Validity of Birth Certificate Data for the Outcome of the Previous Pregnancy, Georgia, 1980–1995

Melissa Adams

From the World Health Organization Collaborating Center in Perinatal Care and Health Services Research in Maternal Child Health, Division of Reproductive Health, Centers for Disease Control and Prevention, Atlanta, GA.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The author evaluated the validity of four historically based variables collected on Georgia birth certificates: outcome of preceding pregnancy, history of delivery of a low- (<2,500 g) or high- (>4,000 g) birth-weight infant, and death of the baby resulting from the preceding pregnancy. Data were derived from birth and fetal death certificates that were linked for the first and second deliveries of 231,075 women in Georgia from 1980 through 1995. Deaths that occurred during the infant's first year of life were also linked to the birth certificate. For all but the survival variable, the outcome of the first birth as reported on the certificate for the second birth was compared with the outcome recorded on the certificate for the first birth, which was assumed to be correct. Except for ascertainment of death of the firstborn infant, sensitivities for the history of poor outcomes were low. Furthermore, sensitivities were higher when an extremely adverse outcome occurred in the first pregnancy or an adverse outcome recurred. The only high sensitivity was for past infant death (85.4%). These results suggest caution when using these variables to identify high-risk subsets for further research or control for confounding.

birth certificates; sensitivity; specificity

Abbreviations: HBW, high birth weight (>4,000 g); LBW, low birth weight (<2,500 g); PVN, predictive value negative; PVP, predictive value positive; RR, relative risk


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In the United States, researchers in perinatal epidemiology rely heavily on birth certificates because they are the only source of uniformly collected data on birth outcomes and associated demographic and medical factors. Validation studies, however, have shown that some of the items collected on the confidential medical portion of the birth certificate have low sensitivity and specificity (1GoGoGoGoGoGoGoGoGoGo–11Go). Most validation studies have compared birth certificate information with other information recorded close to when the birth certificate was completed. Sources for this contemporaneous information include medical records (1GoGoGo–4Go, 7Go, 8Go, 10Go, 11Go), hospital discharge summaries (5Go), and maternal interviews (9Go). While use of these sources may be appropriate for validating events that occur during the index pregnancy, one may question their relevance for evaluating events, such as the outcome of the previous pregnancy, that occurred before the index pregnancy. To validate this kind of information, one needs to compare the current report with the medical record or birth certificate for the earlier event.

The availability of a maternally linked database of birth outcomes in Georgia permitted evaluation of the validity of four historically based variables: 1) history of delivery of a low-birth-weight ((LBW) <2,500 g) infant, 2) history of delivery of high-birth-weight ((HBW) >4,000 g) infant, 3) outcome of the preceding pregnancy, and 4) survival of previous livebirths. This report describes our evaluation of these variables and the implications for their use.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Data
All of the data for this study were derived from fetal death and birth certificates that had been linked for successive deliveries to an individual woman, creating individual reproductive histories (12Go). This database included certificates registered in Georgia from 1980 through 1995. All deaths in first year of life were linked to the infant's birth certificate. Details of the linkage methodology, which used deterministic and probabilistic methods, as well as its evaluation have been reported elsewhere (12Go, 13Go). Because linkage is most complete for first and second deliveries, I limited this analysis to these events. I further restricted the analysis to singleton infants with gestational ages 20–44 weeks or, if gestational age was unknown, to infants with birth weights of 500–5,000 g.

I used two kinds of standards to assess the accuracy of the four historical variables reported on the birth certificate for the second pregnancy. For LBW, HBW, and outcome, I assumed that the information on the birth certificate of the first delivery was accurate, a generally reasonable assumption for birth weight (1Go, 8Go). For survival of the previous livebirth, I used the presence or absence of a fetal or infant death certificate as the standard. I also assumed that the linkage of infant death certificates with birth certificates was complete and accurate.

I analyzed previous LBW and HBW for records of the second delivery from 1989 through 1995, previous outcome for 1980–1988, and survival of previous livebirth for the combined period (1980–1995). The Georgia birth certificate changed substantially in January 1989, and our analysis reflects that change. Only for second deliveries that occurred in 1980–1988, for example, was it feasible to study previous outcome. In those years, the birth certificate included the item "Result of the Last Pregnancy." Respondents were instructed to "check one box only" and were given the following choices, presented as they appeared on the certificate: I defined premature as less than 37 weeks of gestation. I excluded the 12.4 percent of mothers for whom the gestational age of the first infant was unknown or implausible, given the infant's birth weight.



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Previous LBW and HBW could be studied only from 1989 onward because, beginning then, the medical portion of the Georgia birth certificate contained a 19-item check-box section entitled "Medical Risk Factors For This Pregnancy." Instructions on the certificate directed the informant to "check all that apply." I assumed that the absence of a check mark indicated that the condition was absent. Check boxes were provided for the following two historical items, shown as listed on the certificate: To validate these items, I compared the presence or absence of a check mark on the second certificate with the birth weight on the first certificate. The second item encompasses three overlapping conditions, for which I created two subgroups of infants in supplemental analyses: 1) LBW (<2,500 g) and 2) LBW, preterm (<37 weeks), or small for gestational age. Analyses of these two subgroups yielded nearly the same results, but use of the second group required me to exclude the 12.4 percent of mothers whose first births had missing or implausible gestational ages. Assuming that information on history of a previous preterm, small-for-gestational age, or LBW infant usually came from a maternal report and that mothers would be more likely to correctly report birth weight than gestational age (14Go, 15Go), I present the results for LBW only.



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Analyses of the survival of a previous livebirth was possible for both time periods, and I performed computations for each as well as the two combined. From 1980 through 1988, informants were asked to state the number of previous livebirths "now living" and "now dead" by two weight categories: 2,500 g or 51/2 pounds or less and more than 2,500 g or 51/2 pounds. From 1989 onward, informants were simply asked to state the number of previous livebirths now living and now dead. Collapsing the 1980–1988 data across birth weight and using data from the second delivery, I created a variable indicating the reported vital status of the first delivery. I compared this variable with survival of the first infant as determined by the presence or absence of a death certificate. Because I lacked certificates for deaths that occurred after the first year of life, I anticipated that I would verify fewer deaths than were reported. I judged this omission to influence computation of sensitivity minimally. Because predictive value positive (PVP) was based on all past deaths regardless of age when they occurred, I was concerned that postinfancy events would distort our results. Thus, I limited computation of PVP to pairs of infants 2 or fewer years apart. I judged that, for these sibling pairs, most of the deaths among firstborn children occurred during infancy, when the risk of death is considerably higher than during the second year of life.

Statistical analysis
I computed the sensitivity, specificity, PVP, and predictive value negative (PVN) (16Go) of all four historical variables. I hypothesized that the accuracy of reporting on the second certificate would be associated with the extent of morbidity for the first and second births. For example, I thought that reporting a true history of HBW would be more likely if either or both infants weighed more than 4,250 g. I used stratified analyses to explore this hypothesis.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
I identified 231,075 women who met the criteria for inclusion in the study. The great majority (98.4 percent) had delivered live infants in their first and second pregnancies; for 1.6 percent, either their first or second pregnancy had resulted in a stillborn infant. Birth weight was missing for 0.5 percent of the first deliveries and 0.8 percent of the second deliveries.

Outcome of last pregnancy
Analysis of the outcome of the previous pregnancy was based on 85,418 women whose second delivery occurred from 1980 through 1988 and for whom a plausible gestational age was available for their first delivery. I excluded records for 1,363 other women because the outcome of the first pregnancy was missing. In general, sensitivities were low for poor outcomes (table 1), a pattern repeated throughout the results. For example, the overall sensitivity for premature delivery of a liveborn infant in the preceding birth was only 28.9 percent, i.e., just 28.9 percent of the births identified as preterm on the first birth certificate were identified as such on the certificate for the mother's second child. Sensitivities for stillbirths were 49.7 percent (for >=37 weeks) and 67.0 percent (for 20–36 weeks), but these results must be interpreted cautiously because 42 percent of the stillbirths were excluded because of missing data on gestational age.


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TABLE 1. Accuracy of outcome of last pregnancy reported on birth certificate for the second-born infant, Georgia, 1980–1988

 
Sensitivity was higher for first-delivery prematurity when either the first or the second pregnancy resulted in an adverse outcome (data not shown). Sensitivity for premature birth in the first delivery was 47.3 percent for the 1,265 women whose second baby was LBW. The comparable sensitivity was 62.5 percent for the 72 women whose second infant died in the early neonatal period (<8 days); it was 40.6 percent for the 64 women whose second infant died postneonatally (28–365 days). The PVP for prematurity was 67.8 percent; for stillbirth at 20–36 weeks, it was 70.4 percent; and for stillbirth at >=37 weeks, it was 82.8 percent. In other words, 67.8 percent of firstborn infants identified as premature on the second birth certificate were actually premature, according to the first certificate. Both specificity (98.4 percent) and PVN (92.6 percent) were high when the first birth was full term.

History of LBW or HBW
Analysis of previous LBW or HBW included 130,806 women whose second delivery occurred from 1989 through 1995 and for whom the infant's birth weight was known. None of the infants had certificates on which the boxes for both LBW and HBW were checked. Sensitivities for a history of either LBW or HBW were very low (table 2). Stratified analysis showed that sensitivity for reporting LBW increased as the birth weight of the first child decreased or when the second child also was LBW (figure 1). Comparable patterns were found for reporting HBW. The PVPs for LBW (70.9 percent) and HBW (77.8 percent) were high, as was the PVN for normal birth weight (86.0 percent).


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TABLE 2. Accuracy of low or high birth weight for the first birth, as reported on the certificate for the second-born infant, Georgia, 1989–1995

 


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FIGURE 1. Sensitivity of history of prior delivery of a low-birth-weight or high-birth-weight infant on the certificate of the second-born sibling, by birth weight of the firstborn and second-born sibling, Georgia, 1989–1995.

 
Death of previous liveborn infants
Analysis of the survival of the first livebirth included 229,276 women; an additional 26 women were excluded because information on the number of past livebirths, as indicated by lack of entries in the boxes, was missing. Both sensitivity (85.4 percent) and PVP (77.6 percent) for death of the first infant were at least reasonably high (table 3), and both increased from 1980–1988 to 1989–1995 from 82.8 percent to 88.2 percent for sensitivity and from 74.5 percent to 82.1 percent for PVP. In both periods, sensitivity was slightly higher if the death occurred in the early neonatal period (87.6 percent) rather than in the late neonatal (83.9 percent) or postneonatal (83.4 percent) periods. Sensitivity did not vary appreciably by survival of the second infant or birth weight of the first or second infant. It was not related to the number of years between births in a consistent pattern. Among infants born less than 2 years after the first birth, the PVP for infant death was 87.4 percent for 1980–1988 and 96.4 percent for 1989–1995.


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TABLE 3. Accuracy of report of vital status of the firstborn infant on the certificate for the second-born infant, Georgia, 1980–1995*

 

    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In this evaluation of four historically derived birth certificate variables, I observed very low to moderate sensitivities, except for infant death, for which sensitivity was quite high (85.4 percent). In contrast, PVPs were all moderately high, i.e., if there was a report on the second birth certificate that a particular adverse outcome had occurred in the first pregnancy, chances were fairly good that the first birth certificate confirmed the report. Except for infant death, the sensitivity for each historical measure I studied was higher for women who had that adverse outcome (e.g., LBW) in either pregnancy than for those who did not. Finally, specificities were high (>98 percent) for past normal outcomes (full term or normal birth weight), i.e., the chances were excellent that they would be reported as normal.

Several methodological considerations may influence interpretation of these results. Our study is enhanced by the availability of population-based data and linking certificates for successive births to the same woman. Use of information from the first certificate as the gold standard for LBW, HBW, and outcome, however, might be challenged. Although birth weight on birth certificates is generally considered to be very accurate, it is not perfect (1Go, 8Go). In addition, a number of studies have raised questions about the accuracy of gestational age on the birth certificate (4Go, 9Go). Because I lacked information about deaths that occurred after infancy, I elected to compute PVP only when the interval between the first and second births was less than 2 years. The result for this interval does not generalize to longer intervals. Finally, only for report of past infant death and history of a HBW infant is Georgia's experience likely to generalize to other states. Result of the last pregnancy, collected on the Georgia certificate from 1980 to 1988, was unique to the state. The item "Previous preterm or small-for-gestational age infant," recommended for the 1989 US Standard Certificate, differs from the Georgia version, which adds previous births of less than 2,500 g.

What are the implications of these findings for clinicians and researchers? A likely use of these historical variables is to estimate risks of occurrence and recurrence and the percentage of occurrence of a phenomenon in the second pregnancy (e.g., LBW) that is associated with recurrence. Because of the low sensitivities for most of these variables, they will substantially underestimate occurrence risks. For example, according to the first birth certificate, the true risk of LBW in the first pregnancy is 6.8 percent, but historical information on the second birth certificate indicates that it is only 1.0 percent. Similar results were observed by Green et al. (6Go) in an evaluation of the accuracy of the historical variable "vaginal birth after past cesarean section." The low sensitivity of this variable resulted in a substantial underestimate of the true rate for vaginal birth after cesarean section.

On the other hand, because of the relatively high PVPs of these historical variables, most will estimate recurrence risks reasonably well. For example, using information from the first certificate, I computed the recurrence risk for LBW as 22.5 percent. Using history of LBW as reported on the second certificate, I estimated the recurrence risk as only about one third higher, or 30.5 percent. Similarly, the recurrence risk for infant death is 2.7 percent when computed using the presence of a death certificate to ascertain survival of the first infant and 2.2 percent when computed from information on the second certificate.

To set priorities for public health, one may also want to identify the percentage of an adverse outcome that is associated with recurrence, but in most cases, low sensitivity would distort the estimate. For example, using data from the first birth certificate, I estimated that 28.9 percent of second-born infants with LBW are associated with a history of LBW. Using data on the history of LBW recorded on the second birth certificate, I estimated that 5.6 percent of the second-born infants with LBW are associated with a history of LBW.

Another use of these historical variables would be to assess interactions or to adjust for confounding effects. Here, even for variables with a high PVP, low sensitivity could distort the findings. Consider the example of the association between birth weight of the second infant (<2,500 vs. >=2,500 g) and maternal education (coded as 9–11 years and >=16 years), for which one might want to assess whether the weight of the mother's first infant modified or confounded this association. The relative risk (RR) is 1.7 for the crude association between education and birth weight of the second infant. Using infant weight recorded on the first birth certificate, stratum-specific RRs are 1.3 (<2,500 g) for mothers whose first baby was LBW and 1.7 (>=2,500 g) for mothers whose first baby was normal birth weight; when weight of the first baby is adjusted for, the RR is 1.5. In this case, the Breslow-Day test for homogeneity of stratum-specific effects is highly significant (p < 0.001). Using history of LBW recorded on the second birth certificate, however, the comparable RRs are 1.5, 1.7, and 1.7, and the Breslow-Day test is not significant (p < 0.449). In a parallel example, I considered death of the first infant as the potential effect modifier or confounder. In this case, the analysis yielded the same results, regardless of whether death was ascertained from a death certificate linked to the first birth certificate or from the history on the second certificate. Here, however, the history of infant death recorded on the second birth certificate has both a high sensitivity and a high PVP.

Finally, these historical variables might be used to identify high-risk subgroups for subsequent analysis. Because of the low sensitivity and bias in recording history of adverse pregnancy outcomes other than infant death, however, use of these variables would be unlikely to lead to generalizable results.

What are the implications of these findings for vital registration? Given the variety of methods by which natality data are collected, the prospect of substantially improving the accuracy of these historical variables and many of the other data elements collected on birth certificates seems daunting. One possibility seems to be wider adoption of check boxes, which have been shown to result in better ascertainment of birth complications than an open-ended query about these conditions (17Go). Limiting the amount of information requested may improve data quality as well, as suggested by our findings that ascertainment of past infant death was better when this information was obtained by asking only two items on the birth certificate (during 1989–1995) than when it was obtained through four items (1980–1988). In addition to retaining the check box approach, I propose empirically evaluating the possibility that the quality of the data on the confidential medical portion of the certificate can be improved by combining a reduction in the number of data elements collected with ongoing training and periodic audits that emphasize the importance of accuracy.


    NOTES
 
Correspondence to Dr. Melissa Adams, 1696 Council Bluff Drive NE, Atlanta, GA 30345-4176 (e-mail: melissaadams{at}mediaone.net).

Editor's note: An invited commentary on this article appears on page 889.


    REFERENCES
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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Received for publication September 11, 2000. Accepted for publication August 28, 2001.