3. Fertility Assumptions, 2010-Based National Population Projections

Introduction

This report provides detailed information on the principal and variant fertility assumptions used in the 2010-based national population projections.

For England, and for Wales, the long-term average completed family size is assumed to be 1.85 children per woman. A higher level of 1.95 is assumed for Northern Ireland and a lower level of 1.70 is assumed for Scotland. These assumptions are the same as the 2008-based projections for Northern Ireland, Scotland, England and Wales.

Principal assumptions

The numbers of births for the projections are obtained by applying the appropriate fertility rate to the average number of women at each age during each year of the projection period. Because cohort fertility rates are more stable than those for calendar years (period rates), the fertility rates used in the projections are derived from assumptions relating to the year in which women were born. The assumptions about completed family size are based on family building patterns to date and other relevant evidence.

Table 3-1 shows the average completed family size by year of birth of woman for the UK and constituent countries. Tables 3-2 and 3-3 show births per 1,000 women, achieved family size, and average age at motherhood, by year of birth of woman, for the UK. The equivalent tables for the constituent countries of the UK, England & Wales and Great Britain can be found in the data download links for these tables.

Table 3-1

Completed family size for the constituent countries of the United Kingdom

Table source: Office for National Statistics

Table notes:

1. Figures in bold are partly or wholly projected

Table 3-2

Births per 1,000 women by age, United Kingdom

Table source: Office for National Statistics

Table notes:

1. Figures in bold are partly or wholly projected

Table 3-3

Achieved family size by exact age, and average age at motherhood, United Kingdom

Table source: Office for National Statistics

Table notes:

1. Figures in bold are partly or wholly projected

For the UK as a whole, completed family size has been falling steadily from an average of around 2.45 children for women born in the mid 1930s. Women born in 1965, effectively the most recent cohort to have completed their fertility achieved an average of 1.91 children per woman.

The family sizes to be achieved by women currently in their twenties or younger are highly conjectural, but there is some evidence that falls in cohort fertility could be slowing down. In particular, women born in 1980 have had more children on average by age 30 than those born in 1975 and, because fertility rates at older ages are projected to remain high, the 1980 cohort is projected to have 2.00 children on average, a higher completed family size than women born in the 1970s. For women born after 1980, it has been assumed that average completed family size for the UK as a whole will fall back below two children and eventually level off at 1.84 children for women born after 2010.

For England, and for Wales, the long-term average completed family size is assumed to be 1.85 children per woman. A higher level of 1.95 is assumed for Northern Ireland and a lower level of 1.70 is assumed for Scotland. These assumptions are the same as the 2008-based projections for Northern Ireland, Scotland, England and Wales.

Between 2002 and 2008, total fertility rates increased in all constituent countries of the UK, followed by a dip in 2009 (see Figure 3-1). All countries except Scotland then showed a recovery in 2010. For the latest projections, the total fertility rate for the UK has been assumed to increase from 2010 to 2013 then decrease gradually to reach the long-term level around 2027.

Over the past eight years (excluding the dip), fertility rates have been rising faster among women in their thirties and forties than for women in their twenties, so mean age at childbirth has continued to rise. The average age at motherhood for the UK as a whole is projected to increase from 28.4 years for women born in 1965 (the most recent cohort to have completed their childbearing) to its long-term level of 29.9 years for those born from 2007 onwards.

Figure 3-1 shows the total fertility rate (TFR) and average completed family size (CFS) for the UK between 1971 and 2035. The equivalent figures for the constituent countries of the UK can be found in the Appendices A to D. Figure 3-2 shows the assumed ultimate (long-term) age-specific fertility rates for the constituent countries of the UK.

Figure 3-1

Total fertility rate and average completed family size, United Kingdom, 1971-2035

Notes:

1. Completed family size relates to cohort born 30 years earlier - 30 years being the approximate mid-point of the childbearing ages. Projected completed family size is given for cohorts who have not yet completed childbearing.

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Figure 3-2

Assumed ultimate (long-term) age specific fertility rates

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Figure 3-3

Actual and assumed average completed family size, women born 1941-2005

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Figure 3-4

Actual and assumed total fertility rates, 1971-2035

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Assumptions for fertility variants

For the standard variants, fertility rates are assumed to move gradually from current levels to those assumed for the long-term. For the UK, England, Wales and Northern Ireland, the high variants assume a sharp increase in total fertility rates to 2012-13, followed by a more gradual increase for a further two years, before a gradual decline to the long-term 'high' level. For Scotland, the high variant also assumes a sharp increase in total fertility rates to 2012-13, but then levels off without a decline. For the UK and its constituent countries, the low variants assume that total fertility rates fall quite sharply for several years after 2010-11, before bottoming out at their projected long-term levels.

For the special case scenarios, the fertility levels given in the table are applied throughout the projection period, that is from 2010-11 onwards.

Table 3-4 shows the assumed long-term total fertility rates for the standard variants and special case scenarios for the UK and its constituent countries.

Table 3-4

Assumed long-term total fertility rates for the standard variants and special case scenarios

Table source: Office for National Statistics

Table notes:

1. Constant fertility is taken from the 2010-11 total fertility rate and applied throughout the projection period

Figure 3-5 shows actual and assumed total fertility rates for the UK between 1971 and 2035 for the principal projection and high and low variants. The equivalent figures for the constituent countries of the UK can be found in Appendices A to D.

Figure 3-5

Actual and assumed total fertility rates, United Kingdom, 1971-2035

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Background notes

1. Details of the policy governing the release of new data are available by visiting www.statisticsauthority.gov.uk/assessment/code-of-practice/index.html or from the Media Relations Office email: media.relations@ons.gsi.gov.uk

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

Appendix A: England Charts

Figure 3-1

Total fertility rate and average completed family size, England, 1971-2035

Source: Office for National Statistics

Notes:

1. Completed family size relates to cohort born 30 years earlier - 30 years being the approximate mid-point of the childbearing ages. Projected completed family size is given for cohorts who have not yet completed childbearing.

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Figure 3-5

Actual and assumed total fertility rates, England, 1971-2035

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Appendix B: Wales charts

Figure 3-1

Total fertility rate and average completed family size, Wales, 1971-2035

Notes:

1. Completed family size relates to cohort born 30 years earlier - 30 years being the approximate mid-point of the childbearing ages. Projected completed family size is given for cohorts who have not yet completed childbearing.

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Figure 3-5

Actual and assumed total fertility rates, Wales, 1971-2035

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Appendix C: Scotland charts

Figure 3-1

Total fertility rate and average completed family size, Scotland, 1971-2035

Notes:

1. Completed family size relates to cohort born 30 years earlier - 30 years being the approximate mid-point of the childbearing ages. Projected completed family size is given for cohorts who have not yet completed childbearing.

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Figure 3-5

Actual and assumed total fertility rates, Scotland, 1971-2035

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Appendix D: Northern Ireland

Figure 3-1

Total fertility rate and average completed family size, Northern Ireland, 1971-2035

Notes:

1. Completed family size relates to cohort born 30 years earlier - 30 years being the approximate mid-point of the childbearing ages. Projected completed family size is given for cohorts who have not yet completed childbearing.

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Figure 3-5

Actual and assumed total fertility rates, Northern Ireland, 1971-2035

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