Methodological Notes – Fertility Module

Updated: January 2019

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 complete expulsion or extraction from its mother of a product of conception, irrespective of the duration of the pregnancy, which, after such separation, breathes or shows any other evidence of life such as heartbeat, umbilical cord pulsation, or definite movement of voluntary muscles, whether the umbilical cord has been cut or the placenta is attached.

WHO Reproductive Health Indicators, 2006, p.32,

The simplest fertility indicator to compute is the crude birth rate, expressed as births per 1000 persons. 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 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 remarkably consistent 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, a 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.

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 fertility rates (ASFR), expressed as births per 1000 women of a specific age or age group. Mother’s ages are usually aggregated into 5-year age groups. The under-15 and 50+ groups are not consistently reported by statistical agencies. The 15-49 age group is generally taken arbitrarily as the “reproductive age” range. The 15-19 age group is often used to assess childbirth among young girls and adolescents, even though girls younger than 15 do give birth.

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)

ASFRs can be aggregated into the total fertility rate (TFR), which 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 births per 1,000 women. However, TFR is more often expressed as births per woman, in which case the above quantity needs to be divided by 1,000. The TFRs reported by CircHOB are calculated from the ASFRs. They may differ slightly from the TFRs published by some statistical agencies.

Data Sources and Limitations

Birth data are collected from birth certificates and recorded by vital statistics agencies, a legal requirement in all circumpolar countries. Near complete (although not always timely) registration of all births can be expected, even in the most remote communities. Births are attributed to the mothers’ usual region of residence.

United States

Data on the sex of the livebirth and age of the mother (and also several other characteristics) are available from CDC Wonder


Livebirths by age of mother are available from Statistics Canada: Table 13-10-0416-01 (formerly CANSIM 102-4503)  and by sex in Table 13-10-0422 (formerly CANSIM 102-4509)


The number of livebirths by sex and age of mother is available from Statistics Denmark’s Statbank Table FOD


Birth data are available from Statistics Greenland > Population > Vital statistics >Births > Livebirths (BEEBBL0) – Note that the 45-49 age group actually refers to age 45+.

Faroe Islands

The number of births by and sex and age of mother are from Statistics Faroe Islands  > Population and elections > Vital statistics > Livebirths by mother’s age IB02020


Birth data from Iceland are obtained from Statistics Iceland  > Births > Births > Livebirths by age of mother MAN05101

> Births > Births > Livebirths and late fetal deaths by sex MAN05100


Birth data are available from Statistics Norway
Livebirths, by sex in Table 4231
Livebirths, by mother’s age in Table 6990


Birth data are available from Statistics Sweden
> Population > Population statistics > Livebirths > Livebirths by region, mother’s age and child’s sex


Birth data are available from Statistics Finland’s Statfin database  > Population > Births > Livebirths by sex, age of mother, and area – 006


The number of livebirths by sex for Russia and its regions and age-specific-fertility rates can be found in the Federal State Statistics Service interactive database in Russian only.