no effect. The momentum of past and current demographic trends may also take several generations to work itself out. A change to replacement level fertility therefore leads to zero population growth only in the long run. The size of the population at which population growth levels off will usually differ from the current population size. It follows that fertility level is not, in itself, a reliable guide to population growth, and it is instead better to examine actual or projected population growth directly, then subsequently relating such growth to fertility, mortality, and migration. Sections describe measuring a population's actual fertility level, interpreting cohort measures, interpreting period level measures, and projected population growth in the UK. For all birth cohorts in the UK from 1950 onwards, fertility will or already has been limited to below replacement levels, although population size is projected to grow until approximately 2027. Population in that year is projected to be 4.3 million individuals larger than the UK population of mid-1992.