# Methodological Notes – Fertility Module

*Updated: October 2011*

### Concepts and Definitions

A population changes in size and composition by the number of people who are born into it, die in it, and move in and out of it. The **number of livebirths** provides the basic information from which various measures of fertility can be constructed. Most statistical agencies have similar definitions of a “livebirth”, based on the WHO one:

The simplest and easiest fertility indicator to compute is the **crude birth rate**. Its denominator, however, is the total population, thus including those who do not contribute to births such as females outside their reproductive age and males.

`Crude birth rate = (number of livebirths during year X) / (mean population of year X)`

The rate is expressed as per 1, 000 persons. The denominator for Table B-2 was extracted from Table A-1. Rates for the 2000-2004 period were calculated by dividing the sum of births during the 5-year period by the sum of the 5 annual mean populations. This differed slightly from averaging the 5 single-year rates.

Children are not born to women equally throughout their reproductive careers, and populations differ in terms of the peak ages of childbearing. The number of **livebirths by mother’s age** is needed to generate the **age-specific and total fertility rates**.

There are two approaches to “mother’s age”: age at last birthday (ie. number of completed whole years of life) and age reached during the year (ie. the actual duration of life lived in years and months). Many statistical agencies produce data for both, which are very similar. Where a choice is available, it is the latter definition that is used here.

Mother’s ages were aggregated into 5-year age groups in the table. The under-15 and 50+ groups were not consistently reported and they have not been included. Moreover, it is the 15-49 age group that is generally taken arbitrarily as the “reproductive age” range.

`Age-specific fertility rate = (number of births to women in age group i during year X) / (mean number of women in age group i during year X)`

The rate is expressed as per 1, 000 women.

The total fertility rate (TFR) can be interpreted as the mean number of children that would be born alive to a woman during her lifetime if she were to progress through her childbearing years experiencing the age-specific fertility rates (ASFR) of the population in a given year. It is therefore a purely hypothetical rate. A TFR of 2.0 is considered the replacement level for the population, since a couple will need two children to replace themselves (when childhood mortality is taken into account, a population will need a TFR of 2.1 or 2.2 to replace itself). A population at replacement level will eventually stop growing, if there is no immigration. The TFR can be calculated as:

`Total fertility rate = [sum of age-specific fertility rates for age group (15-19), (20-24)… (45-49)] x 5 `

Since the ASFR for each 5-year age group is in fact the average of the ASFR for 5 single years of age, to obtain the sum of all single-year rates for ages 15 to 49, one needs to multiply by 5. The formula as stated above produces a TFR expressed as per 1, 000 women. However, TFR is more often expressed as per woman, in which case the above quantity needs to be divided by 1, 000. The TFRs presented are calculated from the ASFRs in the table. They may differ slightly from the TFRs published by some statistical agencies.

The ratio of male to female livebirths is known as the **secondary sex ratio** and is usually expressed as the number of males per 100 females (sometimes also simply as the number of males or per female).

`Secondary sex ratio = (number of male livebirths) / (number of female livebirths) x 100`

It is remarkable consistently across populations, between 105-107 males for every 100 females born. In the absence of selective termination of pregnancy favouring one sex over another for sociocultural reasons, substantial deviation of the ratio from this “norm” is suggestive of threats to human reproductive health from the external environment or health conditions in the parents.

### Data Sources and Limitations

The number of livebirths is available from the national statistical agencies, either in their interactive databank websites or in published reports. Births are attributed to the mothers’ usual region/territory of residence. Ultimately birth data are collected from birth certificates, required by law in all circumpolar countries, and near complete (although not always timely) registration of all births can be expected, even in the most remote communities.