1. Introduction

For the UK as a whole, the main measure used in setting the fertility assumptions in the national projections is average completed family size; 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. This has been falling from a peak of nearly 2.5 children per woman for women born in the mid-1930s and the projections assume that this will level off at 1.89 children per woman for women born in 2015 and later. This long-term assumption is unchanged from the 2012-based projections at the UK level.

The assumptions made about completed family size, which underlie this projection round, are based on an analysis of recent trends in fertility and an assessment of their implications for future completed family sizes, together with other relevant information such as the views of the expert advisory panel. These assumptions about future levels of fertility are set for each of the UK’s constituent countries separately and then combined to obtain the assumption for the UK as a whole.

This chapter discusses past trends in fertility and summarises the resulting assumptions adopted for the 2014-based projections.

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3. Fertility assumptions for the UK

In the 2014-based projections, the long-term completed family size is assumed to be 1.89 children per woman. This is the same as the level assumed in the 2012-based projections, but is below the “replacement level”. The “replacement level” family size of 2.075 represents the approximate number of children per woman needed for the population to replace itself in the long-term (in the absence of migration)8. The total fertility rate (TFR) in the UK has been below replacement level since the early 1970s and the completed family size assumed for the long-term falls around 9% below replacement level.

Table 3.1 and Figure 3.2 show the achieved family sizes of selected cohorts at successive ages. From 1950, for those aged 25, 40 and 45, each subsequent cohort has had fewer children by each age than earlier cohorts. For those in their 30s, the most recent data show a slight increase on previous cohorts. Over time there have been some fluctuations in the achieved family size for those aged 20 but a downwards trend for successive cohorts born since 1980.

There is also evidence of strong recuperation at older ages for women born between 1960 and 1970. These cohorts delayed their fertility at younger ages but have been experiencing relatively high rates at older ages compared with earlier cohorts. For example, Table 3.2 shows that women born in 1965 had, on average, 0.22 children between the ages 35 to 39, compared with 0.16 children for the 1955 cohort for this age group. Thus, the completed family sizes of more recent cohorts will not be as low as they would have been had their fertility at older ages stayed at levels experienced by earlier cohorts.

Figure 3.3 also shows this recuperation in terms of differences in selected cohorts relative to the 1965 cohort, who completed their fertility with an average of 1.91 children per woman. Although the 1970, 1975 and 1980 cohorts fell increasingly behind the 1965 cohort during their 20s, the curves for these cohorts after around age 28 rose steeply towards the 1965 level due to higher fertility at older ages, with the 1970s cohorts set to catch up with the completed family size of the 1965 cohort.

Women born in 1980 have followed a very similar fertility trajectory to the 1975 cohort up to age 25, but are showing higher fertility from age 27 onwards. This represents a marked difference from the previous pattern where successive cohorts born between the 1940s and the 1960s achieved lower fertility by each age than their predecessors, and suggests that falls in cohort fertility are bottoming out. However, women born in the late 1980s onwards have experienced slightly lower teenage fertility than those born in the 1970s and early 1980s, so they will have further to catch up at older ages if they are to match the achieved family sizes of their predecessors.

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4. Fertility assumptions for the constituent countries

Figure 3.4 and Figure 3.5 show the estimated and assumed trends in the total fertility rate (TFR) and completed family size for the constituent countries of the UK. All 4 countries show a similar trajectory over time, though Scotland’s TFR declined from 2008 onwards compared with the roughly stable TFRs that England, Wales and Northern Ireland had between 2008 and 2012. In 2013, the TFR fell in all 4 countries of the UK (Figure 3.4). In 2014, the TFRs for England and Wales were 1.83 and 1.78 children per woman, respectively. Northern Ireland has historically had higher fertility than the rest of the UK and in 2014 its TFR was 1.97. Scotland has had lower fertility than England since the early 1980s and in 2014 its TFR was 1.62.

Recent trends do not provide any strong evidence of convergence in the overall levels of fertility between the individual countries, so current differentials are reflected in the completed family sizes assumed for the long-term (Figure 3.5).

The achieved family sizes to date for the individual countries of the UK for selected cohorts are shown in Table 3.3. For the 1964 and 1969 cohorts – who can now be effectively regarded as having completed their childbearing – average family sizes were lowest in Scotland and highest in Northern Ireland. In the 1964 cohort, Wales had a larger completed family size than England and this pattern continues to the 1969 cohort; the most recent cohort to complete childbearing. While the 1964 and 1969 completed family size (CFS) for England are similar, there were declines for UK, Scotland and Northern Ireland, and an increase for Wales between these cohorts. The relative achieved family sizes across the countries of the UK for the 1974 and 1979 cohorts are similar to the 1969 cohort. For younger cohorts, the achieved family size for Northern Ireland is lower than for England and Wales and, for the 1994 cohort, is lower than England, Wales and Scotland. This reflects the older age pattern of child bearing in Northern Ireland.

For the 2014-based projections, the long-term fertility assumptions for England, Wales, Northern Ireland and Scotland have been slightly raised when compared with the 2010-based projections. The assumptions for England, Wales and Northern Ireland are the same as the 2012-based projections whereas the long-term assumption for Scotland has been set slightly lower. The assumed long-term completed family size is 1.90 children per woman for England and for Wales, 2.00 for Northern Ireland and 1.70 in Scotland. Table 3.4 illustrates, for each constituent country of the UK, the assumed progression in completed family size from cohorts who have recently finished childbearing to those who have not yet started. The CFS is assumed to rise slightly for the cohorts between 1975 and 1985, before declining back down to the long-term trend.

Between 2002 and 2008, total fertility rates increased in all constituent countries of the UK, followed by a dip in 2009. All countries except Scotland then showed a recovery in 2010, followed by a fluctuating pattern in TFR in the short-term. This is reflected in the latest projections, as the total fertility rate for the UK also fluctuates in the short-term and levels off at the long-term assumption of 1.89 by 2032.

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5. Fertility assumptions age and sex distribution

Assumed age pattern of fertility

Table 3.5 summarises assumed fertility rates for the UK by 5 year age groups. The age pattern is projected to change slightly over the projection period, with fertility rates for women aged 40 and over increasing and rates for women aged under 20 decreasing slightly. Fertility rates for women in their 20s are also assumed to decrease slightly and this is offset by slight increases for women in their 30s.

The mean age at motherhood for the UK is assumed to rise gradually from 28.4 years for the 1965 cohort to its long-term level of 30.6 years for those born from 2005 onwards. Among the constituent countries of the UK, the mean age at motherhood assumed for the long-term varies from 29.9 years in Wales to 30.3 years in Scotland, 30.6 in England and 30.4 years in Northern Ireland.

Assumed sex ratio at birth

It is assumed that there will be 105 boys for every 100 girls. This is in line with the estimated sex ratios recorded in the UK since the 1987-based projections. The UK sex ratio fluctuates each year with no clear trend over time and these annual fluctuations are relatively larger in Wales, Scotland and Northern Ireland compared with England; therefore it was decided during the 2004-based round not to use different sex ratios for the 4 countries of the UK.

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6. Distribution of completed family size

The assumptions for these projections have been informed by the use of a birth order probability model for England and Wales as a whole, maintained by the Office for National Statistics (ONS)9,10,11. This model also provides details of a distribution of women by number of children that is consistent with the fertility assumptions used for the 2014-based projections.

Table 3.6 shows that the proportion of women who remain childless by age 45, in England and Wales as a whole, has been increasing in recent years, from an estimated 14% of the 1950 cohort to 20% of women born in 1965. The rise in childlessness was the main factor in the reduction in completed family size for cohorts born in the 1950s through to the early 1960s, since the average number of children for women who were not childless remained fairly stable for these cohorts at around 2.4.

In the long-term, for cohorts born from the mid-1990s, it is assumed that 18% of women will remain childless. The fall in completed family size, from 1.98 for the 1960 cohort to the 1.90 assumed for those born from the mid-2000s onwards, is consistent with a decrease in the average complete family size of women who have children from 2.45 to 2.33. The family size distribution consistent with the 2014-based projections is similar to the distribution produced alongside the 2012-based projections.

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7. Future fertility levels

For the 2006-based projections, the fertility assumptions were raised for the first time since the 1960s, with the long-term level of completed family size for the UK increasing from 1.74 to 1.84 children per woman. For the 2008-based projections, the long-term assumptions remained unchanged following a review of the available evidence, except in Scotland where the assumption was raised slightly. In 2010, the assumptions were maintained at the 2008-based levels. In 2012, the long-term fertility assumptions in the individual countries were increased from the 2006, 2008 and 2010 based rounds.

Our review prior to the 2014-based projections proposed maintaining the assumptions in line with the following arguments:

  • the expert panel cautioned against frequent changes of long-term assumption, stating that stability is desired by users
  • four (of nine) of the expert panel predicted a long-term total fertility rate (TFR) of between 1.90 and 1.93.
  • both Eurostat and the UN assume higher TFRs for the UK than the Office for National Statistics (ONS); decreasing the assumption will increase this gap
  • the current achieved family sizes for the 1970 to 1985 cohorts suggests that they are catching up with the achieved family sizes of the 1968 cohorts and may exceed the completed fertility of the 1968 cohort by the time they have finished childbearing, if the projected rises in older age fertility rates occur; the completed family size (CFS) for the 1968 cohort was 1.91, near to the 2012-based long-term projected TFR, so this supports maintaining the assumption
  • net migration levels remain high despite political will to decrease them, and women born outside the UK tend to have higher fertility levels than UK-born women
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  1. Smallwood S (2002). The effect of changes in the timing of childbearing on measuring fertility in England and Wales. Population Trends 109. Available at: http://webarchive.nationalarchives.gov.uk/20150904113534/http://ons.gov.uk/ons/rel/population-trends-rd/population-trends/no--109--autumn-2002/index.html

  2. Jefferies, J (2008). Fertility assumptions for the 2006-based national population projections. Population Trends 131, Available at: http://webarchive.nationalarchives.gov.uk/20150904113534/http://ons.gov.uk/ons/rel/population-trends-rd/population-trends/no--131--spring-2008/index.html

  3. Tromans, N, Jefferies, J and Natamba, E (2009). Have women born outside the UK driven the rise in UK births since 2001? Population Trends 136. Available at: http://webarchive.nationalarchives.gov.uk/20150904113534/http://ons.gov.uk/ons/rel/population-trends-rd/population-trends/no--136--summer-2009/index.html

  4. Hoorens S., Clift J., Staetsky L., Janta B., Diepeveen S, Morgan Jones M. and Gran J (2011) Low fertility in Europe: Is there still reason to worry? RAND Corporation. See www.rand.org/pubs/monographs/MG1080.html and a summary at www.rand.org/content/dam/rand/pubs/monographs/2011/RAND_MG1080.sum.pdf

  5. ONS (2012) Childbearing of UK and non-UK born women living in the UK. Available at: http://webarchive.nationalarchives.gov.uk/20160105160709/http://www.ons.gov.uk/ons/rel/fertility-analysis/childbearing-of-uk-and-non-uk-born-women-living-in-the-uk/2011/index.html

  6. ONS (2014) Childbearing of UK and non-UK born women living in the UK, 2011 Census data. Available at: http://webarchive.nationalarchives.gov.uk/20160105160709/http://www.ons.gov.uk/ons/rel/fertility-analysis/childbearing-of-uk-and-non-uk-born-women-living-in-the-uk/2011-census-data/index.html

  7. Robards, J, Berrington, A (2015) The fertility of recent migrants to England and Wales: interrelationships between migration and birth timing. ESRC Centre for Population Change Working Paper 65. Available at: http://www.cpc.ac.uk/publications/cpc_working_papers/pdf/2015_WP65_The_fertility_of_recent_migrants_to_England_and_Wales.pdf

  8. Smallwood S and Chamberlain J (2005). Replacement level fertility, what has it been and what does it mean? Population Trends 119. Available at: http://webarchive.nationalarchives.gov.uk/20150904113534/http://ons.gov.uk/ons/rel/population-trends-rd/population-trends/no--119--spring-2005/index.html. The estimate of replacement level fertility (2.075) in this article has not been updated since but is not expected to have materially changed.

  9. Smallwood S (2002). New estimates of trends in births by birth order in England and Wales. Population Trends 108. Available at: http://webarchive.nationalarchives.gov.uk/20150904113534/http://ons.gov.uk/ons/rel/population-trends-rd/population-trends/no--108--summer-2002/index.html

  10. For application in population projections, see also Smallwood S (2003). Fertility assumptions for the 2002-based national population projections. Population Trends 114. Available at: http://webarchive.nationalarchives.gov.uk/20150904113534/http://ons.gov.uk/ons/rel/population-trends-rd/population-trends/no--114--winter-2003/index.html

  11. Since May 2012, information on previous children has been collected from all women at birth registration, so from 2013 onwards, birth order is no longer estimated from the General Lifestyle Survey for births outside marriage.

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.Background notes

  1. The 2014-based Population Projections for the UK and constituent countries were published 29 October 2015 (main release) and the extra variants were published 26 November 2015.

  2. 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