Source: Statistics New Zealand
Guide to fertility measures – methods paper
17 December 2025
This guide describes a range of fertility measures, discusses the strengths and limitations of these measures, and identifies where published measures are available.
Measuring the number of births and rates of birth (fertility) is of long-standing demographic and statistical interest. Births – along with deaths (mortality) and migration – is one of the three fundamental processes that change the size and composition of populations.
In a demographic context, fertility is the actual level of reproduction of a population, not the biological capacity of a population to bear children (fecundity). Measures of fertility are important for measuring average family size, the extent to which a population is replacing itself over time, and for population estimation and projection.
The number of births depends on the number and age of females in the population (although the number of males is also relevant), how many babies that females have during their lifetime, and when they have them. However, changes in annual births and cross-sectional (period) fertility rates are not always a good indication of changes in family size.
