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Definition

In its broadest sense, fertility refers to the performance of a population in bearing children. It should not be confused with fecundity, the biological capacity of the population to bear children.

Fertility is defined as the actual level of reproduction of a population based on the number of live births that occur. Fertility is normally measured in terms of women of childbearing age, usually defined as 15–49 years of age, although births to women outside this age range can, and do, occur. In the Census of Population and Dwellings and, if required in other surveys, the fertility measure of number of children born alive is usually collected from all women aged over 15 years (that is, with no upper age limit).

There are a number of ways of measuring the broad concept of fertility. They are listed below with the source(s) from which they were derived.

Age-specific fertility rate

The age-specific fertility rate is the number of live births to women of a particular age divided by the number of women of that age. Derived from:

  • birth registrations
  • resident population estimates.

Completed fertility rate

The completed fertility rate is the average number of live births that a woman born in a particular year has had by the end of her reproductive life. It is also known as completed family size or the cohort total fertility rate. Derived from:

  • birth registrations
  • resident population estimates
  • number of children born alive question.

Crude birth rate

The crude birth rate in a particular year is the number of live births divided by the mean estimated total resident population. (This population includes both males and females). Derived from:

  • birth registrations
  • resident population estimates.

Cumulative fertility rate

The cumulative fertility rate is the average number of live births that a woman born in a particular year has had by the time she reaches a particular age. Derived from:

  • birth registrations
  • resident population estimates
  • number of children born alive question.

Ex-nuptial birth rate

The ex–nuptial birth rate is the number of live births to not–legally–married mothers divided by the mean number of not–legally–married women aged 15–49 years.
(See Glossary and references section for definition of 'mean not–married population').Derived from:

  • birth registrations
  • resident population estimates.

General fertility rate

The general fertility rate is the number of live births divided by the mean number of women aged 15–49 years of age. Derived from:

  • birth registrations
  • resident population estimates.

Gross reproduction rate

The gross reproduction rate in a particular year is the average number of live daughters a woman will bear during her reproductive life, assuming that the age-of-mother specific birth rates experienced during that year continue to apply. Derived from:

  • birth registrations
  • resident population estimates.

Known pregnancies

The number of known pregnancies includes live births, stillbirths and legally induced abortions. It does not include miscarriages, as miscarriages are not a notifiable event in New Zealand and as such there is no information available on the number of miscarriages in a given year. Derived from:

  • birth registrations
  • Abortion Supervisory Committee Report to Parliament.

Net reproduction rate

The net reproduction rate in a particular year is the average number of live daughters a woman will bear during her reproductive life, assuming that the age–of–mother specific birth rates and age–specific mortality rates experienced during the year continue to apply. Derived from:

  • birth registrations
  • resident population estimates
  • life tables.

Number of children born alive

Number of children born alive is defined as the number of children ever born alive to each woman resident in New Zealand, aged 15 years and over.

Foetal deaths and stillborn children are excluded, as are stepchildren, adopted children, foster children and wards of state. Derived from:

  • number of children born alive question.

Nuptial birth rate

The nuptial birth rate is the number of live births to legally-married mothers divided by the mean number of legally married women aged 15–49 years. Derived from:

  • birth registrations
  • resident population estimates.

Parity-specific birth rate

The parity–specific birth rate is the number of births of specified birth order to the number of women who have borne one child less than the number indicated by the specified birth order. For example, the second–parity birth rate is the number of second births divided by 1,000 women who have already had one child. Derived from:

  • birth registrations
  • resident population estimates
  • number of children born alive question.

Registered confinements

Registered confinements are confinements (live and still) registered in New Zealand to mothers resident in New Zealand and mothers visiting from overseas.

The number of registered confinements is the number of pregnancies resulting in either live or stillborn children. Such a pregnancy is counted as one confinement irrespective of whether a single or multiple birth results. Derived from:

  • birth registrations.

Registered live births

Registered live births are live births registered in New Zealand to mothers resident in New Zealand and mothers visiting from overseas.

It excludes late registrations under Section 14 of the Births, Deaths and Marriages Registration Act 1995. These are births which are not registered in the ordinary way at the time the birth occurred. Such registrations can occur as late as retirement age. Derived from:

  • birth registrations.

Replacement level

Replacement level fertility is the long–term total fertility rate required by any population to replace itself without migration. The replacement level is about 2.1 births per woman. Changes in population structure and mortality by age and sex can affect fertility rates at a particular point in time without affecting the long-term patterns.

Stillbirth rate

The stillbirth rate is the number of stillbirths per 1,000 total births (live and still). Derived from:

  • birth registrations.

Total fertility rate (or fertility rate)

The total fertility rate (TFR) (or fertility rate) in a particular year is the average number of live births a woman will have during her reproductive life if she is exposed to the age–specific fertility rates characteristic of various childbearing age groups in that year. The TFR is the sum of the single year of age–specific fertility rates. Derived from:

  • birth registrations
  • resident population estimates.

Total paternity rate (or paternity rate)

The average number of live births that a man would have with women during his life if he experienced the age–specific paternity rates of that year. It excludes the effect of mortality. It is derived from the sum of the age-specific paternity rates relating to a given year and is subject to annual fluctuations in births. While they represent each year's experience, they do not necessarily represent the lifetime reproductive experience of real generations or cohorts of men.

As used in ethnic population projections, the total paternity rate is the average number of live births that a man of a given ethnicity would have with women of other ethnicities during his life:

Mäori population is the rate applied to the Mäori male population to produce the number of births between Mäori males partnered with non–Mäori females.

Pacific population is the rate applied to the Pacific male population to produce the number of births between Pacific males partnered with non–Pacific females.

Asian population is the rate applied to the Asian male population to produce the number of births between Asian males partnered with non–Asian females.

European population is the rate applied to the European male population to produce the number of births between European males partnered with non–European females. Derived from:

  • birth registrations
  • resident population estimates

The following supporting concepts are defined under the heading Glossary and references:

  • abortion
  • birth
  • birth rate
  • children ever born
  • confinement
  • estimated resident population
  • foetal death
  • issue
  • live birth
  • mortality rate
  • mean not–married population
  • parity (previous issue)
  • registered marriage
  • registrar–general
  • resident population concept
  • stillbirth.
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