Guide To Fertility Measures

Stillbirth rate

The stillbirth rate is the number of stillbirths in a year per 1,000 total births (stillbirths plus live births). These are generally based on the registrations of stillbirths and live births in a year, rather than when those births occurred.

A stillbirth (or late fetal loss) is defined as a child who showed no signs of life at the time of birth and either:

• weighed 400g or more at birth, or
• was born after the 20th week of pregnancy.

Before September 1995, a stillbirth was defined as "a child who was not alive at the time of birth and was issued from its mother after the 28th week of pregnancy".

Fetal and Infant Deaths web tool provides fetal death rates based on stillbirths (Health New Zealand / Te Whatu Ora publication). Not all stillbirths are registered, with some information collated from stillbirth cause of death data, resulting in a slightly longer lag in timeliness than the Stats NZ data.

Crude birth rate

The crude birth rate (CBR) is the number of live births during a year per 1,000 people in the total population. The CBR is the simplest measure of the rate at which a population is being replenished by births. The CBR is calculated by dividing the number of live births in a year by the mean estimated population over that year, and then multiplying by 1,000.

The CBR is easy to calculate, does not require detailed population data, and is widely used as a population indicator. However, because the sex and age structure of the population is not taken into account, CBRs are a limited measure of fertility over time or between populations.

For example, the CBR decreased from 15.7 per 1,000 in 1983 to 12.7 per 1,000 in 2014. The total fertility rate (which takes age and sex into account) was 1.92 births per woman in both 1983 and 2014. This suggests that the decrease in the CBR between 1983 and 2014 was largely a result of changes in the population rather than changes in family size. Indeed, females aged 15-39 years made up 20 percent of the New Zealand population in 1983, compared with 17 percent in 2014.

General fertility rate

The general fertility rate (GFR) is the number of live births occurring during a year per 1,000 women in the childbearing ages. Usually the female population aged 15 to 49 years is used as the denominator, but 15 to 44 years, or 10 to 49 years, are sometimes used in different sources.

Stats NZ does not regularly publish this measure but it can be calculated by dividing the number of live births in a year by the mean estimated population aged 15-49 years over that year, and then multiplying by 1,000.

The GFR takes account of the size of the female population of childbearing age, but takes no account of the age composition within that population. The GFR is therefore a more refined measure than the crude birth rate, but less intuitive than the total fertility rate.

Age-specific fertility rates

Age-specific fertility rates (ASFRs) measure the number of live births that 1,000 women in a particular age group have in a given period (usually a year).

The youngest and oldest age groups used for ASFRs can vary because they often involve small numbers of births. Within the youngest and oldest age groups, aggregated births are divided by an appropriate estimated population. For example, the fertility rate for the age group 45-49 years often includes all births in the 45+ age group relative to the estimated number of females aged 45-49 years. Similarly, the fertility rate for the age group 15-19 years often includes all births in the under-20 age group relative to the estimated number of females aged 15-19 years.

ASFRs show a big drop to women aged in their twenties, especially during the 1960s and 1970s (figure 3). Fertility rates for women aged in their thirties also decreased during the 1960s and 1970s, but increased in the 1980s, 1990s, and 2000s. However, women aged in their thirties still have lower fertility rates in the 2020s than in the early 1960s.

Despite the decrease in ASFRs, the number of live births registered in 2008 (64,340) was only slightly lower than in 1962 (65,010). This is because New Zealand's population increased from 2.5 million in 1962 to 4.3 million in 2008. Together, the number of births and the ASFRs tell us that, in 2008, there were more women, but they were having fewer babies than in the early 1960s.

Figure 3

Fertility rates are also calculated for each year of age (Stats NZ publishes them for women aged 11-49 years and 50+ years). Single-year-of-age fertility rates show variation in fertility within age groups, especially for women beginning or nearing the end of the childbearing years. For example, there were 10 births per 1,000 women aged 15-19 years in 2024. However, within this age group, fertility rates increase with age, from 1 birth per 1,000 women aged 15 years, to 24 births per 1,000 women aged 19 years in 2024. For women aged 40-44 years, fertility rates decrease with increasing age.

ASFRs are important for understanding trends and levels of childbearing. However, it is important to note that these rates measure the fertility of an age group at a point in time. They are not a measure of generational change in overall fertility or family size.

Age-specific nuptial and ex-nuptial fertility rates

Age-specific nuptial and ex-nuptial fertility rates relate live births to the legally married/unmarried female population in each age group respectively, which gives comparisons over time or between populations. Historically, nuptiality was a strong predictor of childbearing, but less so in recent decades. Many ex-nuptial births are to women in de facto relationships.

In the early 2020s, 1 in 2 births were ex-nuptial, compared with 1 in 4 in the mid-1980s, and 1 in 11 in the early 1960s.

Total fertility rate

The total fertility rate (TFR) is the average number of live births that women would have during their life if they experienced the age-specific fertility rates of a given period (usually a year). It excludes the effect of mortality and is usually expressed as the number of births per woman.

The TFR allows us to summarise ASFRs into a single-number indicator of fertility. However, annual fluctuations in the TFR do not necessarily indicate changes in family size, but may indicate changes in the timing of births. In addition, the TFR is distorted by changing patterns of childbearing and can hide significant changes in age-specific fertility rates. Demographic indicators such as the TFR assume that the fertility patterns of the population are stable and that there is little or no migration in and out of the population. New Zealand, however, has a highly mobile population and has experienced a significant shift over the last century in both family size and the age at which women have children.

The TFR was estimated at around 3.0 births per woman in the early 1920s, but declined to about 2.3 births per woman in the mid-1930s (figure 4). From the mid-1940s the TFR increased dramatically, peaking at 4.3 births in 1961. New Zealand then experienced decreasing fertility rates, with the TFR dropping to 2.1 in the late 1970s. This drop was largely driven by fewer women in their twenties having babies. From the late 1970s to the early 2000s, the TFR was relatively stable, fluctuating around 2.0 births per woman. A 'baby blip' in 2007─2012 saw the TFR rise to nearly 2.2, but it then dropped steadily to under 1.6 in 2025.

Figure 4

Although the TFR is sometimes used as a proxy for family size, this can be misleading. The difficulty with the TFR is explicit in the definition: it assumes that women will experience the ASFRs of a particular period throughout their lives. However, this assumption rarely holds, as ASFRs often change significantly during the reproductive life of women.

In general, if there is a significant trend towards having children at a younger age, the TFR tends to overstate the number of live births that women will have over their lifetime. Conversely, if there is a significant trend away from having children at a younger age, the TFR tends to understate the number of births that woman will have.

This impact of changing age at first birth on New Zealand's TFR in NZ is indicated in figure 4. This graph shows the TFR in given years compared with the completed fertility rate (CFR) of cohorts born 28 years before the year for which TFR is shown. A 28-year lag is chosen to approximate the average age at birth of mothers over this long period of time, although it has varied (for example, from a minimum of 25.4 years in 1974 to a maximum of 30.9 years in 2023), so the comparison is only indicative.

The TFR and following reproduction rates indicate what current fertility (and mortality) imply for the size of future generations.

Gross reproduction rate

The gross reproduction rate (GRR) is the average number of daughters that a woman would have during her life if she experienced the age-specific fertility rates of a given period (usually a year). It excludes the effect of mortality.

Net reproduction rate

The net reproduction rate (NRR) is the average number of daughters a woman would have during her life if she experienced the age-specific fertility and mortality rates of a given period (usually a year). A NRR of 1.0 means that a woman would exactly replace herself.

Replacement-level fertility

Replacement-level fertility is the average number of children a woman needs to produce one daughter who survives to childbearing age. Replacement-level fertility is also described as the total fertility rate required for the population to replace itself in the long term, without migration.

The internationally accepted replacement-level fertility for developed countries is 2.1 births per woman. Replacement-level fertility allows for child mortality (children who die before reaching reproductive age) and the birth of more boys than girls. On average, 105 boys are born for every 100 girls, but this ratio can vary from year to year, and from country to country.

In New Zealand, the sex ratio at birth has averaged 105.5 boys for every 100 girls over the period 1900 to 2024. Over that period it ranged from 103 to 108 boys per 100 girls.

The actual replacement-level fertility will vary slightly from country to country, depending on the sex ratio at birth and child mortality rates. As mortality rates drop, the replacement-level fertility will also drop to converge with the sex ratio at birth.

Using the sex ratio at birth, fertility rates, and period life tables, the replacement level in New Zealand was 2.07 births per woman in 2022-2024. While an interesting exercise for demographers, a specific measure of replacement-level fertility is not very useful. The concept of replacement level assumes that there is no migration and is subject to the limitations of the total fertility rate.

It is misleading to equate below-replacement fertility with declining population. New Zealand has had fertility rates close to, or below, replacement since the mid-1970s. However, births outnumbered deaths by about two to one in most of those years until the 2020s.

The impact of below-replacement fertility is unlikely to be felt until the first generations which experienced sub-replacement fertility reach old age and die in significant numbers. Nationally, deaths are unlikely to exceed births before the 2050s (National population projections: 2024(base)-2078), although subnationally a growing number of areas already experience natural decrease (Subnational population estimates: At 30 June 2025).

Age-specific paternity rates

Age-specific paternity rates (ASPRs) measure the number of live births that men in a particular age group have (with women) in a given period (usually a year). Paternity rates are not commonly derived because the focus of fertility analyses is usually on women. Moreover, birth registrations tend to have more complete information of the mother (partner 1) than the father (or partner 2).

However, paternity analyses indicate similar trends for males as for females, such as declining rates at younger ages, and increasing rates at older ages (Parenting and fertility trends in New Zealand: 2018).

In New Zealand, ASPRs are most often calculated as an input to ethnic population projections. These account for the extra contribution to ethnic population growth that arises from births between men of a given ethnicity and women without that ethnicity. For example, Māori ASPRs measure the number of live births that 1,000 Māori men in a particular age group have with non-Māori women during a year.

Total paternity rate

The total paternity rate (TPR) is the average number of live births that men would have (with women) during their lifetime if they experienced the ASPRs of a given period (usually a year). It excludes the effect of mortality and is usually expressed as births per man.

Paternity rates are not commonly derived because the focus of fertility analyses is usually on women. However, paternity analyses indicate similar trends for males as for females, such as declining average family size over time.

In New Zealand, TPRs are most often calculated as an input to ethnic population projections. These account for the extra contribution to ethnic population growth that arises from births between men of a given ethnicity and women without that ethnicity. For example, the Māori TPR measures the average number of live births that Māori men would have with non-Māori women during their life, if they experienced the ASPRs of a given year.

Age-specific pregnancy rates

Age-specific pregnancy rates relate pregnancies (live births, stillbirths, and induced abortions) to the female population in each age group, which gives meaningful comparisons over time or between populations. They exclude miscarriages.

Pregnancy rates provide clearer understanding of pregnancy trends across different age groups in the population. They can help understanding of needs for health services or policies that provide targeted support, including for maternal health, sexual health education, and contraception services. They also help understanding of changes in pregnancies as well as births, and proportions of pregnancies that end in birth (both live or still), and induced abortion.

Data published by Stats NZ from 2020 onwards has a longer lag than other births data due to abortion data being published by the Ministry of Health. Abortion data up until 2019 was published by Stats NZ (Abortions).

Total pregnancy rate

Total pregnancy rates are an indication of the average number of pregnancies that women will have during their lifetime, measured by summing age-specific pregnancy rates. It is a cross-sectional measure based on births and induced abortions to different birth cohorts that are 'registered' in a specific period. For example, some induced abortions in a year will have been counted as births in the following year.

Cohort completed fertility rate

The cohort fertility series traces the fertility experience of women born in a particular year. The series measures the average number of births that women born in a particular year have at each age from 13 to 49 years of age. Summing the cohort age-specific rates gives the completed fertility rate (CFR) by year of birth.

The CFR indicates that women born in the early 1930s averaged the largest families. Their CFR was about 3.5 births per woman, well below the peak of 4.3 births suggested by the TFR (figure 4). In contrast, women born during the 1950s had a CFR of about 2.4 births per woman, and women born during the 1970s had a CFR of about 2.1 births per woman.

While the TFR hovered around 2.0 from the mid-1970s to the mid-2010s, CFRs have tended to decline with successive cohorts born since the mid-1930s.

Cohort fertility gives us important insights into generational changes in childbearing and family size. Despite being one of the more robust measures of fertility, the CFR cannot be calculated until women have ended their childbearing. Hence the CFR and other cohort fertility measures can only give a picture of past trends unless complemented by projections of remaining fertility experience.

Number of children born (including childlessness)

Most New Zealand censuses since 1981 have asked all females (based on self-identified 'sex' or 'sex at birth') aged 15 years and over about the number of children they have had. Up to the 2013 Census, the question excluded stillbirths, stepchildren, adopted children, foster children, and wards of the state. In a change from previous censuses, the 2018 and 2023 Census questions did not explicitly exclude stillborn children.

This information allows us to combine information on fertility with other socio-economic characteristics (such as education, income, relationship status, and religious affiliation) not available from birth registrations. See, for example, tables in Aotearoa Data Explorer from the 2018 Census and 2023 Census.

For information on 'number of children born' by iwi see Births | All Māori | Te Whata or Aotearoa Data Explorer.

Females aged 45-54 years (around the end of their childbearing) in 2023 had averaged 2.03 births during their lifetime, compared with 2.31 births for this age group in 2006, and 3.26 for this age group in 1981. Declining completed family size over time is evident among females of all major ethnic groups (figure 5).

Figure 5

The census is also the only direct source of data on females who remain childless - either voluntarily or involuntarily - throughout their lives. 'Childless' refers to females who have never given birth, and therefore excludes females who had a child that later died.

Of those females aged 45-54 years in 2023, 17 percent were childless, up from 13 percent in this age group in 2006, and 10 percent in 1981. Increased childlessness since 1996 is evident among females of all major ethnic groups (figure 6).

Figure 6

As rates of childlessness increase, those females having children would need to have more children if New Zealand was to achieve current levels of fertility. For example, if one in five females remain childless, the remaining females would have to average:

• 2.6 children to achieve a total fertility rate of 2.1 births per woman (Replacement-level fertility)
• 2.0 children to achieve a total fertility rate of 1.6 births per woman.

Census data also gives information on the proportion of females who have had one, two, three, or more children. For example, the 2023 Census showed 16 percent of females aged 45-54 years have had one child, 37 percent two children, 19 percent three children, and 12 percent four or more children (figure 7). In principle, parity data (the number of previous births prior to the current birth) from birth registrations is limited to children of the current relationship. The census 'number of children born' question is therefore an important source of parity data.

Figure 7

Among females of Māori ethnicity aged 45-54 years, the 2023 Census showed 13 percent have had one child, 25 percent two children, 20 percent three children, and 28 percent four or more children (figure 8).

Figure 8

Other information

Births and deaths

Births and deaths provides the latest births/fertility releases.

For additional data on annual births, see Health New Zealand / Te Whatu Ora:

• Report on Maternity web tool - based on live birth numbers occurring in December years, regardless of when the births were registered
• Fetal and Infant Deaths web tool - the denominator for death rates is based on live and total birth numbers registered in December years, regardless of when the births occurred.

Induced abortions

For induced abortions data see:

• for 1980─2019, Infoshare > Health > Abortion statistics...
• for 2020 onwards, Ministry of Health
• Abortion Trends in New Zealand 1980─2007 (Stats NZ Storehouse).

Definitions and metadata

Glossary of Common Terms − Stats NZ DataInfo+ provides definitions.

Population statistics - user guide gives an overview of the different population statistics produced by Stats NZ, including census counts, population estimates, and population projections.

Most fertility indices use population estimates as denominators.

• Infoshare > Population > Population estimates > provides the latest population estimates.
• For 'flow' data, such as births that occur over a period of time, 'mean year ended' population estimates are the preferred denominator.

Mortality measures - user guide (in development) provides a discussion of what mortality measures are available and their strengths and limitations.

ISBN 978-1-991307-97-2 (online)