Annex A Fertility Assumptions

Fertility assumptions are agreed in two stages. The long term assumption is decided as part of the consultation process between the UK countries and the Government Actuary’s Department. Then there is a detailed assumption setting to produce the age-specific fertility rates for each year of the projection period that are consistent with this long-term assumption.

Fertility assumptions of long term average completed family size for the 2004-based projections are the same as those used in the previous 2002-based and 2003-based projections. The assumptions for Scotland and other constituent countries of the UK are given in Table A1.

Table A1 Assumptions of long-term average completed family size 2002, 2003 and 2004-based projections

Recent data have shown rises in fertility, that at ages under 30 are not in line with previous downwards trends, and at older ages are stronger than recent trends. It is too early to know whether these changes are a temporary phenomenon or indicative of a sustained higher level. In holding the assumptions for this projections round at the previously assumed level it is recognised that if these changes in trends continue for the next two years then there is likely to be a case for raising the fertility assumption in the next projection round. However, in view of the changes that we have seen in recent years, although the long-term assumption has been held constant the path to the assumption has been substantially changed.

Fertility assumptions are formulated in terms of the average number of children that women born in particular years will have. This cohort measure of fertility is more stable than the analogous calendar year or period measure (the total fertility rate). This is because it is affected only by change in the total number of children women have and not by the timing of births within their lives. Period rates 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 data.  For the UK as a whole, completed family size has been falling steadily from 2.45 children per women born in the mid 1930s. It is assumed that this will continue to decline until the 1985 cohort and then level off at 1.74.

The trends in age specific fertility for Scotland are shown in Figure A1. Up until 2002, there is a general pattern of falling fertility at younger ages coupled with rises in fertility at older ages. From 2002, with the exception of the under 20 group there have been increases in fertility.

Around 1980 the fertility levels in Scotland were similar to the UK, but they have shown a more rapid decline in age specific fertility at ages between 20 and 29. The fertility rates in the under 30s are currently 90 per cent of those in the UK as a whole.

At times when women are delaying the age at which they have children the total fertility rate will underestimate the real fertility levels. The current evidence of increasing fertility at later ages, and past declines in fertility among women in their twenties, may be indicative of postponement. As such, the increases in fertility of more recent years may reflect a catching up of postponed births and the recent trend may not continue. Figure A2 illustrates the impact of increasing fertility at ages over 30, on the completed family size assuming age specific fertility rates a) remain constant at 2004 levels or b) increase in line with trends in the most recent 10 years (shown as dotted lines). These are compared with actual data and the assumptions for the previous projections made in 2003 (bold line). Note that cohorts up to 1959 are complete; family size for women born after that year contain an increasing element of projection.

This demonstrates that for women in the second half of their childbearing careers the previous assumptions appear to have been at risk of underestimating those women’s completed family size. Although not as steep, the trend is still downwards and there is  insufficient evidence for raising the long-term assumption.

As part of the ESRC/Scottish Executive demography research programme some research is ongoing to investigate factors in the low fertility in Scotland. See section 6.3 for more details.

More details on the background of how the fertility assumptions are derived can be found in 'Population Trends No 114' on the Office for National Statistics' website.