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Chapter 3: Fertility, 2010-based NPP Reference Volume This product is designated as National Statistics

Released: 29 March 2012 Download PDF

Introduction

For the UK as a whole the key assumption for the future is that average completed family size – which has been falling from a peak of nearly 2.5 children per woman for women born in the mid-1930s – will level off at 1.84 children for women born in 2010 and later. This long-term assumption is unchanged from the previous 2008-based projections.

The assumptions underlying the fertility projections are set for each of the UK’s constituent countries separately, and then combined to obtain the assumption for the UK as a whole. Assumptions are based on analysis of recent demographic trends and an assessment of their implications for future completed family sizes. This is summarised in the ‘Assumption setting’ section. The long-term assumptions are unchanged from the previous 2008-based projections for England, Wales, Northern Ireland and Scotland.

Fertility assumptions for the UK

Fertility assumptions are formulated in terms of completed family size – the average number of children that women born in particular years will have. As Figure 3.1 shows, this cohort measure of fertility is more stable than the total fertility rate (TFR), the calendar year (period) measure. This is because the completed family size is affected only by changes in the total number of children women have and not by the timing of births within women’s lives. The TFR, in contrast, may rise or fall if births are brought forward or delayed for any reason.1   The TFR measures the average number of children that a group of women would each have if they were to experience the age-specific fertility rates of the year in question throughout their childbearing lives. The TFR for the UK fell sharply from the ‘baby boom’ peak of just under three children in 1964 to a trough of 1.69 in 1977. During the 1980s, it stayed relatively stable at around 1.8 children then fell to around 1.7 in the second half of the 1990s. The turn of the century saw further falls with the lowest figure ever recorded, 1.63, in 2001. Since then, fertility rates have risen each year, except 2009, and by 2010 the TFR had reached 1.98.

The completed family size is plotted against the year in which the women were, or will be, aged 30 (the approximate mid-point of the childbearing ages). Average completed family size reached around 2.45 children per woman among those born in the mid-1930s, who would have been in their peak childbearing ages in the early to mid-1960s. Since then the completed family size in the UK has fallen steadily, with women born in 1965 – the most recent cohort for whom there are data up to age 45 – having on average 1.91 children.

Figure 3.1: Actual and assumed total fertility rate (TFR) and average completed family size (CFS), 1951-2035, United Kingdom

Actual and assumed  Total Fertility Rate (TFR) and average completed family size (CFS), 1951-2035, United Kingdom
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.
  2. This figure is presented on a calendar year basis.

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In the 2010-based projections, the long-term completed family size is assumed to be 1.84 children per woman. This is the same level assumed in the 2008-based projections, but is still below ‘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). The 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 11 per cent below replacement level.

Table 3.1 and Figure 3.2 show the achieved family sizes of selected cohorts at successive ages. From 1950, each subsequent cohort has generally had fewer children by each age than earlier cohorts. For example, the 1975 cohort had averaged 0.98 children each by their 30th birthday, 0.11 children fewer on average than the 1970 cohort at the same age. 

Table 3.1: Average achieved family size by age (exact years) and year of birth of woman, women born 1950-1990, United Kingdom

  20 25 30 35 40 45 Final
1950 0.23 0.93 1.56 1.93 2.06 2.09 2.09
1955 0.22 0.78 1.43 1.84 2.00 2.03 2.03
1960 0.16 0.68 1.31 1.75 1.94 1.98 1.98
1965 0.13 0.59 1.18 1.64 1.85 1.91 1.91
1970 0.15 0.57 1.09 1.56 1.84
1975 0.15 0.51 0.98 1.53 :
1980 0.15 0.50 1.02  :
1985 0.13 0.49
1990 0.13  :

Table source: Office for National Statistics

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Figure 3.2: Average achieved family size by age and year of birth of woman, women born 1950-1990, United Kingdom

Average achieved family size by age and year of birth of woman, women born 1950-1990, United Kingdom
Source: Office for National Statistics

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There is evidence of strong recuperation at older ages for women born between 1960 and 1975. 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 1970 had on average 0.28 children between the ages 35 to 39, compared with 0.19 children for the 1960 cohort. Thus the completed family sizes of these 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. 

Table 3.2: Average number of children between given ages by year of birth of woman, women born 1950-1990, United Kingdom

  Up to 20 20–24 25–29 30–34 35–39 40–44   45 and over
1950 0.23 0.70 0.63 0.36 0.13 0.03 0.00  
1955 0.22 0.56 0.65 0.40 0.16 0.03 0.00  
1960 0.16 0.53 0.63 0.44 0.19 0.04 0.00  
1965 0.13 0.46 0.59 0.45 0.22 0.05 0.00  
1970 0.15 0.42 0.52 0.47 0.28  :  :  
1975 0.15 0.36 0.47 0.55  :  :  :  
1980 0.15 0.35 0.52  :  :  :  :  
1985 0.13 0.36  : : : : :  
1990 0.13  : :  :  :  :  :  

Table source: Office for National Statistics

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Figure 3.3 shows this recuperation more clearly. The fertility of selected cohorts is shown relative to the 1965 cohort, who completed their fertility with an average of 1.91 children per woman. Although the 1970 and 1975 cohorts fell behind the 1965 cohort during their twenties, the curves for these cohorts after age 30 rise towards the 1965 level.

Women born in 1980 have followed a very similar fertility trajectory to the 1975 cohort up to age 25, but are now showing higher fertility by age 30. The 1980 cohort had 0.52 children on average between ages 25 to 29 compared with 0.47 for the 1975 cohort. This represents a marked difference from the previous pattern where successive cohorts achieved lower fertility by each age than their predecessors, and suggests that falls in cohort fertility could be stopping. Women born in the late 1980s have experienced slightly lower teenage fertility than those born in the 1970s and early 1980s and so they will have further to catch up at older ages if they are to match the achieved family sizes of their predecessors.

Figure 3.3: Difference between average achieved famly size by age and year of birth of woman, 1965 cohort compared with women born 1950-1990

Difference between average achieved famly size by age and year of birth of woman, 1965 cohort compared with women born 1950-1990

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Fertility assumptions for the constituent countries

Figure 3.4 and Figure 3.5 show the actual and assumed trends in the completed family size and TFR for the constituent countries of the UK. All four countries have seen an upturn in the TFR since 2002, although the trends have varied since 2009. In 2010 the TFRs for England and Wales were 2.00 and 1.98 children respectively. Northern Ireland has historically had higher fertility than the rest of the UK and in 2010 its TFR was 2.06. Scotland has had lower fertility than England since the early 1980s and in 2010 its TFR was 1.75.

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

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

Notes:

  1. This figure is presented on a calendar year basis

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Figure 3.5: Actual and assumed total fertility rates, 1971-2035

Actual and assumed total fertility rates, 1971-2035

Notes:

  1. This figure is presented on a calendar year basis

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The achieved family sizes to date for the individual countries of the UK for selected cohorts are shown in Table 3.3. For the 1960 and 1965 cohorts – who can now be regarded as having completed their childbearing – average family sizes were lowest in Scotland, slightly higher in Wales than in England and highest in Northern Ireland. These patterns persist among younger cohorts, where achieved family size to date is higher in Wales, and lower in Scotland, compared with England. But, because fertility rates at ages 30 and above are currently higher in England than in either Wales or Scotland, the differential between England and Wales for younger cohorts can be expected to narrow, whereas the differential between England and Scotland is likely to widen. In Northern Ireland, achieved family size is relatively high for all but the youngest cohorts, reflecting the fact that fertility rates for women in their thirties are consistently higher in Northern Ireland than elsewhere in the UK.

For the 2010-based projections, the long-term fertility assumptions for England, Wales, Northern Ireland and Scotland have remained the same as those used in the 2008-based projections; for England and for Wales the assumed long-term completed family size is 1.85 children per woman, 1.95 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.

Table 3.3: Achieved family size attained by 2010, constituent countries of the UK, women born 1950-1990

  Achieved  to age United Kingdom   England   Wales   Scotland Northern Ireland
1950 Complete 2.09 2.06 2.10 2.08 2.87
1955 Complete 2.03 2.02 2.05 1.95 2.65
1960 Complete 1.98 1.98 1.99 1.87 2.42
1965 Complete 1.91 1.91 1.96 1.80 2.22
1970 Age 40 1.86 1.87 1.91 1.71 2.08
1975 Age 35 1.62 1.62 1.70 1.47 1.72
1980 Age 30 1.15 1.15 1.25 1.01 1.24
1985 Age 25 0.58 0.59 0.66 0.52 0.53
1990 Age 20 0.19 0.19 0.23 0.19 0.17

Table source: Office for National Statistics

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Table 3.4: Actual and assumed average completed family size for the constituent countries of the UK, women born 1950-2010

  United  Kingdom   England   Wales   Scotland Northern Ireland
1950 2.09 2.06 2.10 2.08 2.87
1955 2.03 2.02 2.05 1.95 2.65
1960 1.98 1.98 1.99 1.87 2.42
1965 1.91 1.91 1.96 1.80 2.22
1970 1.91 1.91 1.95 1.74 2.12
1975 1.91 1.92 1.95 1.74 2.03
1980 2.00 2.01 2.00 1.78 2.17
1985 1.99 2.01 2.00 1.78 2.04
1990 1.96 1.98 1.99 1.75 1.99
1995 1.90 1.92 1.92 1.72 1.96
2000 1.87 1.88 1.88 1.71 1.95
2005 1.85 1.86 1.86 1.70 1.95
2010 and later 1.84 1.85 1.85 1.70 1.95

Table source: Office for National Statistics

Table notes:

  1. Figures in bold are partly or wholly projected

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Fertility assumptions: age and sex distribution

Assumed age pattern of fertility

Table 3.5 summarises assumed fertility rates for the UK by five-year age group. Fertility rates for women aged 30 and over are projected to remain at fairly high levels, while fertility rates for younger women are assumed to fall slightly from current levels in the long-term. As a result, 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.

The mean age at motherhood for the UK is assumed to rise gradually from 28.4 years for the 1965 cohort – the most recent to have completed their childbearing – to its long-term level of 29.9 years for those born from 2010 onwards. Among the constituent countries of the UK, the mean age at motherhood assumed for the long-term varies from 29.3 years in Wales, to 29.9 years in both England and Scotland, and 30.1 years in Northern Ireland.

Table 3.5: Actual and assumed births per 1,000 women by age and year of birth of woman, women born 1950-2010, United Kingdom

Rates per 1,000 women
  Under 20 20-24 25-29 30-34 35-39 40 and over Average completed family size (number of children)   Mean age at motherhood (years)
1950 231 699 634 365 132 28   2.09   26.4
1955 221 561 650 403 163 36   2.03   27.1
1960 156 527 630 438 190 43   1.98   27.8
1965 133 457 594 454 216 57   1.91   28.4
1970 152 418 522 466 278 70   1.91   28.9
1975 147 361 471 546 315 72   1.91   29.4
1980 154 349 519 598 309 68   2.00   29.4
1985 135 357 538 591 300 67   1.99   29.4
1990 127 365 524 576 298 68   1.96   29.4
1995 108 347 507 571 300 68   1.90   29.6
2000 101 333 494 571 301 68   1.87   29.8
2005 97 322 490 571 301 68   1.85   29.8
2010 and later 93 319 490 571 301 68   1.84   29.9

Table source: Office for National Statistics

Table notes:

  1. Figures in bold are partly or wholly projected

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Assumed sex ratio at birth

It is assumed that there will be 105 boys born for every 100 girls. This is in line with the actual sex ratios recorded in the UK over the decade 2001 to 2010, which averaged 105.2. The average levels in each constituent country of the UK are similar, although there is substantial year-on-year fluctuation, particularly in Scotland, Wales and Northern Ireland. Varying the sex ratio to reflect small changes over time or any differences between countries would have a very small effect on the resultant UK population projections. Thus the ratio of 105.0 assumed in the 2008-based projections has been maintained in all individual countries of the UK.

Distribution of completed family size

Distribution of completed family size

The assumptions for these projections have been informed by the use of a birth order probability model for England & Wales maintained by the Office for National Statistics (ONS).3,4 This model also provides details of a distribution of women by number of children that is consistent with the fertility assumptions used for the 2010-based projections.

Table 3.6 shows that the proportion of women who remain childless by age 45 in England & Wales has been increasing in recent years, from an estimated 14 per cent of the 1950 cohort to 20 per cent 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.

For the 1980 cohort, childlessness is projected to fall back to 14 per cent, reflecting recent increases in first births among childless women. As this trend is not expected to continue indefinitely, childlessness is projected to increase again in the long-term, reaching 19 per cent among those born in 2005 and later.  The most common family size is projected to remain at two, with around four in ten women having two children exactly. 

Table 3.6: Actual and assumed distribution of women by number of children and year of birth of woman. Consistent with 2010-based projections, women born 1950-2010 & later, England and Wales

Average family size all women Average family size of women who have children   Number of children (percentages)
Cohort born 0 1 2 3 4 or more
1950 2.07 2.39 14 13 44 20 10
1955 2.02 2.41 16 13 41 19 11
1960 1.98 2.45 19 12 38 20 11
1965 1.91 2.39 20 13 38 19 10
1970 1.92 2.31 17 18 37 18 10
1975 1.92 2.32 17 18 38 17 11
1980 2.01 2.35 14 17 39 18 11
1985 2.01 2.35 15 17 40 18 11
1990 1.98 2.34 16 17 39 18 11
1995 1.92 2.31 17 17 39 17 10
2000 1.88 2.30 18 18 38 17 10
2005 1.86 2.29 19 18 38 16 9
2010 & later 1.85 2.28  19  18  38  16  9

Table source: Office for National Statistics

Table notes:

  1. figures in bold are partly or wholly assumed

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Assumption setting

The assumptions about completed family size which underlie this projection round are based on an analysis of recent trends in fertility, together with other relevant information such as the views of the expert advisory panel.

Recent trends in fertility

The UK has seen increases in the TFR in recent years, from 1.63 in 2001 to 1.96 in 2008. In 2009 the TFR fell slightly to 1.94, a dip likely to be related to the economic recession. This was short-lived as the longer-term increase resumed in 2010 resulting in the UK TFR reaching 1.98, its highest level since 1973.  There was some variation between UK countries during 2009 and 2010, with a stabilisation then strong recovery in England, while in Scotland the TFR fell in both years, and Wales and Northern Ireland both saw small recoveries in 2010.

Fertility rates among women in their thirties and forties in the UK have continued to rise at a fast pace since the turn of the century, reaching levels last seen during the 1960s baby boom. This increasing fertility among older women continued in 2009 and 2010.  Since 2002 there have also been smaller increases in fertility among women in their late twenties and a stabilisation among women in their early twenties, following declining fertility in these age groups during the 1990s. However the increases in fertility rates for women in their twenties stalled in 2009 and 2010. The combination of trends in these two age groups has led to the rise in overall fertility over the decade, as well as further small increases in the mean age at childbirth.

Apart from the recuperation in fertility at older ages by women born in the late 1960s and 1970s mentioned above, other factors that could be associated with recent increases in period fertility include the increasing proportion of women of childbearing age born outside the UK (who have above average fertility), and the possible role of changes relating to support for families (such as tax credits or maternity and paternity leave) – see references 6 ,7,8 for further discussion of these factors.

These increases in period fertility have started to have an impact on the family sizes achieved to date by cohorts of women who have not yet completed their fertility. Figure 3.2 shows that, for example, women born in 1975 have almost ‘caught up’ with the achieved family size of the 1970 cohort by their late thirties, as shown by the convergence in the two lines. This reflects the continued increases in the fertility of women in their thirties. In addition, women born in 1980 have achieved a slightly larger family on average by their 30th birthday than women born in 1975. This represents a marked difference from the pattern seen previously, where successive cohorts achieved slightly lower fertility at each age than their predecessors.

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. The review by ONS prior to the 2010-based projections proposed keeping the long-term assumptions stable, given the uncertainty around the likely direction of fertility change in both the short- and long-term. This recommendation was accepted in line with the arguments detailed below.

The NPP advisory panel was asked their views on the likely level of fertility in 2034 and eight out of ten experts thought that the UK TFR would be 1.80 or 1.85 in 2034. This suggests that experts do not believe fertility is likely to maintain its current period level in the long-term.

When considering likely factors affecting future fertility, some could put downward pressure on fertility levels, for example continued increases in female employment and higher education that raise the opportunity costs of childbearing, and changes in socio-economic conditions such as housing cost and availability. Others factors could put upward pressure on fertility in the long-term; these include the continuing in-migration of women from countries with higher fertility than the UK and perhaps the increased ability of women to realise their fertility intentions, for example by more flexible working patterns for parents. The uncertainty inherent in future trends in these factors, particularly in the prevailing economic and social climate, makes it difficult to judge whether those having an upward or downward influence will have the stronger influence on fertility in the long-term.

In order to decide on plausible assumptions for long-term fertility, the completed family sizes resulting from different scenarios for possible trends in fertility at different ages were examined. As agreed in consultation with key users, the final projection for the UK is broadly based on a long-term scenario where fertility rates for women in their twenties are somewhat lower than in 2010, fertility rates for women in their thirties are slightly lower than 2010 levels, but fertility among women aged 40 and over is slightly higher than in 2010 – this long-term pattern is achieved by the late 2020s and stabilises from then on. Broadly similar scenarios were used in each of the four UK countries, with the assumed long-term total family sizes kept in line with recent country differentials in fertility.

For the short-term, fertility projections have been based around the latest trends in age-specific fertility. For example, a small increase in the TFR up to 2013 has been projected for England, which experienced the largest increase in the TFR from 2009 to 2010. In contrast the TFR in Scotland is not projected to increase further in the short-term. These differing short-term trends are incorporated to ensure the projected path from current fertility to the long-term level is plausible.  However short-term changes in period fertility, such as those related to the economic downturn in 2009 are not expected to have a large impact on completed family size in the long-term unless they continue for a longer period.

References

1.  Smallwood S (2002).  The effect of changes in the timing of childbearing on measuring fertility in England and Wales. Population Trends 109: www.ons.gov.uk/ons/rel/population-trends-rd/population-trends/no--109--autumn-2002/index.html

2.  Smallwood S and Chamberlain J (2005). Replacement level fertility, what has it been and what does it mean? Population Trends 119: www.ons.gov.uk/ons/rel/population-trends-rd/population-trends/no--119--spring-2005/index.html

3.  Smallwood S (2002).  New estimates of trends in births by birth order in England and Wales. Population Trends 108: www.ons.gov.uk/ons/rel/population-trends-rd/population-trends/no--108--summer-2002/index.html

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

5.  True birth order estimates are based on the incomplete information on previous children collected at birth registration combined with information from the General Household Survey. Information on family size can be found in Cohort Fertility: www.ons.gov.uk/ons/rel/fertility-analysis/cohort-fertility--england-and-wales/2010/index.html

6.  Jefferies, J (2008). Fertility assumptions for the 2006-based national population projections. Population Trends 131: www.ons.gov.uk/ons/rel/population-trends-rd/population-trends/no--131--spring-2008/fertility-assumptions-for-the-2006-based-national-population-projections.pdf

7.  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: www.ons.gov.uk/ons/rel/population-trends-rd/population-trends/no--136--summer-2009/have-women-born-outside-the-uk-driven-the-rise-in-uk-births-since-2001-.pdf

8.  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

Background notes

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