1. Introduction

This article provides detailed information on the principal and variant fertility assumptions used in the 2016-based national population projections. For England and Wales the long-term average completed family size is assumed to be 1.85 children per woman, a decrease of 0.05 from the 2014-based projections. Northern Ireland has seen no change from the 2014-based projections; it is still assumed to be 2.00 children per woman. The assumption for Scotland has been reduced from 1.70 to 1.65, a difference of 0.05 from the 2014-based projections.

Completed family size (CFS) is the average number of live-born children per woman which a group of women born in the same year have had by the end of their childbearing years. In general, measurement and analysis of fertility in terms of women born in a particular year is referred to as cohort fertility.

The age-specific fertility rate (ASFR) is the average number of children per woman, born to a group of women of a particular age in a particular year, normally expressed per thousand women.

The total period fertility rate (TFR) is the average number of children per woman that would be born to a group of women if they experienced the current year's age-specific fertility rates for each year of their childbearing years. This measure is referred to as the total fertility rate, or TFR, in this article.

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2. Principal assumptions

The numbers of births for the projections are obtained by applying the appropriate fertility rate to the number of women at each age during each year of the projection period. Because cohort fertility rates are more stable than period rates, the fertility rates used in the projections are derived from assumptions relating to the year in which women were born. Cohort fertility rates are more stable because they are affected only by changes in the total number of children women have and not by the timing of births within women’s lives. Period rates, the total fertility rate, in contrast, may rise or fall if births are brought forward or delayed for any reason.

The assumptions about completed family size are based on family-building patterns to date and other relevant evidence. Discussion papers showing the background information used in setting the fertility assumptions are available. Consultation papers

Tables 3.1 to 3.3 show estimated and assumed: average completed family size, births per 1,000 women, achieved family size and average age at motherhood, by year of birth of woman.

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4. Assumptions for fertility variants

Standard variants look at the effect of varying one assumption at a time from the principal projection.

For example, the high fertility variant uses mortality and migration assumptions consistent with the principal projections, but assumes a higher rate of fertility.

History shows that there can be quite sudden changes in period fertility. It is therefore important to demonstrate the effect of significant short-term changes, as well as the long-term effects that would result from sustained levels of fertility significantly above or below that assumed in the principal projection.

For the standard variants, fertility rates are generally assumed to move gradually from current levels to those assumed for the long term.

The 2016-based projections are the first to use asymmetric variants for the fertility assumptions.

Figure 3.5 shows that the principal projection assumes the total fertility rate for the UK decreases in the short term from the 2016 level of 1.79 to 1.77 in 2017, then increases gradually over the years. In the long term, the high and low fertility variants assume total fertility rates of 1.94 and 1.64 children per woman for the UK.

Figure 3.5 shows estimated and assumed total fertility rates for the UK between 1973 and 2041 for the principal projection and high and low variants.

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5. Background notes

  1. Discussion papers showing the background information used in setting the fertility assumptions are available on our website.

  2. Details of the policy governing the release of new data are available from the UK Statistics Authority website.

  3. These National Statistics are produced to high professional standards and released according to the arrangements approved by the UK Statistics Authority.

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Contact details for this Compendium

Andrew Nash
Telephone: +44 (0) 1329 44 4661