Implausible Birth Weight for Gestational Age

K. S. Joseph1,2, Michael S. Kramer3, Alexander C. Allen2, Leslie S. Mery1,4, Robert W. Platt3 and Shi Wu Wen1

1 Bureau of Reproductive and Child Health, Laboratory Centre for Disease Control, Ottawa, Ontario, Canada.
2 Perinatal Epidemiology Research Unit, Departments of Obstetrics and Gynecology and of Pediatrics, Dalhousie University, Halifax, Nova Scotia, Canada.
3 Departments of Pediatrics and of Epidemiology and Bio-statistics, McGill University, Montreal, Quebec, Canada.
4 Cancer Bureau, Laboratory Centre for Disease Control, Ottawa, Ontario, Canada.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Various rules have been proposed to identify and exclude live births with implausible values of birth weight for gestational age from large perinatal data sets. The authors carried out a preliminary evaluation of common rules by examining the frequency and nature of rule-based exclusions among live births in Canada (excluding Ontario) between 1992 and 1994. There were 625 (0.09%), 133 (0.02%), 170 (0.02%), and 2,858 (0.40%) live births identified for exclusion by a median birth weight for gestational age ±4 standard deviations (SD) rule, a ±5 SD rule, a rule based on expert clinical opinion, and a modification of Tukey's rule, respectively. The birth weight and gestational age distribution of the exclusions depended on the particular rule used; for example, 12.1% and 0.3% of live births of >=4,500 g were excluded under Tukey's rule and the rule based on expert opinion, respectively. Infant mortality rates among those excluded were 8–13 times higher than among all live births. Current rules for identifying implausible birth weight for gestational age tend to flag live births at high risk for infant death. Such rules may erroneously attenuate temporal trends in important perinatal outcomes.

birth weight; databases; epidemiologic studies; gestational age; infant; mortality

Abbreviations: SD, standard deviation


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Large, population-based perinatal data sets invariably contain implausible values of birth weight for gestational age in a small fraction of records. Various rules have been proposed to identify and exclude such live births, including statistical rules (1Go, 2Go), rules based on complex algorithms (3GoGo–5Go), and rules based on a combination of expert (clinical) opinion and statistical observation (6Go). These rules have been used to edit data prior to creating growth curves (1Go, 3GoGoGo–6Go) or testing hypotheses (2Go, 7Go).

Evaluation of available rules is hampered by the absence of readily available standards for the identification of erroneous values. The problem is compounded by the large number of observations typically encountered in the data sets at issue. We carried out a preliminary evaluation of various rules by examining the frequency and nature of exclusions with particular reference to rule specificity, that is, the ability of a rule to distinguish random data error from true pathology (two potential causes of extreme birth weight for gestational age).


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
We used data from Statistics Canada's linked databases of live births and infant deaths. Information in these databases is collated from live birth and death registrations (8Go). Information on birth weight and gestational age in the live birth registration is obtained from the responsible physician or the mother. Our primary analysis was restricted to live births of >=22 weeks of gestation born between 1992 and 1994. Births from the province of Ontario were excluded because of documented problems with data quality (9Go). Data for the years 1985–1987 were also analyzed as a second step in order to examine temporal differences with regard to exclusions.

We assessed the numbers of live births that would be excluded using 1) a birth weight cutoff of ±4 standard deviations (SD) from the median birth weight for gestational age, 2) a median birth weight ±5 SD cutoff, 3) a rule based on expert opinion (6Go), and 4) modification of Tukey's rule (1Go). The rule based on expert opinion specifies ranges of plausible birth weight for each gestational age (e.g., 375–3,000 g at 30 weeks, 375–3,250 g at 31 weeks) and was developed using expert clinical opinion and observed patterns of birth weight for gestational age (6Go). Under Tukey's rule, observations were excluded if the birth weight was more than twice the interquartile range above (or below) the 75th percentile (or 25th percentile) of birth weight for gestational age (1Go). We also contrasted the exclusions under each rule in terms of birth weight- and gestational age-specific infant mortality.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Between 1992 and 1994, there were 718,844 live births who were >=22 weeks of gestational age. Of these, 625 (0.09 percent) were identified for exclusion under the median birth weight ±4 SD rule; 102 (16.3 percent) live births were excluded as their birth weight was too low for the gestational age, while for 523 (83.7 percent) live births the birth weight was too high for the recorded gestational age. Similarly, 133 (0.02 percent), 170 (0.02 percent), and 2,858 (0.40 percent) live births were identified for exclusion by the median birth weight ±5 SD rule, the rule based on expert opinion, and Tukey's rule, respectively (table 1). Many more live births were identified as "too large" (as against "too small") for gestational age; all exclusions based on expert opinion were "too large." The more stringent rules excluded significant proportions of babies in the macrosomic range; for example, 12.1 percent of those who weighed >=4,500 g were excluded by Tukey's rule (table 1).


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TABLE 1. Numbers of live births in Canada, 1992–1994, and among those with implausible birth weight for gestational age under different rules

 
Infant mortality rates among those with an implausible birth weight for gestational age were 8–13 times higher than those among all live births (table 2). The infant mortality rate among exclusions under the median birth weight ±5 SD rule was 75.2 per 1,000 live births (95 percent confidence interval: 36.6, 133.9) compared with 5.8 per 1,000 live births (95 percent confidence interval: 5.7, 6.0) among all live births. Infant mortality rates were higher among both those identified as "too large" and those identified as "too small" for gestational age. Birth weight- and gestational age-specific comparisons of infant mortality revealed complex patterns (table 2). Among those with birth weights of <1,000 g, infant mortality rates were unexpectedly low for those excluded, suggesting birth weight (not gestational age) errors. Among the exclusions based on expert opinion, birth weight-specific comparisons revealed excess mortality, though gestational age-specific comparisons did not. Under the ±4 SD rule, the ±5 SD rule, and Tukey's rule, excess rates of infant death were observed among exclusions in several birth weight and gestational age categories (table 2).


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TABLE 2. Infant mortality rate (per 1,000 live births) in Canada (excluding Ontario), 1992–1994, and among exclusions

 
Examination of the causes of infant death among those identified as having implausible birth weight for gestational age identified plausible explanatory pathology in a few instances (e.g., microcephaly as the explanation for "too small" for gestational age). Sex- and plurality-specific analyses showed essentially the same results. Results using data for 1985–1987 were generally similar to those for 1992–1994, although fewer live births were identified as "too large" and more live births were identified as "too small." Exclusions identified as "too small" by the ±5 SD rule, the ±4 SD rule, and Tukey's rule all had higher rates of infant mortality in 1985–1987 compared with 1992–1994, although the differences were not statistically significant.


    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Our study shows wide variation in the frequency of exclusions under the different rules used to identify implausible values of birth weight for gestational age. Moreover, infant mortality among those deemed to have implausible values of birth weight for gestational age was substantially higher than that among all live births. If implausible values of birth weight for gestational age occur due to random errors (e.g., during data transcription), most errors would be among term normal weight births (given their relative frequency). The mortality experience of implausibles, under such a scenario, would be similar to that of all live births. The high rates of mortality among the exclusions identified by the different rules suggest that this is not the case, however. It is noteworthy that the infant mortality rate among live births with missing gestational age information was relatively low in 1992–1994 and moderately high in 1985–1987 (5.2 vs. 18.3 per 1,000 live births in 1992–1994 and 1985–1987, respectively). The infant mortality among those with missing birth weight was substantially higher during both periods (27.3 and 48.4 per 1,000 live births in 1992–1994 and 1985–1987, respectively) (10Go).

Gestational age errors that are due to one or more amenorrheic cycles will result in infants who are too small for gestational age, while spotting at the time of the first missed period will result in infants who are too large for gestational age. These explanations can account for the gestational age peaks observed at approximately 1 month before and/or 1 month after the modal gestational age, a phenomenon that is observed at lower birth weights (11Go). Faulty recall of gestational age or intentional underreporting of gestational age (to mask premarital conception) are other potential causes for implausible values of birth weight for gestational age. Errors in gestational age of ±1 month cannot account for the extreme values of birth weight for gestational age observed in our study, however.

An interesting finding of our study was the wide variation with regard to the birth weight and gestational age distribution of exclusions. The rule based on expert opinion did not find any live births to be too small for gestational age; no live birth of <1,500 g and only six live births of <2,500 g were excluded by this rule (table 1). This rule appears to identify potential birth weight errors when the birth weight is too high for gestational age; indeed, this was a primary motivation behind its formulation (6Go). Tukey's rule identified a large number of macrosomic live births for exclusion. Given that an increase in live births of >=4,500 g is part of a secular trend in fetal growth (12Go, 13Go), mechanistic use of such a rule can lead to an attenuation of real trends in fetal growth. Similarly, our finding that babies deemed to be "too small" for gestational age were more frequent and had a higher mortality in 1985–1987 (relative to 1992–1994) is suggestive of a temporal increase in obstetric intervention and/or an improvement in the neonatal care of extremely growth-restricted babies. Alternative explanations include changes in the modality of gestational age ascertainment (i.e., increasing use of ultrasonography). Exclusion of live births based on a mechanistic application of such rules may lead to the attenuation of these trends as well.

A limitation of our study was that we were not able to verify the birth weight and gestational age information among those with implausible combinations. However, it has been observed that, among fetal deaths, implausible birth weight for gestational age (>2 SDs from the mean) remained implausible even after correction with information in medical charts in 79 percent of instances (14Go).

In conclusion, the rules used to exclude implausible values of birth weight for gestational age typically exclude small numbers of live births. However, they tend to flag live births at high risk for infant death and may erroneously attenuate temporal trends in pregnancy outcomes. The potential impact of using any rule is likely to depend on the nature of the analysis. Deploying such a rule should be an ad hoc decision made on a study-by-study basis after considering the potential for introducing bias.


    ACKNOWLEDGMENTS
 
This study was carried out under the auspices of the Fetal and Infant Health Study Group of the Canadian Perinatal Surveillance System. Contributing members included Leslie Forrester, Russell Wilkins, Dr. Arne Ohlsson, and Dr. Reg Sauvé. Dr. Joseph is supported by a Research Investigator award from the IWK Grace Health Centre and a Clinical Research Scholarship from the Faculty of Medicine, Dalhousie University. Dr. Kramer is a Distinguished Scientist of the Canadian Institutes of Health Research.


    NOTES
 
Reprint requests to Dr. K. S. Joseph, Division of Neonatal Pediatrics, IWK Grace Health Centre, 5980 University Avenue, Halifax, Nova Scotia, Canada B3H 4N1 (e-mail: ks.joseph{at}np.iwkgrace.ns.ca).


    REFERENCES
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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Received for publication September 21, 1999. Accepted for publication May 1, 2000.