births in England and Wales in 2000, no information on previous births is collected. .... Changes in society since have made ... This will allow the law governing ...
Population Trends 108
Summer 2002
New estimates of trends in births by birth order in England and Wales
Steve Smallwood Population and Demography Division Office for National Statistics
INTRODUCTION
A greater understanding of past, present and future trends in fertility can be gained from analysing trends in birth order; that is whether a birth is a first, second, third or higher order birth. However, under current legislation, birth order information is not collected at registration for births outside marriage and birth order recorded within marriage is not the true birth order. This article presents revised and updated estimates of true birth order. It discusses the construction of the new estimates and presents analysis relating the births by true birth order to the population of women by parity on both a period (fertility in a particular year) and cohort (fertility of women born in a particular year) basis. The new true birth order figures are also compared to the previous set of estimates.
National Statistics
Information on live-births is recorded by the civil registration system. The Population Statistics Acts of 1938 and 1960 specify that when the birth is being registered within marriage the number of previous children born to the woman ‘by her present and any former husband’ should be recorded. This should therefore include previous births that took place outside marriage where the father was the woman’s present or any former husband, but exclude previous births outside marriage where the woman had never been married to the father. For births occurring outside marriage, which comprised nearly 40 per cent of births in England and Wales in 2000, no information on previous births is collected. Figure 1 illustrates how the number of births outside marriage has grown since 1941. Box two gives further information on the question asked at registration and the difficulties in interpreting the resulting information from the question. Two previous articles have presented estimates of true birth order (see Glossary in Box one for definition of term). In 1986, Werner used information from the 1979 to 1982 General Household Survey (GHS) to estimate true birth order and the distribution of women by parity for births occurring in the period 1938 to 1985.1 Then in 1992 Cooper and Jones updated the estimates to 1990 using data from the 1986 to 1989 GHS.2 Published estimates since then have used the 1986–89 GHS data to adjust registration birth order. The results presented in this article are based on GHS data for the years 1986–96, 1998 and 2000. The method used to produce true birth order estimates, which is broadly similar to that used by Werner, Cooper and Jones, is described in the following section. In an appendix to the article other sources of data, including the ONS Longitudinal Study (LS), which has also been used to look at birth order,3 are briefly discussed.
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Population Trends 108
Figure 1
Number of births inside and outside marriage, 1941–2000
Summer 2002
THE USE OF GENERAL H OUSEHOLD S URVEY DATA TO CONVERT BIRTHS REGISTRATION DATA TO TRUE BIRTH ORDER
England and Wales 900 Births in marriage 800
Outside marriage
Births (thousands)
700 600 500 400 300 200 100 0 1941
1951
1961
1971 Year
1981
1991
Box one GLOSSARY OF TERMS True bir th order (TBO) – The number of a live-birth (first, second, third, etc) counting all the mother’s previous live-bir ths. Registration bir th order – The birth order of a child, based on the answer to the question asked at registration (see Box two). Marital bir th order – The bir th order of a child counting only previous live-born children born within marriage . Parity – The number of children a woman has had. A woman with no children would have a parity of zero, a woman who has had one child would have a parity of one and so on. Parity progression ratio – The probability of a woman moving from parity x to parity x+1. Total fertility rate (TFR) – The sum of the age specific fertility rates in a single year, it can be thought of as the average number of children a woman would have if the age patterns of fertility in that year persisted throughout her childbearing life . It provides a single figure measure of the level of fertility in a year, controlled for the age distribution of women of fertile age. Total period parity fertility rate (TPPFR) – Similar in concept to the total fertility rate, a one number summary of the level of fertility produced by combining period parity progression ratios (see Box three for an explanation of how the TPPFR is calculated). It provides a single figure measure of the level of fertility in a year, controlled for the age and parity distribution of women.
The family information section of the GHS provides information on the birth and marital histories of women respondents in Great Britain. This information can be used to produce a dataset of births including the marital status of the mother at the time of birth. Previous true birth order estimates1,2 used only the latest three-years GHS data available at the time of making the estimate. In this exercise data from the 1986 to 1996, 1998 and 2000 surveys, thirteen in all, have been used. Combining data from a number of surveys increases the sample size and therefore reduces the random sampling error. This has also allowed the data used to be restricted to women in England and Wales at the time of the GHS interview. Thus the data used should more closely match with the registration data, which are also for England and Wales, although there is no guarantee that the women interviewed were resident in England and Wales throughout their childbearing history. The 1986 to 1996 GHS datasets contained a derived variable giving marital status at the time of each birth. For the 1998 and 2000 GHS datasets, the derived variable had to be created from the raw birth and marital histories. The data can be arranged to calculate the relationship between the order of births in marriage5 and true birth order, and the true birth order of births outside marriage. Increasingly couples may have a child together prior to marrying. As can be seen from the example birth history given in Box two, marital birth order may therefore not be the same as the birth order recorded at registration. Using information given on cohabitation prior to marriage in the GHS6 the factors applied to births within marriage have been calculated so that they more closely resemble the recording of birth order at registration. When counting the ‘registration’ order for births in marriage the births occurring in a period of cohabitation prior to marriage, which are very likely to have been to a current or former husband,7 have been included in the count. Making this change has a significant effect on the true birth order estimates; for example, it increases the estimate of the overall number of first births in 2000 by five per cent and reduces the number of second and higher order births by three and a half per cent. As in the estimates by Cooper and Jones2, factors to adjust registration birth data to true birth order were calculated by single year of age from age 15 up to age 34. From age 35, five-year age groups were used. Individual age factors were smoothed using 5-point moving averages over cohorts (carried out by summing the numerator and denominator separately) and then 3-point moving averages by age. For the 5-year age band data individual cohort rates were calculated then smoothed using 5-point moving averages. Using more recent survey data has meant that adjustments made to births up to 1998 for cohorts born up to 1978 were based on new survey results. The factors are then held constant.
REGISTRATION BIRTH ORDER AND MULTIPLE BIRTHS Multiple births affect the registration birth order recorded. At a multiple birth the same order is recorded for each live-born child (see Box two). Thus, for example, where the first marital maternity is twins, as is currently the case in just under half of all multiple maternities within marriage, the number of first births recorded at registration will be
33
National Statistics
Population Trends 108
Summer 2002
overestimated, crucially affecting the statistic of the number of childless women resulting from the true birth order estimates. There will also be corresponding underestimation for second births and above. Adjusted registration birth numbers by order have therefore been produced for births back to 1963, the earliest year for which data are available electronically. Adjustments for the years 1938 to 1962 have been constructed.8 Figure 2 shows the resulting percentage adjustments to each birth order.
Box two THE CURRENT INFORMATION COLLECTED AND FUTURE PROSPECTS
Schedule 1 of both the Population (Statistics) Acts of 1938 and 1960 give the following particulars that may be required at the registration of a bir th: On registration of a birth (including a still-birth) (a) in all cases the age of the mother; (b) where the name of any person is to be entered in the register of births as father of the child, the age of that person; (c) except where the bir th is of an illegitimate child – (i) the date of the parents’ marriage; (ii) whether the mother had been married before her marriage to the father of the child; (iii) the number of children of the mother by her present husband and by any former husband, and how many of them were born alive or were still-born. Bir th order information for births within marriage is collected under Section 1(c)(iii). At the time the question was framed in 1938, births outside marriage were some 4 per cent of total births. Thus most births were within marriage and the question on order would have counted most previous births. Changes in society since have made the information collected in the question considerably less comprehensive and relevant. Not only does the question not collect parity information when births outside marriage are registered, but the interpretation of what is to be recorded for births within marriage has become more complex. The following example helps illustrate the complexity. The table below describes a hypothetical birth history and gives for each bir th the true birth order and registration birth order (see the glossary in Box one for explanation of terms). Bir th history
True birth order 1
Registration birth order Not recorded
Marital birth order Not applicable
Second birth while married to man B
2
1
1
Third birth while cohabiting with man C
3
Not recorded
Not applicable
Fourth birth after marriage to man C
4
3
2
First birth cohabiting with man A
National Statistics
34
Appendix A gives more information on the GHS sample and discusses other data sources for birth order data. Appendix B presents Tables 10.3 and 10.5 from the Birth Statistics Annual Reference Volume comparing the resulting new estimates of true birth order with the previously published figures.9
For completeness the marital birth order is also shown. The basis of the GHS adjustment factors for birth order recorded at registration in previous estimates of true birth order 1,2 was the relationship between marital birth order and true bir th order derived from the GHS birth histories. Guidance for registrars specifically forbids them raising questions of parentage of previous children but does indicate that ‘previous children’ includes children born to the mother by any husband before marriage. The registration process only identifies a birth as being within marriage if the woman is married to the father of the child (except where a woman is widowed between conception and birth). Thus a birth to a woman who is separated, but not divorced, where the child’s father is not the estranged husband will not be counted as a bir th within marriage and no birth order information will be recorded. The birth order information collected is affected by the method of recording multiple births. The guidance to registrars states that ‘previous children’ excludes any other children born in the same confinement. Thus, for example, a married mother whose first live maternity leads to live-born twins would have both births registered as order 1 births (previous children 0). However, the result of that mother later registering a second maternity within marriage would be an order 3 birth (previous children 2). The Civil Registration System is currently under review, A White Paper Civil Registration: Vital Change was published on 22 January 2002 4 and a consultation document will be published later this year. In future , the primary legislation will provide the power to collect statistical information, with the details set out in subordinate legislation. This will allow the law governing what is collected to be updated to meet changing needs whilst retaining the protection offered by a statutory framework. It is likely that this flexibility will be used to collect birth order for all births. However, true birth order data from this source would only be available for the year 2005 onwards and such data would be unlikely to be available until the end of 2006 at the earliest.
Population Trends 108
Figure 2
Percentage adjustment to registration birth order because of multiple births, 1938–2000
Table 1
England and Wales
Percentage change to birth registration order
3
Summer 2002
Percentage distribution by true birth order of births inside and outside marriage; percentage distribution by true and (multiple birth adjusted) registration birth order of births inside marriage; distribution by registration birth order as recorded and adjusted for multiple births. 1980, 1990 and 2000
England and Wales
Percentages
2
Birth order 1st
2nd
3rd
4th and higher
Total Births (Thousands)
1 1980 All births Births outside marriage Births inside marriage Total 1st 2nd 3rd 4th and higher
0
-1
True birth order 41.6 34.6 59.0 20.8 39.2 95.0
36.5 4.3 96.0
15.4 11.1
8.5 9.1
656.2 77.4
15.9 0.6 3.6 95.4
8.4 0.1 0.4 4.6 100.0
578.9 238.9 209.2 87.4 43.4
14.9 15.1
7.3 7.5
578.9 578.9
15.4 12.0
8.7 7.0
706.1 200.0
16.8 0.9 3.5 95.4
9.4 0.1 0.6 4.6 100.0
506.1 197.8 185.7 80.2 42.4
15.6 15.9
8.2 8.4
506.1 506.1
15.1 13.2
8.9 7.8
604.4 238.6
16.3 0.7 4.2 94.3
9.6 0.0 1.0 5.7 100.0
365.8 143.6 135.6 56.1 30.5
8.1 8.3
365.8 365.8
Registration birth order -2 1938
1948
1958
1968 Year
Order 1
Order 3
Order 2
Order 4
1978
1988
1998
Order 5
Dotted lines indicates reconstructed data
DIFFERENCES BETWEEN BIRTH ORDER RECORDED AT REGISTRATION AND TRUE BIRTH ORDER
Table 1 shows the resulting estimated distributions of true birth order built up from the registration data. The distribution of births inside marriage compared to outside marriage is very different. Many more of the births outside marriage are first rather than second births, although the proportion of births outside marriage that are first births has fallen from three in five to under a half, between 1980 and 2000. It is estimated, that for births outside marriage, second or higher order births started to exceed first births in 1994, with 107.6 thousand first births and 107.9 thousand second or higher order births taking place outside marriage. The shaded area in Table 1 shows the adjustments made to the registration birth order data for marital births to produce their true birth order. Of births recorded as first births inside marriage in 2000, 7.1 per cent have been adjusted to a higher order. This compares with 5 per cent in 1980 and 6.5 per cent in 1990. Adjustments are smaller for birth order 2, but then increase slightly for birth order 3. Taking account of births not to a current or former husband means that in 2000, 40 percent of births within marriage were recorded at registration as first births (although once adjusted for multiple births this is reduced to just over 39 per cent), but only 36.5 per cent births in marriage were actually first live-births. It is clear that that overall distribution by true birth order is a weighted average between two quite different distributions for births inside and outside marriage, the weighting having changed considerably from a ratio of approximately 9:1 in 1980 through to a ratio of 3:2 in 2000.
All births inside marriage recorded at registration Adjusted for multiple births
41.6 41.3
36.1 36.1
1990 All births Births outside marriage Births inside marriage Total 1st 2nd 3rd 4th and higher
True birth order 41.9 33.9 55.6 25.4 36.5 93.5
37.3 5.4 95.9
Registration birth order All births inside marriage recorded at registration Adjusted for multiple births
39.6 39.1
36.6 36.7
2000 All births Births outside marriage Births inside marriage Total 1st 2nd 3rd 4th and higher
True birth order 41.3 34.8 48.7 30.3 36.5 92.9
37.6 6.4 94.8
Registration birth order All births inside marriage recorded at registration Adjusted for multiple births
40.0 39.2
36.8 37.1
15.0 15.3
BIRTH ORDER OUTSIDE MARRIAGE Cooper and Jones2 noted that between 1980 and 1990 second and higher births outside marriage had increased at a faster rate than first births. Figure 3 provides further insight into the trends in true birth order for births outside marriage. It shows for first births and second or higher order births the percentage that took place outside marriage. In 1980 only around 17 per cent of first births took place outside marriage. By 2000 this figure had risen to nearly 47 per cent.
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National Statistics
Population Trends 108
Figure 3
Summer 2002
The percentage of births outside marriage by true birth order, 1980, 1990 and 2000
England and Wales 100 (38)
90
1st birth
(7)
2nd or higher birth
(46)
80 Numbers in brackets give total number of births, both inside and outside marriage, by parity (thousands) 70
(55)
Percentage
(9) (52)
60 50
(50)
(250)
(94)
40
(296)
(86)
(355)
(72) (99) (24)
30
(76) (60)
(10)
20
(101) (152)
(106)
(273) (113)
(50)
(6)
(29)
(82) (141)
(410)
(121)
(43)
(96)
10
(12)
(384)
(35)
(101)
0 1980
1990 2000
1980
15–19
1990 2000
1980 1990
2000
Over the period 1990 to 2000, for all age groups, it is notable that the rise in the percentage of second births outside marriage has increased more rapidly than the rise in the percentage of first births outside marriage. For women aged 25–34 in 2000, the proportion of second or higher order births born outside marriage now exceeds the proportion of first order births born outside marriage. For 15–19 year olds the proportion of first births outside marriage had increased to over 90 per cent in 2000. Of the 7 thousand second or higher order births in 2000 in the 15–19 age group, the proportion outside marriage has risen to a level similar to first births.
Figure 4
1980 1990
Births by true birth order, 1950 to 2000
Figure 5
Births (thousands)
1980 1990
2000
All ages
Trends in components of the Total Period Parity Fertility Rate by true birth order, 1965–2000
England and Wales
1977 lowest year for total births
1977 lowest year for total births
1.2
300
1st births
250
2nd births
200
150 3rd births 100
50
2000
35+
Figure 4 shows the number of births by true birth order. The late 1980s saw a rise in the number of first births to a level approaching that of the 1960s baby boom. Smaller rises occurred for second and higher order births. The 1990s saw a fall in births at each order; between 1990 and 2000 the numbers of first and third births each fell by 16 per cent and the number of second order births fell by 11 per cent. Of course, the trend in the number of births is in part a reflection of changes in the number, age distribution and previous childbearing history of women of childbearing age. Figure 5 shows the trends in the components of the total period parity fertility rate. The construction of the data used is
TPPFR components by parity (per woman)
1964 peak year for total births
1980 1990
TRENDS IN FERTILITY BY TRUE BIRTH ORDER
England and Wales
350
2000
30–34 25–29 Age of mother at birth
20–24
4th births
1 1st births 0.8
2nd births
0.6 3rd births 4th births 0.4
0.2
0
0 1950
1960
1970
1980
2000
1950
1960
1970
1980 Year
Year
National Statistics
1990
36
1990
2000
Population Trends 108
Box three THE METHODOLOGY USED FOR CONSTRUCTING THE POPULATION AT RISK AND CONSTRUCTING PARITY PROGRESSION RATIOS
Construction of the female population by parity Female population by parity can be constructed by combining population estimates with numbers of live births by birth order. When a woman gives birth to her nth child she moves out of the population group at risk for that order and into that at risk for n+1. Thus for the population by age x in year y the number of births of order n at age x in year y can be subtracted from those of parity n-1 to leave the number age x+1 year olds with parity n-1, the remainder now being parity n. The resulting proportions of women at each parity can then be assumed to apply to the total number of women aged x+1 in year y+1. This process is repeated for each age from 15 to 45 until the cohort is completed.
Summer 2002
imply women would have more than one child of a particular order. 10 The interpretation of the measure is similar to the TFR in that the statistics produced describe the fertility of a group of women should they conform to the fertility behaviour (by birth order) of the period. 11 Firstly, given a set of age specific rates for moving from parity 0 to parity 1 a single probability of moving from parity 0 to parity 1 can be constructed by applying the probabilities of a first birth at each age to a notional number of people aged 13 (a radix of 1 has been used), as described below: To calculate the progression ratio from parity 0 to parity 1 construct a table analogous to a life table by decreasing a notional population at each age: Let PZ yx be the population of parity z at age x in year y Let BZ yx be bir ths to those of parity z at age x in year y Let LZ x be the synthetic population for parity z at age x.
Age 13 Age 14
L0 1 y L0 13 - (L13 x B0
