Fertility & Its Various Measurements

Fertility is the natural capability to produce offspring. As a measure, fertility rate is the number of offspring born per mating pair, individual or population. Fertility differs from fecundity, which is defined as the potential for reproduction. It is measured in terms of 

Crude Birth Rate (CBR): Crude birth rate is measured as the number of live births in a given year per 1,000 people alive at the middle of that year. One disadvantage of this indicator is that it is influenced by the age structure of the population.

General Fertility Rate (GFR): The General Fertility Rate is given by division of Number of Births in a Year to the Number of Women Aged 15-44 multiplied by 100. It focuses on the potential mothers only, and takes the age distribution into account.

Child-Woman Ratio (CWR): CWR is given by division of Number of Children under 5 to the Number of Women 15-49 and multiplied by 100. It is especially useful in historical data as it does not require counting births. This measure is actually a hybrid, because it involves deaths as well as births. (That is, because of infant mortality some of the births are not included; and because of adult mortality, some of the women who gave birth are not counted either.)

Total Fertility Rate (TFR): The total number of children a woman would bear during her lifetime if she were to experience the prevailing age-specific fertility rates of women. TFR equals the sum for all age groups of 5 times each ASFR rate. 

Gross Reproduction Rate (GRR): The number of girl babies a synthetic cohort will have. It assumes that all of the baby girls will grow up and live to at least age 50.

Net Reproduction Rate (NRR): The NRR starts with the GRR and adds the realistic assumption that some of the women will die before age 49; therefore they will not be alive to bear some of the potential babies that were counted in the GRR. NRR is always lower than GRR, but in countries where mortality is very low, almost all the baby girls grow up to be potential mothers, and the NRR is practically the same as GRR. In countries with high mortality, NRR can be as low as 70\% of GRR. When NRR = 1.0, each generation of 1000 baby girls grows up and gives birth to exactly 1000 girls. When NRR is less than one, each generation is smaller than the previous one. When NRR is greater than 1 each generation is larger than the one before. NRR is a measure of the long-term future potential for growth, but it usually is different from the current population growth rate.