1 Department of Clinical Nutrition, Göteborg University, Göteborg, Sweden.
2 Göteborg Pediatric Growth Research Center, Institute for the Health of Women and Children, Göteborg University, Göteborg, Sweden.
3 Department of Medicine, Sahlgrenska University Hospital, Göteborg, Sweden.
4 Department of Primary Health Care, Vasa Hospital, Göteborg, Sweden.
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ABSTRACT |
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birth weight; middle age; recall; women
Abbreviations: SD standard deviation
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INTRODUCTION |
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The purpose of this study was to evaluate agreement between self-reported birth weights and those obtained from original delivery records of adult women. To our knowledge, such an assessment has not been reported previously for middle-aged women.
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MATERIALS AND METHODS |
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Self-reported birth weight
Participants in the 1974/1975 examination (aged 4466 years) were asked how much they weighed at birth. The question was part of a questionnaire completed at home before attending the health examination. The question read: "How much did you weigh when you were born?" Possible answers were 1) "I weighed __.__kg" or b) "I don't know." The questionnaire was reviewed with the interview nurse on the day of examination.
Birth weight from original delivery records
Delivery records (both home and hospital births) for the births of all participants were sought in city, regional, county, and health center archives on the basis of the date of the subject's birth and the mother's name. Church records were examined to confirm the mother's name, age, place of residence, and parity.
Data retrieval
To examine whether there was a bias in delivery record retrieval, parental sociodemographic characteristics at the time of the birth of the participants were compared for those with and those without traced records. Information on maternal marital status, maternal age, maternal parity, and parental social group at the time of the participant's birth was obtained from the extensive recording system of the Church of Sweden and directly from the delivery records. Social group at birth was based on the father's occupation at the time of the birth or, if the parents were not married, the mother's occupation and were classified into five categories as previously described (20).
Characteristics of self-reporters and nonreporters
Sociodemographic characteristics in adulthood and at birth that may influence reporting were compared in self-reporters and nonreporters with traced records. Level of education attained was defined as primary school only or primary school plus further education. The participant's marital status was categorized as ever married or never married. Parity was defined as the total number of pregnancies minus miscarriages. Maternal living status at the time of reporting birth weight was coded as living or not living. Social group was defined by the husband's occupation if married, cohabiting, or widowed or by the participant's own occupation and classified as described above. Characteristics at birth of maternal age, parity, marital status, and parental social group were also assessed.
Application
To examine the consequences of using self-reported birth weight on outcome measures in adulthood, the association between birth weight and cardiovascular risk factors (serum triglycerides, serum cholesterol, serum glucose, waist-to-hip ratio, body mass index, and hypertension) was analyzed. The risk factors were measured at age 60 years during the 24-year population study follow-up period. Procedural details of measurements are presented elsewhere (21).
Data analysis
The Fisher exact test and the 2 test for trend were applied to compare proportions. Probit analysis (normal plot), as described by Altman (22
), was used to examine the distribution of self-reported birth weights (reporters), their corresponding birth weights from delivery records, and the remaining population with delivery records (nonreporters). A plot as described by Bland and Altman (23
) was used to examine agreement. The difference (self-reported birth weight minus birth weight from original records) was plotted against the mean of the two methods (self-report + original)/2. Limits of agreement were calculated as mean difference plus 2 standard deviations (SD), and 95 percent confidence intervals were calculated to assess the precision of the estimated limits of agreement (23
). Since the format of the self-reported birth weights was in kilograms plus one decimal, "no difference" between reported birth weight and birth weight from original records was defined as the difference ±50 g to allow for rounding. Univariate and logistic regressions were used to examine the relation between the independent variable birth weight (self-reported or from original record) and dependent cardiovascular risk factors. All analyses were carried out by using the SAS software release 6.12 statistical package 19891996 (SAS Institute, Inc., Cary, North Carolina).
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RESULTS |
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Representativeness of subgroups with and those without traced delivery records
Original delivery records could be traced for 693 (61 percent) of the participants. There were no statistically significant differences in parental sociodemographic characteristics between those with and those without traced delivery records at the cohort level. Power for the tests at the cohort level did not exceed 50 percent. With all cohorts combined, participants with traced records had slightly younger mothers (p = 0.02) and were from lower social classifications (p = 0.001).
Reporters/nonreporters
A total of 192 (71 percent) of the self-reported birth weights could be matched with original birth weight records. A random selection (12 percent) of the original questionnaires of nonreporters (n = 501) were reviewed to examine the response "I don't know" to the question of weight at birth. Only one form was not properly filled out, thus limiting speculation of low response due to poor questionnaire completion.
Table 1 presents the age distribution at self-reporting and sociodemographic characteristics of reporters and nonreporters at the 1974/1975 examination. There was a statistically significant difference in the level of education, with self-reporters having higher education compared with nonreporters (p = 0.005). A total of 41 percent of the participants with traced original delivery records had a living mother at the time of reporting. Self-reporters were more likely to have a living mother (p < 0.001).
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Of those women for whom original delivery records were traced, 28 percent (192 of 693) could report a birth weight. The proportion of self-reporting increased with lower age at reporting (2 = 41.1, p < 0.0001, test for trend). In the 1930 cohort, 36 percent could report a birth weight, while in the 1914 cohort, only 9 percent could do so.
Figure 1 presents the distribution of self-reported birth weights, their corresponding birth weights from original records, and birth weights of nonreporters from original records by probit analysis. The distribution of birth weights from original records of self-reporters did not differ from the population as a whole. However, the distribution of self-reported birth weights deviates, indicating overreporting of lower and higher birth weights.
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Application
Serum triglycerides and waist-to-hip ratio were found to be positively and significantly related to weight at birth when self-reported birth weights were applied (table 3). However, when original birth weights of the self-reporters or those of the total population with original records were analyzed, no such relation was found (table 3). Hypertension was negatively and significantly related to birth weight when assessed with original records with and without adjustment for body mass index. This relation was not found with self-reported birth weights.
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DISCUSSION |
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We examined whether there was selection bias in the tracing of original delivery records. We found no significant differences in parental sociodemographic characteristics at the cohort level. Low power may be a limitation at the cohort-specific level. The statistically significant difference at the combined cohort level of maternal age (29.7 and 30.6 years) cannot be of biologic importance. Those with retrieved records tended to be from lower social groups at birth. This may reflect that missing original birth data were for home births and a higher proportion of nonurban births.
We also examined whether sociodemographic composition differed by reporting status. Reporters had a higher level of education, a higher birth order, and younger mothers. Those women with reported birth weights were not a random sample as such. They were those in the entire study population during the 1974/1975 examination who could reply to the question of self-reported birth weight. The fact that only 24 percent of all participants in the examination (28 percent in the group with traced original delivery records) could report a birth weight at all in itself suggests that self-reporting is not an appropriate means of determining birth weight. Poor response rates alone could lead to bias in the resulting database.
The low reporting rate may be seen as a limitation of this study. However, in comparison with previous studies on self-reported birth weight, our study was not limited by poor retrieval of original birth weight records. We were able to trace original birth weight records for 84 percent of the participants born in Göteborg and 61 percent for the entire study population. The limiting factor here was the low reporting rate.
Was the information requested of the respondents too detailed? Reporters thought they knew their birth weight, and yet, we still found discrepancies of up to 2,000 g. Some studies have analyzed birth weight as a categorical variable (10, 12
15
). Categorization of birth weight, however, could result in loss of power as well as sensitivity to detect intracategoric differences (24
, 25
).
Categorized, self-reported birth weights from younger women (aged 2744 years) (n = 220, 0.2 percent of the study population) were examined in a previous study (14). The authors found a Spearman correlation coefficient of r = 0.74 and concluded that this showed good agreement between self-reported birth weight and original birth records. However, the correlation between methods is only a preliminary step in determining whether there is agreement between two methods of measurement (23
, 26
). Correlation provides a measurement of the strength of a relation between variables, while perfect agreement will occur only if all points lie on the line of equality (23
).
In another validation study, moderate agreement was observed between reported and recorded birth weights by using the kappa measure of agreement between categorical assessments (n = 79. 8 percent of population) (10). A later validation study on a larger portion of their population (n = 661, 36 percent of population) found a Spearman correlation coefficient of approximately r = 0.82 for cases and controls combined for categorized self-reported birth weights and birth weights from birth certificates for women less than age 44 years (15
). That study found differential measurement error between cases and controls, which resulted in biased odds ratios despite the high correlation (15
).
The objective of a study of method comparison is to determine whether exposure estimates are valid, characterized by accuracy of the self-reported exposure compared with a standard measurement (original records) evaluated by measuring agreement (27). In our study, we found a Spearman correlation between self-reported and recorded birth weights similar to that found in a previous study (14
) (r = 0.76 and r = 0.74, respectively). We, however, took the analysis one step further to look at the agreement between the two measures. The plot of difference versus mean showed that there was poor agreement between the two methods of determining birth weight (figure 2). The estimated limits of agreement found that reported birth weights may be -1,028 g below to 1,038 g above the true birth weight, which is clinically unacceptable. More than 50 percent of the reported birth weights were positively or negatively discordant with the recorded birth weight by 250 g or more (table 2). This could lead to misclassification and potential errors in conclusions drawn based on self-reported birth weight data.
In this study, we were unable to test for differential recall or recall bias or for whether the recall (self-report) was simply impaired (28, 29
). What was the source of information for self-reporting? In the case of our study, the questionnaire was sent home prior to the date of examination at the study center. The participants therefore could have had access to personal records and, for the younger cohorts, even had the possibility of questioning their mothers. We expected that participants with a living mother would be more likely to report a birth weight and to report accurately. We found, however, that only 40 percent of those with a living mother reported a birth weight at all and that, of those reporting, nearly half of the birth weights were discordant by 250 g or more. Therefore, possible maternal assistance was of limited value in this study. One might speculate that the majority of reporters approximated a birth weight based on hearsay, such as "you were extremely small/big or average." Figure 1 and 2 support this speculation.
What are the implications of applying self-reported birth weight in further analysis? We found conflicting results, whether based on self-reported birth weights or on original records, with respect to known important risk factors for cardiovascular disease. This analysis illustrates the risk of drawing unfounded conclusions based on self-reported birth weights.
Current interest lies in clarifying mechanisms underlying the relation between birth weight and later disease. Imprecise self-reporting of birth weight, as found in our study, can result in false relations or overshadow important findings. It is quite clear that the ideal situation would be that all studies in this area be based on original birth records. In most countries, this is simply not possible, at least with data on births long enough ago in time that implications in later adulthood can be studied. If the alternative is to use some form of self-reporting, then the validity of such methods must be carefully assessed. Correlation coefficients alone do not provide a valid argument. A plot of the difference versus the mean of the reported and original birth data will provide a visual guide to the interpretation of the relation. Even with a relatively high correlation coefficient, as is seen in both this and previous studies (14, 15
), we found that there was poor agreement between methods when assessed in women aged 4460 years. We conclude that self-reported birth data from middle-aged women would not be an adequate replacement for birth weights from original delivery records.
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ACKNOWLEDGMENTS |
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NOTES |
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REFERENCES |
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