Published by Oxford University Press on behalf of the International Epidemiological Association
© The Author 2005; all rights reserved. Advance Access publication 28 February 2005
International Journal of Epidemiology 2005;34:688–695
doi:10.1093/ije/dyi029
Factors affecting a mother’s recall of her
baby’s birth weight
A Rosemary Tate,1* Carol Dezateux,1 Tim J Cole,1 Leslie Davidson2 and
the Millennium Cohort Study Child Health Group1
Accepted
5 January 2005
Background The Millennium Cohort Study of UK babies born this century obtained maternal
report of birth weight and data on the family’s characteristics, including parental
ethnicity, education, and social circumstances. Parental permission to link babies
to their birth registration data provided the opportunity to investigate factors
affecting accuracy of maternal recall of birth weight and to determine possible
causes of error.
Methods
Logistic regression was used to investigate the relationship between maternal
factors and recall of birth weight. Numerical and graphical methods were used to
identify potential causes for birth weight discrepancies.
Results
Data were obtained from the birth registry and Millennium Cohort Study for
11 890 of the 14 294 cohort children born in England and Wales. Weight was
reported in imperial units by 84% of mothers and this was more common in
younger mothers. Accuracy within 100 g was 92% overall, varying from 94%
among British/Irish white mothers to 69–89% for other ethnic groups and was
lower among the long-term unemployed and those living in disadvantaged or
ethnic wards. Explanations (mostly rounding and transcription errors) were
identified for 27% of the discrepancies of 100 g or more.
Conclusion
Mothers’ reports of their infants’ birth weight showed high level of agreement
with registration data, the mean discrepancy being consistently close to zero.
However, the variance of the discrepancy differed according to ethnic group,
ward type, and socioeconomic status. These sources of differential variability
should be taken into account in analyses using birth weight, and possibly other
reported data, from socially mixed populations.
Keywords
MCS, birth weight, maternal recall, ethnicity, transcription errors, data linkage
Birth weight is an important measure for assessing future
growth patterns, and investigating both immediate health
risks and those in later life. It is thus a key variable in any
longitudinal study of child health. The weight is often obtained
from maternal report. Several studies have shown that mothers
recall their infants’ birth weight with reasonable accuracy,
with ~90% of mothers reporting a figure within 200 g of that
recorded on the birth register, even when the mother is
interviewed several years after the birth.1–5 However, the
extent to which accuracy is affected by ethnicity and social
1 Centre for Paediatric Epidemiology and Biostatistics, Institute of Child
Health, London, UK.
2 Mailman School of Public Health, Columbia University, 722 W 168 Street,
Room 1612, New York, NY 10032, USA.
* Corresponding author. Centre for Paediatric Epidemiology and Biostatistics,
Institute of Child Health, 30 Guilford St, London WC1N 1EH, UK.
E-mail: [email protected]
class in socially mixed populations is unclear. The Millennium
Cohort Study (MCS) was disproportionally stratified in order to
focus on characteristics of families living in disadvantaged areas
of England, Wales, Scotland, and Northern Ireland, and in areas
with a high ethnic minority in England. A large proportion of
mothers agreed to linkage with the child’s birth registration
data. This allowed comparison of the birth weight reported by
the mother with the registered weight, and an assessment of the
factors that affect a mother’s report of her child’s birth weight
in a contemporary multicultural sample.
Methods
Study population
The MCS is a longitudinal survey of social, economic, and
health-related factors among babies born in the UK between
September 2000 and January 2002. Stratification was based on
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FACTORS AFFECTING MOTHER’S RECALL OF BABY’S BIRTH WEIGHT
electoral wards, defined as minority ethnic (30% of the total
population ‘Black’ or ‘Asian’), disadvantaged (top quartile of
the ward-based Child Poverty Index) or advantaged (not in the
upper quartile of this index). Full information on the MCS and
its sample design is described elsewhere.6
The cohort comprises 18 819 children (from 18 553 families)
who were living in the UK at age 9 months. In this report we
focus on children born in England and Wales since these are the
only two UK countries where birth weight is recorded on the
birth register. Interviews were conducted with 14 294 families,
11 533 in England and 2761 in Wales. These included 188 sets
of twins and 6 sets of triplets, but, in order that each family was
represented only once, analysis was restricted to the first born
cohort child. Mothers were interviewed when the child was
~9 months old and questioned on a large number of factors
including the child’s birth weight, whether the child was the
first born, mother’s age, socioeconomic status, self-reported
ethnic group and educational qualifications, and whether
English was the only language spoken at home. The mother’s
socioeconomic status was classified in accordance with the
National Statistics Socio-economic Classification (NS-SEC).7
Educational qualifications were categorized as degree, diploma
in higher education, A/AS/S level, O level/GCSE grade A-C,
GCSE grade D–G, other academic qualification (mainly from
overseas), or none of these. Ethnicity was categorized in
accordance with guidelines from the Office for National
Statistics (ONS):8 British/Irish White, Other White, Mixed,
Indian, Pakistani, Bangladeshi, Black Caribbean, Black African,
and ‘Other’ ethnic group. Further information about these
variables is available via the UK Data Archive (http://wwwdataarchireacuk). Parents were given the option of reporting the
weight in pounds or ounces (which was then converted into
metric), or kilograms and grams. Most interviews were
conducted in English, but if necessary and possible bilingual
interviewers were specifically provided. If this was not possible
a member of the family translated the questions.
Data linkage
At the end of the interview the mother was given a form asking
for permission for researchers to have access to birth registration
records. This form was provided in several languages. In England
and Wales birth weight is obtained from the maternity hospital
and recorded (in kilograms) on the birth register. Full details of
the birth registration procedures have been reported elsewhere.9
Birth weight is now measured routinely in metric units usually
to 1 or 2 decimal places. Data linkage, for all those who agreed,
was carried out by the ONS using the NHS number, baby’s name,
and mother’s and baby’s date of birth. Registration birth weights
are routinely checked by the ONS office for implausible values,
which if possible, are corrected in consultation with the registrar.
Statistical analysis
The characteristics of families for whom no registration data
were available were examined and any significant differences
(P 0.05) between the proportions were identified using
Pearson’s chi-square statistic with the Rao and Scott10 second
order correction.
Multiple logistic regression, using a combination of forward
and backward feature selection, was used to relate the odds of
having a discrepancy 100 g with ethnic characteristics and
689
social circumstances. The figure of 100 g was chosen as a
difference in birth weight having physiological significance.
Adjusted Wald tests were used to test for differences in means
and regression coefficients.
In order to determine possible causes of discrepancy, MCS
birth weights, which differed from those on the birth register by
100 g or more, were examined for any obvious recording errors.
A check was made to see if the birth weight had been incorrectly
entered as imperial rather than metric, or vice versa by dividing
birth weights that had been reported in metric by 0.4536, and
those originally recorded as imperial by 1/0.4536 and comparing
the resulting figure with the registration birth weight. To check
if pounds and ounces had been transposed in error, the original
pounds and ounces figures were swapped and re-converted into
kilograms. A scatterplot of the two weights, and a Bland Altman
plot11 of their means and differences, were then examined in
order to identify any other patterns of discrepancy (between the
MCS and the registration birth weight).
All analyses were conducted using STATA 8.2 (Stata
Corporation, Texas, USA), using sample weights and including
SVY commands to allow for the design effect.12
Results
Birth registration data were obtained for 11 919 of the 14 294
children.
Registration data were unavailable at the time of this study if no
valid consent was obtained from the mother (n = 1438), or if there
were administrative (n = 859) or linkage problems (n = 78). Of
those successfully linked, 11 890 had birth weights from both
sources; 14 were missing from MCS and 15 from the birth register.
Overall 86% (weighted) of mothers reported the birth weight
in pounds and ounces, with younger mothers more likely to use
the imperial [mean age 28.7 years (95% confidence interval
(CI) 28.5–29.0)] than the metric measure [mean age 30.2 years
(29.7–30.7)].
Figure 1 shows a scatter plot of the MCS and registration
weights for each baby. The mean weight of 3.36 kg was the
same for both, and the mean difference between the MCS and
registration weights was 1 g (SD 151 g) (Table 1). The mean
MCS birth weight differed significantly between ethnic groups
(P 0.001, Wald test adjusted for gestational age) with white
mothers tending to have heavier babies than the other ethnic
groups. However, within each ethnic group, the mean
difference between the maternal report and registry recorded
birth weight (shown in Table 1 in grams) was very small and did
not differ significantly from zero. These differences were similar
across all ethnic groups (although there was a significant
(P 0.001) difference between the absolute differences).
Examination of histograms of the difference (MCS-registry
weight) revealed that the distributions for all groups were
symmetric, with very long tails with roughly the same number
of negative differences (i.e. underestimates) as positive
differences (overestimates) in each group.
Over 82% of mothers reported their baby’s weight to within
30 g (~1 oz) of the registration weight and 92% reported to
within 100 g (Table 2). There was marked ethnic variation in
the cumulative frequency of these absolute differences. For
example, there was a significant difference (P 0.001) for a
discrepancy of 100 g; with the proportion being 94% among
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INTERNATIONAL JOURNAL OF EPIDEMIOLOGY
Figure 1 Scatter plot showing the MCS and registration weight for each baby with line of identity
Table 1 Mean (SD) of the MCS and registration birth weights and
their difference, MCS-register, by mother’s ethnic group
Table 2 Absolute difference between the MCS and registration birth
weight: frequency and percentage
Ethnic group
Absolute
difference (g)
MCS (kg)
Registration (kg)
Difference (g)
British/Irish white
3.38 (0.58)
3.38 (0.57)
1 (135)
Other white
3.43 (0.54)
3.44 (0.52)
8 (180)
Mixed
3.27 (0.70)
3.27 (0.71)
41 (226)
30
Frequency
Percentage
(weighted)
Cumulative
percentage
9 543
82.4
82.4
30–60
867
7.0
89.4
391
3.0
92.4
Indian
3.04 (0.61)
3.04 (0.66)
14 (207)
60–100
Pakistani
3.12 (0.57)
3.10 (0.55)
10 (254)
100–200
397
2.8
95.1
0 (214)
200–300
197
1.3
96.4
Bangladeshi
3.07 (0.58)
3.11 (0.47)
Black Caribbean
3.17 (0.61)
3.18 (0.58)
15 (222)
300–400
128
0.9
97.3
Black African
3.31 (0.62)
3.33 (0.57)
8 (249)
400–500
116
0.9
98.3
251
1.7
100.0
Other
3.26 (0.60)
3.27 (0.52)
7 (230)
500
Total
3.36 (0.58)
3.36 (0.57)
1 (151)
Total
infants of British/Irish White mothers as compared with 69% of
infants of Bangladeshi mothers (Figure 2), 73% of Black
Africans, 87% of ‘Other White’ (68% of those who stated that
they were European) and 89% of Black Caribbeans.
In 8% of infants the birth weights were discrepant by at least
100 g (referred to here as a discrepant weight), with the MCS
weight lower than the registration weight in 53% of infants.
The percentage of mothers with discrepant weights varied
significantly according to ward type and ethnic group (Table 3)
being highest for mothers in ethnic wards and for those not
from a British/Irish White background. Mothers who had never
worked and those with no academic qualifications had more
discrepant weights, as did those who spoke a language other
than English at home. The percentage of discrepant weights was
11 919
higher among mothers reporting the weight in metric units.
The mean age of mothers with discrepant weights was the same
(29 years) as of those without.
Babies with discrepant weights had a slightly lower mean
register birth weight (3.3 kg vs 3.4 kg (P 0.001)). The
proportion of discrepant weights was significantly (P 0.001)
higher among the 793 babies with registry weight 2.5 kg
(11%) than among the 9857 weighing 2.5–4 kg (8%) or the
1240 weighing 4 kg (5%). Mothers with lighter babies tended
to overestimate the weight and those with heavier babies to
underestimate. The mean difference of MCS-registration weight
was 45 g, 3 g, and 12 g, respectively for the above three
birth weight categories and these differed significantly between
the groups (P 0.001).
FACTORS AFFECTING MOTHER’S RECALL OF BABY’S BIRTH WEIGHT
691
Figure 2 The cumulative distribution of absolute difference between MCS and register (REG) birth weight for the white and Bangladeshi
mothers. The vertical line indicates a difference of 100 g between the birth weights
Table 4 shows adjusted odds ratios for the factors that
significantly predicted a discrepant weight when the above
factors were taken into account. Discrepant weights were
significantly less likely for British/Irish White mothers, but
significantly more likely for mothers who had other children or
who lived in an ethnic ward. The likelihood of a discrepant
weight decreased linearly with an increase in the registered
weight of the baby. Employment was significant, with mothers
who had recently worked in any type of employment being
less likely to have discrepant weights than the long-term
unemployed or never employed, but maternal education level
was not significant after adjusting for socioeconomic status.
The odds of a discrepancy when birth weight was reported in
metric changed from 1.7 (95% CI 1.3–2.1) to 1.2 (1.0–1.5)
when ethnicity was taken into account, since mothers who
were not British/Irish White were more likely to report in
metric. There was no evidence for difference according to
mother’s age at the child’s birth.
When the logistic regression was repeated to include only the
British/Irish White mothers, the odds ratios remained within
one decimal point of those shown in Table 4, with the exception
of the odds ratio for ethnic ward which increased from 1.6 to
2.0 (1.5–2.9). When the analysis was repeated after excluding
the British/Irish White mothers, all the odds ratios that had
been significant in Table 4 remained within one or two decimal
places of the original odds ratios and all were now only
significant at the 0.1 level, except for the odds ratio for
registration birth weight which was no longer significant
(P = 0.5). In addition, it was found that speaking another
language other than English at home was significantly related
to having a discrepant weight among minority ethnic mothers
[OR = 1.62 (1.01–2.60)].
Explanations for birth weight
discrepancies
Possible causes were identified for 295 (27%) of the 1089
discrepant weights (see Figure 3). The most common cause was
rounding or truncating the MCS weight to the nearest kilogram
(n = 52) or pound (n = 86) or half kilogram (n = 6) or half
pound (n = 28). There were 27 cases where the pounds and
ounces appeared to have been transposed and six with the
weight recorded incorrectly as pounds and ounces instead of
kilograms. A further 29 had a leading zero missing after the
decimal point e.g. written as 1.2 kg rather than 1.02 kg. In 59
cases MCS weights had been recorded with a difference of one
or more units from the registration weight; 19 in kilograms and
40 in pounds. This was the cause of the two large outliers at the
top of Figure 1 which had been recorded by interviewers as
14 lb 15 oz and 15 lb 15 oz instead of 4 lb 15 oz and 5 lb 15 oz,
respectively. The other identifiable cause was incorrect linkage
of two children of multiple births due to misrecording of the
birth order, either by the interviewer or the birth register.
Explanations were found for 24% of the discrepant weights
reported in imperial and 37% of those reported in metric.
The only factors found to be independently related to having
an identifiable error (given a discrepant weight) were; speaking
another language at home [OR = 2.0 (95% CI 1.3–3.2)],
reporting in kilograms [1.8 (1.2–2.6)], and mother’s age at MCS
birth [1.2 (1.1–1.4) for every 5 year increase].
Discussion
This large data set from the MCS provided the opportunity to
investigate the influence of ethnic and social factors on mother’s
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INTERNATIONAL JOURNAL OF EPIDEMIOLOGY
Table 4 Results of logistic regression showing odds ratios associated
with an absolute difference 100 g between the registration and MCS
birth weight
Table 3 Total numbers for each factor together with the (weighted)
percentage of mothers reporting a birth weight 100 g different from
that on the birth register
Factor
Total in group
Percentage
with 100 g
difference
Wales
9556
2334
8
6
4695
6
Disadvantaged
5486
8
Ethnic
1709
21
Other white
9563
6
254
13
Mixed
150
13
Indian
325
12
Pakistani
606
18
Bangladeshi
245
31
Black Caribbean
188
11
Black African
269
27
Other
265
23
Intermediate occupations
Small employers
Lower supervisory and technical
3029
7
1966
5
460
8
646
8
Semi-routine and routine
occupations
4371
8
Never worked and long-term
unemployed
1282
17
Degree
1758
Diploma in higher education
1038
6
983
7
4096
7
GCSE grade D–G
1404
6
343
18
2253
12
None of these
Home language
English only
9938
6
Other language
1952
19
Cohort baby first born
4896
6
Cohort baby not first born
6884
9
1777
11
10 113
7
Family order
Measure used
Kilograms and grams
Pounds and ounces
(1.2–2.2)
0.8
(0.7–0.9)
For each 1 kg increase
Ethnic Group
British/Irish white
1
2.3
(1.5–3.2)
Mixed
2.03
(1.1–3.7)
Indian
1.63
(1.0–2.6)
2.1
(1.5–3.1)
Bangladeshi
3.65
(2.4–5.7)
Black Caribbean
1.37
(0.9–2.2)
4.1
(2.8–6.0)
3.21
(2.0–5.1)
0.73
(0.55–0.97)
0.6
(0.4–0.8)
Pakistani
Black African
Other
Socio-economic status
Managerial
Small employers
0.8
(0.5–1.2)
Lower supervisory
0.8
(0.5–1.2)
Semi-routine
0.7
(0.6–0.9)
Never worked and long-term
unemployed
1
Whether first born
Yes
No
1.5
(1.3–1.8)
7
O level/GCSE grade A–C
Other academic qualification
1.6
aAdjusted for the factors shown in this table.
Education
A/AS/S level
(0.9–1.4)
Ethnic
Intermediate
Socioeconomic status
Managerial and professional
1.1
Other white
Ethnic Group
British/Irish White
1
Disadvantaged
Registration birth weight
Ward Type
Advantaged
(95% CI)
Ward Type
Advantaged
Country
England
Adjusted ORa
Missing number of cases for: ward type (0), country(0), ethnic group (40),
socioeconomic status (197), education (49), language (0), family order (216).
recall of birth weight. Over 92% of MCS mothers reported
within 100 g of the weight recorded on the birth register which
compares well with other recent UK studies, albeit that most
studies were carried out when the children were older.
For example, O’Sullivan et al.4 interviewed mothers of 649
children aged 6–15 years and found that 85% recalled the birth
weight to within 100 g of the hospital record weight. They
found no significant association between the difference in birth
weight (mother hospital) with social class, but this may have
been because they looked only at the means of the actual
differences (which were symmetrically distributed around, and
therefore close to, zero), rather than the magnitudes of the
differences. Walton et al.5 reported that 85% of parents of 1015
children aged 12–15 years recalled the weight within 220 g of
the computerized child health record. They looked at absolute
differences and (as we did) found a significant relationship
between accuracy and social class, with parents from manual
occupations more likely to have greater discrepancies. In
common with MCS mothers, parents of children with
FACTORS AFFECTING MOTHER’S RECALL OF BABY’S BIRTH WEIGHT
693
Figure 3 Scatter plot of discrepant birth weights, showing those with an identifiable cause
particularly low (2.5 kg) birth weight reported their child’s
weight less accurately than those of ‘normal’ weight (2.5–4 kg)
and tended to report them as heavier than the recorded weight.
However, unlike MCS mothers, parents of children of high birth
weight (4 kg) were also less accurate.
Ours is the first UK study to consider maternal recall of birth
weight in relation to ethnicity and language. The results suggest
that ethnicity is a major influence, with British/Irish White
mothers much more likely than any other ethnic group to
accurately recall their child’s birth weight. This could be due to
difficulties in written or spoken English in other ethnic groups
which may have caused miscommunication.
For example the midwife may have given the mother an
accurate figure but the mother may have recorded or recalled it
wrongly, or the interviewer may have misunderstood the
mother. Alternatively, it could be due to cultural differences
since remembering the exact weight, for example to report to
family, friends and neighbours, may be more important in some
cultures than others. We could find no reason (e.g. home
language or partner’s ethnicity) why mothers living in ethnic
wards were more likely to have discrepancies than those living
in advantaged or disadvantaged wards and can only surmise
that this may be due to the cultural influences of living in an
ethnically mixed neighbourhood.
Other factors relating to recall were socioeconomic status and
birth order, with mothers who were long-term unemployed and
whose baby was not the first born, being the most likely to have a
significant discrepancy of 100 g or more.
It is surprising that so many, particularly, young mothers still
report the weight in pounds and ounces, despite all babies being
weighed in grams at birth and the fact that metric measures
have been taught in British schools since 1974. One source of
error could be incorrect conversion of the weight from grams
into pounds and ounces, either by the mother or the midwife at
the time of delivery. However, a higher proportion of discrepant
weights were found among mothers who reported the weight in
kilograms. This could be partly explained by the fact that ethnic
groups were more likely to use metric measures. Another
explanation is that rounding errors to the nearest kilogram
would lead to a larger discrepancy between birth weights than
rounding to the nearest pound. Of birth weights reported in
pounds, 0.9% were identified as being rounded up or down to
the nearest pound or half pound as opposed to 3.3% of those
reported in kilograms.
It was not possible in this study to identify whether the
discrepancies between the MCS and registration weight arose
from incorrect recording by the interviewer or incorrect
reporting by the mother. There was also no way of knowing if
discrepancies were due to mother’s incorrect recall, or due to an
erroneous birth weight being provided at the time of birth.
A limitation of this study is that registration weights were
obtained for only 84% of the MCS babies born in England and
Wales. A preliminary investigation of the factors related to
successful linkage (to be reported elsewhere) showed that most
of the factors that were related to discrepancy in birth weight
were also significantly related to non-linkage. For example,
successful linkage differed according to ethnic group being
highest among mothers who were British/Irish White (87%),
and lowest among Asian women (72%).
We hope to have minimized any bias in the results of
the logistic regression due to unequal linkage by including
these factors in our model. There is also the issue of non-response
bias. In an attempt to adjust for this in our analysis, the sample
weights were modified to account for non-response.13
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INTERNATIONAL JOURNAL OF EPIDEMIOLOGY
However, this had very little effect on the odds ratios and their
confidence intervals, indicating that the factors identified as
related to non-response (e.g. social and ethnic factors) were
already accounted for in the original logistic regression analysis.
An issue that needs to be addressed is whether to use the
mother’s or registration birth weight in future analyses of
the MCS data. One could use the registration figure when
available, or alternatively use it just when there is a discrepant
weight. However, this could lead to biased results, since
registration data are available for only 64% of the MCS
children. Also, we have no way of confirming that the
registration weights themselves are correct. The register and
reported weights had very similar mean values within each
ethnic group, so using the reported weight should only
introduce relatively random error rather than systematic bias.
Therefore there is an advantage in using the reported weights
which are more complete. We suggest that the registration
weight should only be used if there is a very obvious error, for
example, the two weights that were identified as having a
discrepancy of 10 lbs, and that discrepant weights could be
tagged so that they can be identified in future analyses.
The results of this study show that although mother’s report
of birth weight is generally very good, accuracy does differ
according to individual factors such as maternal ethnicity,
socioeconomic status and parity as well as the ethnic
composition of the community in which they live. It is possible
that these factors will also affect accuracy of other reported
measures, not only birth weight, (a topic for further research)
and this should be taken into account when designing studies
or analysing data from socially mixed populations.
Acknowledgements
We would like to thank all the Millennium Cohort families who
provided the data for this study, Christine Hockley for providing
us with the linked registration data, Lisa Calderwood for
answering our numerous questions about the MCS data set and
Alison Macfarlane on her helpful explanations of current birth
weight recording processes. We also thank Professor Heather
Joshi, director, and members of the Millennium Cohort Study
Management Team at the Centre for Longitudinal Studies,
Institute of Education, University of London and the ONS and
Birth registration departments. The other members of the
Millennium Cohort Study Child Health Group who contributed
to this work are; Helen Bedford, Neville Butler, Lucy Griffiths,
Catherine Peckham, Lamiya Samad and Suzanne Walton.
KEY MESSAGES
•
In a large birth cohort study of a socially and ethnically diverse population accuracy of birth weight was found to be generally
high with 92% of cohort mothers reporting the weight within 100 g of the registration weight.
•
However, accuracy varies according to socioeconomic status and is significantly less in some ethnic groups.
•
This should be taken into account when designing surveys and analysing birth weight and other data from socially mixed
populations.
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