Determinants of the Availability and Accuracy of Self-reported Birth Weight in Middle-aged and Elderly Women

Diane S. Allen1, George T. H. Ellison2, Isabel dos Santos Silva3, Bianca L. De Stavola3 and Ian S. Fentiman1

1 Department of Academic Oncology, Guy's Hospital, London, England.
2 Social Science Research Unit, Institute of Education, London, England.
3 Cancer and Public Health Unit, London School of Hygiene and Tropical Medicine, London, England.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Associations have been found between birth weight and many diseases in adult life. In most countries, few birth records exist for older adults; therefore, birth weight is usually obtained by maternal recall or self-report. This study examined determinants of the availability and accuracy of self-report in middle-aged and elderly women. Birth weights, recorded at the time of birth, were found in 1999 for a subset of 363 women participating in a long-running cancer research study in the United Kingdom. Questionnaires were sent to the surviving 286 women requesting information on their birth weight and other factors related to their birth family. Twenty-five percent of the 244 respondents were able to report their birth weight to within 4 ounces (113.4 g) of that listed in birth records, 28% reported it inaccurately, and 47% did not know their birth weight. The most important factors determining the availability of self-reported birth weight were having a living mother and a low birth weight (<=6 pounds (2,722 g)). The most important determinants of accuracy, for those who provided a report, were being younger and the eldest child. Research studies relying on self-reported birth weight should take these factors into account.

birth weight; middle age; recall; reproducibility of results; women

Abbreviations: CI, confidence interval; OR, odds ratio; r, Pearson's correlation coefficient; rho; Spearman's rank correlation.


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Over the past 20 years, there has been increasing interest in the effects of prenatal exposures on disease susceptibility in adulthood (1Go). In many of these studies, birth weight was taken as a crude marker of the prenatal milieu. For retrospective studies, the accepted "gold standard" is to obtain birth weight data from birth records (2Go). Nonetheless, several studies have used subject self-report of birth weight (3GoGoGoGoGo–8Go). In the United Kingdom, as in many other countries, self-reported birth weight is, of necessity, based on the mother's recall of birth weight; birth weight was not recorded on birth certificates in the past, and few historical records exist.

Several studies have assessed the validity of maternal recall of birth weight (9GoGoGoGoGoGoGoGo–17Go), but we know of only three that have assessed the validity of self-reports (8Go, 15Go, 16Go). Two compared birth weights obtained from study participants, their mothers, and birth records but excluded participants whose mothers were dead. The impact of maternal vital status on the availability of self-reported birth weight among middle-aged women was examined by Andersson et al. (8Go), who found that only 28 percent of the participants were able to report their birth weight and that these respondents were more likely to have a living mother. However, it was unclear whether this finding simply reflected the age of the participants.

Our study aimed to investigate 1) the determinants of availability and 2) the determinants of accuracy of self-reported birth weight in middle-aged and elderly women and, furthermore, to estimate 3) the separate effects of age and maternal vital status at the time of response.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The participants in this study included a subgroup of women with recorded birth weights who had taken part in a prospective study of hormones and breast cancer, with recruitment between 1986 and 1991. The study was conducted in Guernsey, United Kingdom (18Go, 19Go), a small island in the English Channel with a relatively stable population.

There were two sources of recorded birth weights found in 1999: a midwife's register for the years 1922–1937 and a subset of records from the island's maternity hospital comprising approximately 40 percent of births in Guernsey between 1950 and 1969. The remaining 60 percent had been filed in the mother's hospital case notes and thus were not readily available. Information on date of birth, recorded birth weight, sex, live- or stillbirths, name of the mother, and place of birth was collected from these sources. This information was then matched to that for women from the Guernsey Project, producing 363 matches of whom 286 women were recorded by the Guernsey Project as still being alive (198 born in 1922–1937, 88 in 1950–1957).

In 2000, a postal questionnaire was sent requesting the following information from each woman: date of birth, place of birth and birth weight, her mother's date of birth and name at birth, whether her mother was currently alive, how many children her mother had had, her birth position among those children, and her father's occupation at the time of her birth. Each woman was asked to complete the questionnaire even if she did not know her birth weight. A stamped, addressed envelope was provided and one reminder letter sent.

Information on additional factors that might influence whether a woman had asked her mother her birth weight in the past had been collected during the Guernsey Project, namely, parity (known for all women), number of years of education, and occupational class (both available for about 70 percent of these women). The woman's occupation at the time of the study had been recorded unless she was a housewife, unemployed, or retired, when her last occupation was recorded. The occupation of her partner, or her previous partner if she was widowed, separated, or divorced, was also recorded. From these data, the occupational class of the respondent's household was considered the occupation of either the woman's partner, the woman's previous partner, or her (if single).

The aim of our study was to assess determinants of the availability and accuracy of self-reported birth weights separately. Availability was defined as an informed reply to the questionnaire, that is, an entry for birth weight different from "not known." Women were encouraged to report that they did not know their birth weight rather than offer a guess, since we were interested in whether collecting self-reports of birth weight from middle-aged and elderly women was a feasible option for research studies.

Within the confines of this study, and for the sake of brevity, we defined a self-reported birth weight as being accurate if it was found to be within 4 ounces (113.4 g; 1 ounce = 28.35 g) of the weight listed in birth records. This definition was used because, although the birth weights from both sources of birth records were recorded in pounds (1 pound = 454 g) and ounces, those from the midwife's register were recorded to the nearest quarter pound (4 ounces ({approx} 115 g)). However, we also looked at accuracy to within 8 ounces and 16 ounces.

The most important determinants of availability and accuracy were identified by using univariable logistic regression. Multivariable logistic regression was then used to assess the relative importance of age and maternal vital status.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Replies were received from 244 women, an overall response rate of 85 percent. Data available from the Guernsey Project files showed that nonresponders were more likely to be in the older age group (79 vs. 68 percent) and nulliparous (19 vs. 6 percent). There were no differences by occupational class or number of years of education.

Apart from birth weight, the information collected from the questionnaire was over 98 percent complete, although only 69 percent of the women reported paternal occupation at the time of her birth. Birth weights were reported by 53 percent of the respondents. Those who did give a value differed from those who did not, the most striking differences being their age (40 percent availability in the older group compared with 81 percent in the younger group) and maternal vital status (89 percent of those whose mothers were alive compared with 41 percent of those whose mothers were dead). Low birth weight, small family size, being the eldest child, and having been educated beyond the minimum school-leaving age also increased availability (table 1).


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TABLE 1. Availability and accuracy of self-reports of birth weight,* by characteristics of the 244 respondents and their families, Guernsey, United Kingdom, 1999–2000

 
Table 1 provides an assessment of the accuracy of these self-reported values in terms of the percentage of responses within 4 ounces, 8 ounces, and 16 ounces of the birth weights listed in the original birth records. Only 48 percent of those women who reported a value for birth weight were accurate to within 4 ounces of the recorded value, 67 percent to within 8 ounces, and 89 percent to within 16 ounces. For each definition, accuracy was higher when the respondent was in the younger age group or her mother was still alive.

Table 2 presents the cross-tabulation of availability and accuracy of self-reported birth weights by maternal vital status separately for the two birth periods. In each period, the percentages of respondents who reported their birth weights accurately fell if their mother was dead: from 33 to 10 percent in the older group and from 68 to 26 percent in the younger group. Similarly, in each stratum of mother's vital status, the proportion of women reporting an accurate birth weight decreased with age. The overall Pearson's correlation coef-ficient between self-reported and recorded birth weight was r = 0.86 (p < 0.01; Spearman's rho = 0.83) and was slightly greater for women born in the later period (r = 0.94 vs. r = 0.81; rho = 0.93, 0.73) and for those whose mothers were still alive (r = 0.94 vs. r = 0.82; rho = 0.92, 0.78).


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TABLE 2. Cross-tabulation of availability and accuracy of self-reported birth weight,* by maternal vital status and period of birth, Guernsey, United Kingdom, 1999–2000

 
Maternal vital status, year of birth, size of and position in the birth family, low recorded birth weight, and age at which full-time education ended were all identified in the univariable analyses as statistically significant determinants of the availability of self-reported birth weight (table 3). In particular, a woman was more than 11 times as likely to report a value if her mother was still alive and was more than six times as likely to do so if she was in the younger age group. Multivariable analysis showed that having a living mother (odds ratio (OR) = 17.56, 95 percent confidence interval (CI): 6.71, 45.96); p < 0.0001) and low recorded birth weight (OR = 2.54, 95 percent CI: 0.73, 8.76; p < 0.005) were the only independent determinants of the availability of self-reported birth weight.


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TABLE 3. Univariate logistic regression analysis for availability and accuracy of self-reported birth weight*: odds ratios, 95% confidence intervals, and likelihood ratio test for heterogeneity, Guernsey, United Kingdom, 1999–2000

 
Similar analyses were carried out for determinants of the accuracy of self-reported birth weight to within 4 ounces by using information from only those respondents who had reported a birth weight (table 3). Univariable analyses identified four determinants of accuracy: younger age, a living mother, continuing education beyond the minimum age, and being the eldest child. Multivariable analysis showed that being younger (OR = 10.24, 95 percent CI: 3.38, 31.07; p < 0.001) and being the eldest child (OR = 8.07, 95 percent CI: 2.07, 31.52; p < 0.005) were the only independent determinants of accuracy to within 4 ounces. Multivariable models in which less stringent criteria were used for accuracy (within 8 ounces or 16 ounces) identified the same determinants.


    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Previous studies in which self-reported birth weight has been used seem to have ignored the often-large percentage of respondents unable to report a birth weight. This shortcoming might have led to bias in their results. The purpose of our study was to assess the determinants of the availability and accuracy of self-reports of birth weight relative to birth records in a sample of middle-aged and elderly women.

We found that 47 percent of the participants in our study were unable to report a value for their birth weight. In other studies of women, the percentages of those unable to report their birth weight varied between 8 and 72 percent and increased with age, as follows: Sanderson et al. (5Go), age 21–45 years, 16 percent; Nurses' Health Study II (3Go), age 27–44 years, 8 percent; Andersson et al. (8Go), age 44–60 years, 72 percent; Nurses' Health Study I (3Go), age 46–71 years, 20 percent; and Sanderson et al. (5Go), age 50–64 years, 27 percent. In studies of men, these percentages varied between 41 and 59 percent, as follows: Francois et al. (7Go), age 24–57 years, 59 percent; Frankel et al. (6Go), age 45–59 years, 50 percent; and Health Professionals Follow-up Study (4Go), age 48–83 years, 41 percent.

In our study, the percentage of women unable to report their birth weight may have been higher than in other studies because we did not encourage them to guess. An alternative, which we recommend, is to offer women the opportunity to enter a value and then ask them, in a supplementary question, how certain they are of this value. Nonetheless, our percentage of unknowns was in the range found in other studies of women.

There is little uniformity concerning what constitutes an "accurate" birth weight, and this definition is likely to vary depending on the accuracy required in different studies. The level of accuracy presented here (within 4 ounces), determined by the acuity of the birth records from the midwife's register, was within the range presented in other studies. However, although this definition may be too strict for the fetal origins of disease hypotheses, we found that our results did not change when less stringent criteria were used.

We know of three previous studies that have investigated the accuracy of self-reported birth weight (8Go, 15Go, 16Go). Sanderson et al. (16Go) studied women aged less than 45 years and found good correlation between self-reported and recorded birth weights (rho = 0.85); this correlation was only slightly lower for those who said they did not consult their mothers before responding (rho = 0.82). Troy et al.'s study (15Go), using a subset of women aged 27–44 years from the Nurses' Health Study II, did not take into account whether the subject asked her mother before replying. These authors presented accuracy data within wide birth-weight bands (24 ounces ({approx}700 g)) and found that 70 percent of the subjects reported birth weights in the correct category. However, because they were also investigating maternal recall, both studies excluded women whose mothers had died. The third study, by Andersson et al. (8Go), in which only 28 percent of the participants offered a weight, found that 47 percent of the weights were within 250 g ({approx}8 ounces), despite good correlation between the two (rho = 0.76). We also found good correlation between self-reported and recorded birth weight, which assesses the measurement error in the group with known values. However, we were also investigating those women who were not able to report a value and the effects of both the mother's vital status and age.

Interestingly, we found that the determinants of availability and accuracy are different. Availability was dependent on having a living mother and a low birth weight. This finding suggests that the mother was asked at the time of self-report and that she was more likely to remember if the baby was small, perhaps needing special postnatal care.

Accuracy was found to be dependent on being younger at the time of self-report and being the eldest in the family. Seidman et al. (12Go) found that the birth weight of the eldest child was remembered most accurately by mothers but that this accuracy decreased over time. Both the woman and her mother were more likely to remember the information if they were younger, and the woman's mother was more likely to remember accurately the birth weight of her eldest child.

In summary, our analysis of whether adult women have accurate knowledge of their birth weight found that 53 percent were able to provide a self-report, and 25, 36, and 48 percent were able to report to within 4 ounces, 8 ounces, and 16 ounces, respectively, of the recorded value. We found that 59 percent of women reported not knowing their birth weight if their mothers were dead. We also found that, although both age and maternal vital status were significant determinants of availability and accuracy, the mother's being alive was more important for availability while younger age was more important for accuracy. Thus, these factors should be considered when assessing the feasibility of studies that plan to rely on self-reported birth weight.


    ACKNOWLEDGMENTS
 
The authors are grateful to the Lloyds TSB Foundation for the Channel Islands, which funded this study, and for its continuing funding of the Guernsey Breast Cancer Research Project and the Channel Islands Occupation Birth Cohort Study.


    NOTES
 
Correspondence to Diane S. Allen, Department of Academic Oncology, 3rd Floor Thomas Guy House, Guy's Hospital, London SE1 9RT, England (e-mail: allend{at}icrf.icnet.uk).


    REFERENCES
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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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Received for publication January 23, 2001. Accepted for publication August 15, 2001.