Ann. Hum. Uenet., Lond. 1963), 27, 1

Printed in Great Britain 1

Data on parental age, sibship size and twins

BY ALICE STEWART AND RENATE BARBER Department of Social Medicine, University of Oxford

In the following account two overlapping groups of sibships are described, one from the point of view of parents’ ages and birth intervals, and the other from the point of view of twin births. Both were selected on the basis of a live or still birth during the year 1952, and the first one included 94 % of all such families in the administrative county of Northamptonshire (3751 sibships). The second sample included 249 sibships, each of which contained one set of twins of known sex. Ninety-one of these belonged to Northamptonshire and 138 to other areas included in the original survey of 1952 births (Stewart, Webb & Hewitt, 1955). Both sets of sib- ships had records of mothers’ ages, fathers’ ages and fathers’ occupations, and the twin sibships included 109 sets of MM twins, 70 sets of NIP twins and 70 sets of FF twins; also 141 twin sets which survived infancy intact, 47 in which one twin was lost, and 61 in which both twins were lost. The data were collected at the time of the 1962 births (index cases) and describe the sibships as they then stood.

NOR!CHAMPTONSHIRE FAMILIES

The Northamptonshire families recorded 3707 single births and 44 twin births during 1952; also 4860 single births and 47 twin births during earlier years. Most of the relevant marriages took place during the second half of the 1940s and, when the data were collected, only a third of the parents shared more than two children. There were no sibships with more than one set of twins and none with triplets, but there were 386 perinatal deaths (stillbirths and neonatal deaths) affecting 332 sibships and 44 twins.

Comparisons with other births in 1952 showed that age differences between the survey parents were more typical of all marriages in England and Wales than first marriages, and that the social class distribution of the families was more typical of the Midland region than of the country as a whole (see Table 1).

OCCUPATIONAL CLASS

When the Northamptonshire families were classified according to the fathers’ occupations in 1952 they showed the usual correlation between sibship size and socio-economic status, with a preponderance of small families in social class I, medium-sized families in social classes I1 and 111, and large families in social classes IV and V (see Table 2).

In all five classes there was a wider range of fathers’ ages than mothers’ ages, and over three-quarters of husbands were older than their wives. Exceptions to this last rule were noticeably uncommon in the larger families, but there was no social class in which the average age gap between the two sets of parents was significantly different from the population norm (3.2 years).

The size of the sibships also influenced birth intervals, but, for sibships of equal size, the social class differences were trivial. Late marriages were relatively common in the higher income

I Hum. Gen. 27, I

2 ALICE STEWART AND RENATE BARBER

Table 1. Comparison with regard to wived and husbands' age between the Northamptonshire families and the populations from which t h y were drawn

England Northampton-

Wales Midlands (index cases) and shire

Wives' ages First births (1952) 25'1 25.0 25'3 All births (1952) 27.8 28-1 28.3 1st mmiages (1950) 24'0 All marriages (1950) 26.4 - 23.0*

- -

- - Husbands' ages I st marriages (I 950) 26.4 All marriages (1950) 29-5 - 26.2*

% % % Occupational class? I, I1 17.8 15'3 145

I11 52'9 55'4 57'1 IV, v 29'3 29'3 28.4

* Parents whose *st child waa born in 1952. t Figures for England and Wales and Midlands taken from the 195 I census.

Table 2. N o ~ t ~ ~ t o ~ h i r ~ familia : o c c u ~ a t ~ o n a ~ c k a distr~but~on of deviations from

Father's age

Mother's ctge

Marriage 1st birth 2nd 3rd 4th 5th 6th Later All births

Marriage 1st birth 2nd 3rd 4th 5th 6th Later All births

Age at first birth Father's Mean

Mother's Mean

(S.D.)

(S.D.)

No. of births per sibship yo of 1st births

No. of sibships

population mrm pare&' ages

Deviations from population norms A r \

(Occupational class) I

+ 1.6 + 2'1 + 2'2 + 1.8 + 1'9 + 2'0 - -

+ 1'2 + 2.3 + 2'2 + 1'5 + 1'0 + 2.4 + 0.5 - -

+ 1'4 29.6 (5.65)

26.5 (495) 1.92 52'1

75

I1

+ 1.5 + 1'9 + 2.4 + 2.6 + 2'2 + 2.6 + 1.6 + 3'4 + 2'0 + 1'2 + 1'7 + "5 + 1'9 + 2.4 + 1.7 + 0 3 + 1'0

+ 1.7 29.4

(5'15)

(4'73) 2.19 45'6 506

26.0

I11 - 0.3 - 0 3 - 0.4 - 0.5 - 0'2 - 0'2 + 0.3 0.7

- 04 00

- 0'1 0'0

+ 0'1 - 0.3 +O.I + 0.6 + 07 - 0 1

27.2 (5'10)

24.2 (4'41 1 2.46 44'5

2203

IV - 0.3 - 06 - 0.7 - 0.8 - 09 - 0.9 - 04 - 1'5 - 04 - 0.4 - 0.9 - 1.0 - 1'2 - 0 3 - 0.9 - 0.5 - 0.4 -07

26.9 (5.01)

23'4 (493) 2'52 39'7

585

V + 0 2 - 04 - 0.5 - 0.4 - 04

- 0.9 - 1'0 - 1'0

0'0

- 0 1 - 0.9

- 1.4

- 1-3 - 2.4 - 1-6 - 0.7

27.1

(5.90)

23'4 (493) 2.76 38.8

- 1'1

- 1'2

382

Population norms

(mem age in years)

25'5 27.5 30'3 32'4 34-1 35'5 36-9 39'0 30. I

22.3 24'3 27.1 29.1 30'4 32.1 33'8 36.2 26.9

27'5 (5.15)

24'3 (4.63) 2.31 43'3

375 I

Parental age, sibship size and twins 3 groups, but in this respect there were no differences between the two grades of professional and managerial occupations (I and 11) or between the three grades of manual workers (111, IV and V).

The average number of births per sibship ranged from 1.9 in occupational class I to 2.8 in occupational class 111, and the proportion of first births from 52 to 39 yo. Owing to the numerical importance of manual workers (84% of the population), the norms for parents' ages were virtually determined by occupational classes 111, I V and V. In none of these groups did the mean ages for given sibship position differ significantly from one another, but, compared with professional parents, working-class parents were, on average, 2 years younger at marriage and at the birth of each subsequent child.

Table 3. Parents' ages at first birth according to occupational class in three populations Mean age of parents at birth of first child (in years)

A f \

England Northamptonshire

Wales* 1952 births Older sibs of 1952 births

class Mothers Mothers Fathers Mothers Fathers

11 28.3 27.6 31.1 25.'4 28.7 I11 26.3 25'2 28.1 23'4 26.6 IV 25.6 24.8 27'9 23.1 26.6 V 25'7 249 28.2 23.1 25.8

Total 26.6 25'5 28.7 23.6 26.8

* From Sutherland (1949).

and A f >

Occupational 1939 births. - - I 28.8 27'3 29.6 25'9 29.7

Comparisons between first births in three periods showed that the recent trend towards earlier marriage has affected men and women equally and left unchanged the relative position of professional and working-class families (see Table 3). In a national sample of sibships established in 1939 (Sutherland, 1949) the mean maternal age was 28-6 years. For first births in Northamptonshire during 1952 the corresponding age was 25.5 years and for first births before 1952 it was 23.6 years. In all three groups (one of which contained no one-child families) the mothers in social classes I and I1 were, on average, 2 years older than the other mothers and, in the two Northamptonshire populations, the fathers in each social class were about 3 years older than the mothers.

SIBSHZP SIZE

Though age differences between parents tended to be the same in large and small families, both age at marriage and intervals between births were correlated with sibship size (see Table 4). For instance, at the time of marriage, parents of only children were, on average, 2* years older than parents who had nine or more children; by the time the first child was born this difference had increased to 4 years.

Intervals between marriages and first births averaged 2.5 years in one-child families, 1.2 years in five-child families, and 0.6 years in ten-child families. As a result of these differences, parents with three children had approximately the same average age when their first child was born as the parents with eight children had when their third child waB born.

1-2

Tab

le 4

. N

orth

ccrn

ptm

wh~r

e fam

ilies

pop

ulat

ion

no

rm of b

irth

inte

rvab

in s

ibsh

ip of

di

flere

ent S

km

Aver

age intervals i

n ye

ars

betw

een successive births

Age

at m

arri

ages

-

All

7---

IW

2.5

I '7

"5

"3

2'0

I '2

I '0

1'1

I '2

06

0-

3 I '0

I '0

2'0

3751

2nd -

4'1

3'0

3'1

2.4

2.3

2-4

I -6

I -8

1'

5 1'

3 3'

0 I '0

3'5

2432

3rd

4th -

-

- -

3.8

-

3.1

3.6

2.5

2.8

2-2

2.6

2'2

2.8

2.3

2.4

2'0

2-8

1-8

2-1

1.

6 1-

6 1'

5 1'5

3'3

3.1

1223

58

8

1'0

2'0

* T

win

bir

ths

in b

rack

ets.

t Interval b

etw

een

mia

ge

and

6rst

bir

ths.

$

A =

Fre

nc

e b

etw

een

fath

er's

age

and

the

pop

ulat

ion

norm

of

25.5

yea

rs (

mea

n difference in

yea

rs).

9 See

Tab

le 5

for c

orre

spon

ding

ages

of

pare

nts.

B

= M

emn

ce b

etw

een

fath

er's

and

mot

her's

age

s (mean d

iffe

renc

e in

yea

rs F - M).

birt

hs

-

4'1

3 '4

3'1

2.7

2'5

2-6

2.3

I .8

1'

9 1.

8 1 '4

3'2

4907

2'2

Parental age, sibship size and twins 5

There was a general tendency for the interval between the two most recent births in a sibship to be longer than the intervals between previous births. Thus, in families with three children the second interval averaged 3.8 years and the first 3.0 years. For all families with three or more children the most recent interval averaged 3.5 years and previous intervals 2.6 years. When making these and similar estimates each twin birth was reckoned as a single event, as in Table 4.

Table 5. Bean and standap.d deviatioB of parento1 ages according to sibahip position.

Sibship posit ion

I

2

3 4 5 6 7 8 9

I0 + Total

I 2

3 4 5 6 7 8 9

I0 + Total

No. of births. Index caaea

1319 I 209 635 277 152 72 36 25

16

375 1

Older sibs

2432 1223 588 311 I59 87 5 1 26 16 I4

4907

I 0

Index m e 8 and older sib8

Index cases Older sibs All births --- Mean S.D.

27.8 5'47 31.5 5.66 33'6 5-21 35'3 7'15 36.9 6.12 38.3 7'74 39'6 5.86 41.6 4'09 42'9 5-22 41.8 466

31.7 6.53

25'3 4.98 28-5 5'07 30'7 5-36

34'8 5 '00

37'1 4.78 39'0 6.90 41-0 3-20 37'9 4'05

28.6 5'49

31'5 5'02

33'0 493

Mean S.D.

Father's age in years

26.8 5'31 29.1 5'69 30'9 5-86 32'9 6.10 34'3 6.01 35'6 6.61 37'0 5.83 37.8 5 '04 37.8 5'59 37'8 2.82

29.0 7'52

Mother's age in years

23.6 472 25.8 5.09

29'5 4'87 27'3 5'37

31.2 4'97 32'9 5-12

34'2 5-25

34'0 4'35 34'7 6'34

36'3 3-41

25 '7 5'67

Mean

27'5 30'3 32'4 34' 1

35'5 36.9 38.1 39'6 39'8 39'9 30.1

24'3 27.1 29. I 30'4 32.1 33'8 35'4 36.8 36.7 37'2 26.9

S.D.

5.15 5'70 6.19 5'59 5-91 6.04 5.68 5'25 5-30 4-08

7'95

4.63

4.78 4.96

5.85 4.46

5.10 5-30

5.10 5'35

3'72

5'75

Two factors have probably influenced the figures shown in Table 4, both of which would tend to mask correlations between sibship size and birth intervals. In the first place there was, in the country as a whole, a movement towards earlier marriages during the period when the Northamptonshire sibships were established (R.G. 194052). Secondly, there must have been a large number of incomplete sibships in the survey population. How many of the sibships received later additions is not known, but there is no doubt that the proportion was larger in small families than in large ones.

SIBSHIP POSITION

In the find analysis of parents' ages the children were p l m d according to their sibship positions, and births in 1952 were separated from earlier births (see Table 6 ) . The percentage of firat births wm lower in 1962 (26 %) than in previous years (60%), but the means for parents'

6 ALICE STEWART AND RENATE BARBER ages were higher for all index cases than for all older sibs. This finding reflects the fact that, for each sibship position, the means were higher for the last child in each sibship than for children who had younger sibs. These differences were less marked for children who were placed relatively high in their sibship than for children who had several older sibs. Por each sibship position there was also less difference between the two sets of mothers' ages than between the two sets of fathers' ages, and more variation in the fathers' ages than the mothers' ages.

Table 6. Stillbirths and neonatal deaths in$& births and later births, for twins am?

singletons, index m e s and older sibs

Sibship position

Single births : I st Later

Total

Twin births : 1st Later

Total

All births : 1st Later

Total

Index cases - No. at Deaths per

risk roo at risk

I307 3'9 2400 3'3

3707 3'5

24 20.8 64 10.9

88 13.6

1331 42

3795 3'7 2464 3'5

Older sibs

No. at risk

2415 2445 4860

34 60

94

2449 2505

4954

Deaths per 100 at risk

4'6 4'1

4'4

55'9 21-7

34'0

5'4 45

49

All births - No. at Deaths per

risk IOO at risk

3722 4'4 4845 3'7 8567 4'0

58 41.4 124 16.1

I82 24'2

3780 5 '0 4969 4'0

8749 4'4

The timing of the births and their sibship positions also influenced the risk of a stillbirth or neonatal death (see Table 6). Both for births in 1952 and for earlier births the risk was greater for first-born children than for younger sibs, but, owing to the recent favourable trend of mortality, the combined stillbirth and neonatal death rate for first births in 1952 (4.21 per 100 babies at risk) was lower than the rate for later births in earlier years (4.51).

The figures in Table 7 also show that the difference between being placed first in a sibship or later was more important for twins than for singletons, as was also the diffegence between being born in 1952 or earlier. Thus the rates for twins born in 1952 were 20.8 for first births and 10.9 for later births; for twins born earlier they were 55.9 and 21.7, respectively. Twin death rates were higher for boys than girls, but the relative risks for first births and later births were the same for both sexes.

FATHER'S AQE AND TWIN BIRTHS

Two other aspects of twin births were considered. The first was whether father's age, as such, influenced the frequency of twin births ; the second whether the sex and survivorship of twins influenced sibship size.

Though there is no a priori reason why father's age should influence the twinning rate, nevertheless, a highly significant increase in the rate with father's age (after standardization for mother's age) has been observed in two sets of national statistics (U.S.A. and Egypt). When drawing attention to these findings Bulmer (1969) suggested that they might be due to the fact that, for given age of d€e , the husbands who were younger than their wives had not been

Parentul age, sibship size and twins 7

Table 7. Number of single births, older or younger in relation to twin birth. Swedish and English data

No. of single births L r 7

Sex of

twins Swedish data All twin sibships MM (Edwards, MF 1960) FF

Total

English data All twin sibships MM MF FF Total

Survivore MM rn FF Tot81

Non-survivors$ MM MF FF Total

* Equals no. of sibships.

No. of twin

births*

56

65 72

I93

109 70 70 249

56

46 39

141

53 31 24 I08

Total

262 344 368 974

277

190 189

656

I49 103 I33 385

128 86 57 27 I

No. per sibship * Older? Younger1

3.02 I *66 2-78 2'00 2.95 2-7 I 2-91 2-14

1-57 0.97 1-67 1-03 1'54 1.17 1-59 I -04

2'02 0.64 1.82 0.82 1.80 1.09 1.89 0.84

I '09 1.32 I -48 1-29 I -04 1'33 1.19 1.31

t i.0: older or younger than the twins in the same sibship. $ i.e. one or both twins fail to survive infancy.

married for as long a time as the men who are older than their wives; and might therefore have had less time in which to observe the panty effect on the probability of twin births.

To test this explanation we extracted from the Northamptonshire sibships all those in which the father was at least a year younger than the mother (494 M + families) and paired them off with an equal number of families in which the father was at least a year older than the mother (494 F+ families), each group being matched for age of mother at the time of the 1952 birth. Numbers of single births and twin births were then counted, and expected numbers of twin births for each sibship position were calculated by applying parity-specific twinning rates to the two populations at risk (Waterhouse, 1950).

Observed and expected numbers of twin births in each group tallied very closely, but both were larger in F + families than in M + families, as were also numbers of single births. Thus in the F + families the observed and expected numbers of these were 17 and 17-99, respectively, and in the M + families they were 13 and 13.95, respectively. In the F+ families there were 1394 single births and 35 yo of first births; in the M + families there were 1123 single births and 44 yo of first births.

Shortage of numbers prevented the number of twin births from being significantly larger in the F + families than in the M + families, but a difference of the kind observed was forecast by Bulmer and serves to show how age differences between husbands and wives may produce a spurious association between father's age and the twinning rate in large populations.

8 ALICE STEWART AND RENATE BARBER

TWIN BIRTHS AND SIBSHIP SIZE

When Edwards (1960) examined 193 Swedish sibships, each of which included one set of twins, he found that the sizes of the sibships before the twins were born were roughly the same in families with MM twins as they were in families with M F and FF twins. But the number of younger sibs was smaller for MM twins (1.66 per sibship) than €or M F twins (2.00) or FF twins (2.7 1). Because the number of mothers of MM twins who started their families after the age of 27 years was high (60.7 %) compared with the mothers of MF twins (66.9 yo) and FF twins (41-5 %), Edwards concluded that the two findings were related and that ‘the most reasonable explanation is a varying sex-differential pre-natal mortality amongst twins, but more data are required to confirm the results before any detailed theory need be put forward’. Hence the decision to include all available twin sibships in the present investigation.

The 249 English families with twins were less satisfactory in several respects than the Swedish families, but they had the great advantage that not only the sex of the twins was known but also the number of twins who failed to survive infancy. The Swedish twins belonged to one social class (professional); they were all born before 1926, and their sibships were complete at the time of examination (Edwards & Fraccaro, 1968, 1960). The English twins were biased in favour of working-class families; most of them were born within 6 years of the 1962 survey and it is not known how many of the sibships received later additions. Compared with the Swedish mothers, the English mothers started their families early, and in this respect there were no significant differences between the three sets of twins, the mothers of MM twins being slightly younger than the other mothers.

In spite of these disparities the English twins resembled the Swedish twins inasmuch as the MM pairs had fewer younger sibs (0.97 per sibship) than the MI? pairs (1.03) or the FF pairs (1.17).

When, however, the English twins were classified by survivorship (see Table 7) it was found that the sex differences were confined to sibships in which both twins survived infancy. In these families, which numbered 141, the number of younger sibs was 0.64 for MM twins, 0.82 for MF twins and 1.09 for FF twins. For 108 families in which one or both of the twins failed to survive the corresponding numbers were 1.32 for MM twins, 1.29 for M F twins and 1.33 for FF twins.

When both twins survived there were more older sibs (1.89) and fewer younger sibs (0.84) than there were when one or both of the twins died (1.19 and 1.31, respectively). This difference was due to the fact that the risk of death was greater for first births than later births. As, however, the relative risk was the same for boys and girls, the deficit of younger sibs in the families with MM twins could not be due to this fact. A more likely explanation is that the cost of rearing two boys of the same age was assumed to be greater than the cost of rearing two girls of the same age. Consequently, the parents of MM twins who survived infancy had 8

stronger incentive to practise birth control than the parents of FF twins. If this explanation is correct one might expect to find greater differences between surviving and non-surviving twins in professional families (e.g. the Swedish sibships) than in working-class families (e.g. the English sibships).

Parental age, sibship size and twins 9

SUMMARY

3751 sibships, selected on the basis of a birth during 1952, were examined from the point of view of parents’ ages and birth intervals, and 294 sibships with twin births were examined from the point of view of sibship size.

There were more late marriages and small families in professional than in working-class families but age differences between parents were not affeoted by occupational class or sibship size.

Age at marriage and birth intervals were correlated with sibship size, and intervals between the last two births in a sibship tended to be longer than intervals between previous births.

Women who were younger than their husbands had larger families and more twin births than women of the same age who were married to younger men. This difference was capable of producing a spurious association between father’s age and the twinning rate.

The recent decline in the perinatal mortality rate has affected twins more than singletons, and first births more than later births.

The sex of twins only affected sibship size if both twins survived; in these cases there were fewer subsequent births if the twins were boys than if they were girls.

The sibship data were collected by Public Health Departments in Northamptonshire and the West Riding (all 1952 births in stated regions) ; Northumberland, Salford and Coventry (selected births in 1962).

REFERENCES

BULMER, M. G. (1959). The effect of parental age, parity and duration of marriage on the twinning rate.

EDWARDS, A. W. F. (1980). On the size of families containing twins. Ann. Hum. Genet., Lond., 24, 309. EDWARDS, A. W. F. & FRACCARO, M. (1958). The sex distribution of the offspring of 5477 Swedish ministers

EDWARDS, A. W. F. & FRACCARO, M. (1980). Distribution and sequences of sexes in a selected sample of

REGISTRAR GENERAL (1940-52). stat. Review of E. & w. Part I. civil f&k~tiatics. STEWART, A. M., WEBB, J. W. & HEWITT, D. (1955). Observations on 1078 Perinatal deaths. Brit. J . Prev.

SUTEERLAND, I. (1949). Stillbirths, their epidemiology and social aigni$cance. London : Oxford University

WATERHOUSE, J. A. H. (1950). Twinning in twin pedigrees. Brit. J . SOC. Med. 4, 197.

Ann. Hum. Genet.. Lond., 23, 454.

of religion. Hereditcce, 44. 441.

Swedish families. Ann. Hum. Genet., Lond., 24, 245.

SOC. Med. 9, 57.

Press.