Impact of Prenatal Glucose Screening on the Diagnosis of Gestational Diabetes and on Pregnancy Outcomes
Shi Wu Wen1,
Shiliang Liu1,
Michael S. Kramer2,3,
K. S. Joseph4,
Cheryl Levitt5,
Sylvie Marcoux6 and
Robert M. Liston7
1 Bureau of Reproductive and Child Health, Health Canada, Ottawa, Ontario, Canada.
2 Department of Epidemiology and Biostatistics, McGill University Faculty of Medicine, Montreal, Quebec, Canada.
3 Department of Pediatrics, McGill University Faculty of Medicine, Montreal, Quebec, Canada.
4 Department of Pediatrics, Dalhousie University, Halifax, Nova Scotia, Canada.
5 Department of Family Medicine, McMaster University Faculty of Health Sciences, Hamilton, Ontario, Canada.
6 Epidemiology Research Group, Laval University, Quebec City, Quebec, Canada.
7 Department of Obstetrics and Gynecology, University of British Columbia, Vancouver, British Columbia, Canada.
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ABSTRACT
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The authors examined the impact of universal screening on the diagnosis of gestational diabetes and its complications. All mothers and newborns registered by the Canadian Institute for Health Information from 1984 to 1996 (even-numbered fiscal years only) were included in the analysis. Over this time period, the proportion of women with gestational diabetes increased ninefold (from 0.3% to 2.7%) while the proportion with prepregnancy diabetes fell from 0.7% to 0.4%. As rates of gestational diabetes increased, a corresponding reduction in the risks of complications (polyhydramnios, amniotic cavity infection, cesarean delivery, and preeclampsia) occurred for women with gestational diabetes. The incidence of gestational diabetes fell in Metro-Hamilton (where screening was discontinued in 1989) but remained high in the rest of Ontario (where screening continued in most areas). No related temporal trends for fetal macrosomia, cesarean delivery, or other diabetes-related complications were observed, regardless of screening policy. The authors concluded that the substantial increase in gestational diabetes in Canada is an artifact caused by universal screening, with no evidence of beneficial effects on pregnancy outcomes.
cesarean section; delivery; diabetes; gestational; fetal macrosomia; pregnancy
Abbreviations:
CCP, Canadian Classification of Diagnostic, Therapeutic, and Surgical Procedures; CIHI, Canadian Institute for Health Information; ICD-9-CM, International Classification of Diseases, Ninth Revision, Clinical Modification.
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INTRODUCTION
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In 1985, the Second International Workshop-Conference on Gestational Diabetes Mellitus recommended routine screening for gestational diabetes with an oral glucose challenge test late in the second trimester (1
). This position was reaffirmed in 1986 by the American Diabetes Association (2
). To our knowledge, no study has examined whether these guidelines have influenced the incidence of gestational diabetes or diabetes-related adverse pregnancy outcomes in the general population. In this study, we assessed the impact of these guidelines by analyzing temporal and geographic trends in gestational diabetes and diabetes-related pregnancy outcomes in Canada from 1984 to 1996.
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MATERIALS AND METHODS
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This study was based on hospital admission and/or separation records collected by the Canadian Institute for Health Information (CIHI). In Canada, almost all women give birth in hospitals, and about 78 percent of Canadian hospitals send their records to CIHI. Births captured by CIHI are comparable to all births in Canada with respect to major maternal and infant characteristics (3
). We chose for analysis a time period (April 1, 1984, to March 31, 1997) that included the fiscal years before and after publication of the 19851986 recommendations for routine screening for gestational diabetes (1
, 2
). Since initial analysis revealed that differences between successive years were small, data from only even-numbered fiscal years are presented here. Data on delivering mothers were abstracted from the entire CIHI file by using "case mix group" codes that uniquely define obstetric deliveries. Variables examined in this study included maternal age and relevant diagnoses and procedures. Diagnoses were coded according to the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) (4
). Procedures were coded by using the Canadian Classification of Diagnostic, Therapeutic, and Surgical Procedures (CCP) (5
). Specific diagnostic and procedure codes used to define the study variables were as follows: gestational diabetes, ICD-9-CM code 6488; prepregnancy diabetes, ICD-9-CM code 6480; cesarean delivery, CCP code 86; preeclampsia, ICD-9-CM codes 6425 and 6426; polyhydramnios, ICD-9-CM code 657; and amniotic cavity infection, ICD-9-CM code 6584. In CIHI data, birth weight was coded from a newborn's records. We defined fetal macrosomia as a birth weight of >4,000 g by using the birth weight information abstracted from CIHI's neonatal records. For women and infants from the province of Ontario, region of residence (defined by postal code) was also used in a subanalysis by region.
We first looked for changes in the incidences of gestational diabetes and other pregnancy complications from 1984 to 1996. Relative risks and their 95 percent confidence intervals were then compared for various maternal complications in women with gestational diabetes and in women with prepregnancy diabetes for the fiscal years 1984/1986 and 1994/1996 to test the hypothesis that universal screening resulted in a diagnosis of "milder" cases of gestational diabetes in more recent years. Milder cases identified by screening were expected to have lower risks for diabetes-related complications, leading to lower relative risks.
Finally, we examined the rates of gestational diabetes and diabetes-related complications in two regions of the province of Ontario that have contrasting policies on prenatal screening. In Metro-Hamilton, opinion leaders at McMaster University were strongly against universal screening; in 1989, prenatal glucose screening was discontinued in medical centers covering a large proportion of Metro-Hamilton's population (Dr. Patrick Mohide, personal communication). Prenatal glucose screening was continued in most of the rest of Ontario.
All analyses for maternal variables were age-adjusted by using the direct method, with all mothers registered by CIHI in fiscal year 1984 as the standard. We cross-tabulated diagnostic codes for gestational diabetes and prepregnancy diabetes to assess whether overlap in coding these diagnoses occurred in our data. We also carried out supplementary analyses by including fiscal years 1980 and 1982 to assess the stability of rates in prescreening years and by excluding multifetal pregnancies to assess the potential impact of secular trends in these pregnancies during the study period. SAS software (SAS Institute, Inc., Cary, North Carolina) was used for all analyses.
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RESULTS
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In the seven even-numbered fiscal years studied, 1,729,225 mothers and 1,738,863 newborns were registered by CIHI. A total of 38,274 mothers were diagnosed as having gestational diabetes. The age-adjusted rate of gestational diabetes increased from 0.3 percent in 1984 to 2.7 percent in 1996, while the age-adjusted rate of prepregnancy diabetes decreased from 0.7 percent in 1984 to 0.4 percent in 1996 (figure 1). Overall (i.e., among all pregnant women) rates of several pregnancy complications also increased slightly (figure 2), although cesarean delivery rates decreased slightly and fetal macrosomia rates were stable.
Among women with gestational diabetes, the incidence of diabetes-related pregnancy complications decreased substantially, while these complications rose sharply among women with prepregnancy diabetes (figure 3). We found temporal decreases in the relative risks of diabetes-related pregnancy complications among women with gestational diabetes; in contrast, relative risks for diabetes-related pregnancy complications increased over time among women with prepregnancy diabetes (table 1).
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TABLE 1. Age-adjusted relative risks (95% confidence intervals) of various pregnancy complications in women with gestational diabetes and in women with prepregnancy diabetes, Canadian Institute for Health Information, 19841986 and 19941996
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Cross-tabulation for ICD-9-CM codes defining gestational diabetes (6488) and prepregnancy diabetes (6480) showed no overlap in the two diagnostic codes. Supplementary analyses that included fiscal years 1980 and 1982 or excluded multi-fetal pregnancies yielded similar results (data not shown).
In the seven even-numbered fiscal years studied, CIHI received hospital records for 970,823 mothers and 985,653 newborns from the province of Ontario, for whom the postal codes of 875,318 (90.2 percent) mothers and 889,142 (90.2 percent) newborns were valid. According to the postal codes, 39,137 mothers and 39,706 newborns resided in Metro-Hamilton, and 836,181 mothers and 849,436 newborns resided in the rest of Ontario. Among mothers in Metro-Hamilton and the rest of Ontario, 378 (1.0 percent) and 12,238 (1.5 percent), respectively, had gestational diabetes. Age-adjusted rates of gestational diabetes increased similarly in both Metro-Hamilton and the rest of Ontario until 1990, when a different pattern emerged: rates in the rest of Ontario continued to rise, while rates in Metro-Hamilton decreased (figure 4).

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FIGURE 4. Age-adjusted incidence of gestational diabetes in Metro-Hamilton versus the rest of Ontario, Canada, 19841996.
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Among mothers in Metro-Hamilton and the rest of Ontario, 7,397 (18.9 percent) and 156,879 (18.8 percent), respectively, had a cesarean delivery; 1,372 (3.5 percent) and 24,588 (2.9 percent), respectively, had preeclampsia; 192 (0.5 percent) and 2,458 (0.3 percent), respectively, had polyhydramnios; and 182 (0.5 percent) and 7,614 (0.9 percent), respectively, had amniotic infection. Among newborns in the two regions, 5,025 (12.7 percent) and 106,118 (12.5 percent), respectively, had fetal macrosomia. Rates of polyhydramnios and amniotic infection increased and rates of cesarean delivery decreased during the study period in both regions, whereas rates of fetal macrosomia and preeclampsia fluctuated (figure 5).
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DISCUSSION
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The recorded incidence of gestational diabetes increased almost ninefold in Canada over one decade: from 0.3 percent in 1984 to 2.7 percent in 1996. Many factors, including diagnostic criteria, laboratory testing methods, maternal age, parity, and obesity, can affect the rate of diagnosed gestational diabetes (6
8
). These and some other unmeasured factors may account for part of the increased incidence of gestational diabetes and of several related complications such as polyhydramnios and amniotic infection. However, universal prenatal screening seems to be the most important explanation for the recent increase in the rate of gestational diabetes. This inference is supported by several findings. First, the increase occurred after publication of the guidelines recommending universal screening (1
, 2
). These guidelines were highly influential, as shown by a 1991 survey of 206 obstetric leaders in North America, 97 percent of whom practiced universal screening by using a 50-g glucose challenge test (9
). Second, the rate of prepregnancy diabetes (a condition also characterized by hyperglycemia but diagnosed before pregnancy) decreased during the same time period. We speculate that more inclusive criteria for gestational diabetes may have led to a diagnosis of gestational diabetes in some women who would formerly have been diagnosed with prepregnancy diabetes.
Finally, and most importantly, the secular trends in gestational diabetes in two regions of Ontario corresponded closely to their screening policies. In Metro-Hamilton, where universal screening for gestational diabetes was discontinued in 1989, the rate of gestational diabetes began to decrease in 1990. In the rest of Ontario, however, screening continued after 1989, and the rate of gestational diabetes remained high. This regional difference in screening policy created a natural experiment that provides the most convincing evidence that the dramatic, recent increase in gestational diabetes in Canada is the result of universal screening.
Our comparison of the relative risks for pregnancy complications in women with and without gestational diabetes in earlier versus later years suggests that cases identified by more liberal screening may be "milder." The associations between gestational diabetes and pregnancy complications such as amniotic cavity infection and polyhydramnios observed in the early years of our study (1984 and 1986) are similar to the associations reported in studies conducted before the liberal screening guidelines were published (10
, 11
), while the associations in more recent years (1994 and 1996) are much weaker.
The results from the Ontario regional comparison suggest that universal screening for gestational diabetes may be of limited benefit. Despite the contrasting regional trends in diagnosis of gestational diabetes, no corresponding trends were observed for fetal macrosomia, cesarean delivery, or other pregnancy complications (figures 4 and 5).
Unfortunately, CIHI data do not contain information on such factors as prepregnancy body mass index or gestational weight gain, which could confound the secular trends in gestational diabetes. Administrative data are also subject to coding errors (12
). However, it is unlikely that such large trends as those observed in our study can be attributed to such factors.
Universal screening for gestational diabetes is hotly debated (13
15
). Preliminary findings from our study suggest that a substantial proportion of pregnant women with "mild" degrees of hyperglycemia may have been diagnosed with gestational diabetes as a result of universal screening during the later 1980s and early 1990s, with no apparent benefit in diabetes-related pregnancy outcomes. Randomized trials are needed to shed further light on the relative benefits and risks of universal screening for gestational diabetes.
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ACKNOWLEDGMENTS
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This study was carried out under the auspices of the Canadian Perinatal Surveillance System.
Dr. Kramer is a Distinguished Scientist of the Medical Research Council of Canada, and Dr. Marcoux is a National Health Research Scholar of Health Canada.
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NOTES
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Reprint requests to Dr. Shi Wu Wen, Bureau of Reproductive and Child Health, Health Canada, Tunney's Pasture, Building #7, First Floor, AL0701D, Ottawa, Ontario, Canada K1A 0L2 (e-mail: Shi_Wu_Wen{at}hc-sc.gc.ca).
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Received for publication September 3, 1999.
Accepted for publication August 11, 2000.