The Prevalence of Gestational Diabetes

A Population-Based Analysis of a Nationwide Screening Program

Background: Gestational diabetes mellitus (GDM) is defined as a glucose tolerance disorder that arises during pregnancy. Estimates of its prevalence vary widely because of varying threshold values. Screening of all pregnant women with a two-step test has been available in Germany since 2012. This study is the first population-based, nationwide analysis of the screening coverage and the resulting one-year prevalence.

Methods: Billing data from the outpatient sector were analyzed for all persons covered by statutory health insurance in the two-year period 2014–2015. A cohort of pregnant women, constructed by using pregnancy care billing data, was studied with respect to the screening coverage. The prevalence of GDM was determined from the use of the corresponding ICD-10-GM codes.

Results: 80.8% of 567 191 pregnant women were screened for GDM. Most of them (63.3%) received only the pre-test, and 12.7% received both the pre-test and the diagnostic test. 4.8% received only the diagnostic test. The overall prevalence of GDM was 13.2%. The prevalence rose with age, from 8% to 26% in women aged 45 or older. Younger women more commonly received only the pre-test; the frequency of receiving both tests rose with age.

Conclusion: Screening for GDM is comprehensively implemented. The analysis of billing data reveals a relatively high prevalence that accords with estimates in other countries, implying that earlier prevalence figures for Germany were probably underestimates.

The clinical perspective

Gestational diabetes mellitus (GDM) is defined as a glucose intolerance which is first diagnosed in pregnancy and remains below the cutoff value for manifest diabetes (1, 2). Although asymptomatic in its clinical course, GDM is associated with an increased risk of complications related to pregnancy and childbirth (3–5). In the long term, the risk of developing manifest type 2 diabetes (T2D) is significantly increased in women with GDM in the years following initial diagnosis (6–9). Depending on study design and methods, particularly on follow-up period and risk structure of the study sample, the rates for women developing T2D after GDM vary considerably, ranging between 3% and over 90% (7, 8). GDM during pregnancy is associated with an up to 7-fold increase in the risk of manifest T2D compared with normoglycemic pregnancies (6, 9).

The screening offer for all pregnant women was introduced within the framework of the Maternity Directive of the Federal Joint Committee (G-BA, Gemeinsamer Bundesausschuss) in 2012 (10). The G-BA used an expert report from the Institute for Quality and Efficiency in Health Care (IQWIiG, Institut für Qualität und Wirtschaftlichkeit im Gesundheitswesen) as a basis for this decision (11). According to this report, data from randomized controlled trials showed that pregnant women with GDM who were identified using a two-step screening procedure and received diabetes care/therapy experienced significantly fewer complications related to pregnancy and childbirth (risk reduction for shoulder dystocias: 60%; for preeclampsia: 36%) (3, 5, 11). The screening program defined in the Maternity Directive includes a two-step test procedure to be offered to pregnant women between 24 and 28 weeks’ gestation. This consists of a “pre-test” and a “diagnostic test”. The latter—the oral glucose tolerance test (oGTT)—is performed if the pre-test returns an abnormal result. If the oGTT is positive, the pregnant woman’s further care is provided in close collaboration with a physician qualified in diabetology. The screening procedure and the cutoff values are summarized in the Table.

Table

Screening steps and cutoff values of the two-step test for GDM

The criteria and cutoff values for the diagnosis of GDM differ internationally (12). Depending on the test strategies used, estimated prevalence rates show considerable variation (13–15)—with an increasing trend over the last decades (16–18). Globally, the prevalence of hyperglycemia in pregnancy (GDM and manifest T2D in pregnancy) is estimated to be approx. 15% (15); for Europe, the prevalence is 12.6% (15). Prior to the introduction of the standardized screening program in 2012, GDM screening in Germany was undertaken on the basis of selective contracts; consequently, very different cutoffs and test strategies were used. This is reflected in the wide variation of the prevalence rates estimated during that period which vary between 2% and more than 18% (19–21). The most comprehensive data to be used for prevalence estimation in Germany are collected in maternity hospitals within the “Quality Assurance in Obstetrics” framework (former Perinatal Survey). For this, relevant data from the Mutterpass (German maternity record) are entered into the hospital documentation system. Analysis of these data showed a steady increase in GDM during the years prior to the introduction of screening, from 2.3% in 2005 to 4.3% in 2012 and since then to 5% in 2015 (22). A recent study, evaluating the implementation of standardized screening based on outpatient service data from the North Rhine region arrived at a GDM prevalence of 6.8% for the year 2013/2014 (23). An overview of the national and international GDM prevalence studies is provided in eTable 1.

eTable 1

Summary of selected recent studies on the prevalence of gestational diabetes

This study is the first population-based research to evaluate the nationwide implementation of the two-step screening program and the resulting 1-year prevalence for all pregnant women in Germany covered by statutory health insurance. The following research questions have been investigated:

•  How many pregnant women are tested for GDM during pregnancy and which method is used?
•  What is the 1-year prevalence of GDM?

Methods

Data source

The data source for this study is the nationwide panel doctor billing data set (referred to as “service data” in the following) of all approx. 71 million members of all German statutory health insurances (24) during the observation period from 1 January 2014 to 31 December 2015. This includes information about billed services and diagnoses. Billing of services rendered by panel doctors is based on fee schedule items (GOP) listed in the German Uniform Value Scale (Einheitlicher Bewertungsmaßstab, EBM) (25).

Formation and validation of the pregnancy cohort

For the provision of care to a pregnant woman, the flat fee-per-case ”GOP 01770” can be billed only once per quarter during pregnancy and up to 8 weeks after delivery by the treating doctor (25). For the formation of the pregnancy cohort, all women whose pregnancies started during the period from 1 July 2014 to 30 June 2015 were identified based on the billing of this flat fee in the respective quarter. This quarter of the individual start of pregnancy is here referred to as “index quarter.” To ensure that only new (incidental) pregnancies were included, it was a requirement that this flat fee-per-case had not been billed in the two quarters preceding the index quarter.

GDM screening is to be offered between 6 and 7 months’ gestation. To operationalize the existence of a pregnancy up to this point in time, women were only included in the pregnancy cohort if the flat fee was billed in the index quarter and the two consecutive quarters. Requiring that this flat fee was billed across 3 consecutive quarters was meant to ensure the exclusion of pregnancies which were not maintained to the screening period, for example due to spontaneous or induced abortion. The validation of the pregnancy cohort, based on the official statistics of live births (26, 27), is provided in eTable 2.

eTable 2

Validation of the pregnancy cohort (2010–2014)

Formation of the study cohort

To form the study cohort, those women were excluded who had—according to ICD-10-GM (International Statistical Classification Of Diseases and Related Health Problems, 10^th revision, German Modification)—in at least one of the two quarters preceding the index quarter a confirmed diagnosis of manifest diabetes (E10–E14 as well as O24.0–O24.3 [28]). The study cohort is the population on which the evaluation of the study questions is based.

Operationalization of screening implementation and prevalence rates

Each included pregnancy was individually assessed over three consecutive pregnancy quarters, starting from the index quarter (study period: 1 July 2014 to 31 December 2015). The two-step screening is represented by the GOP with the item numbers 01776 “Pre-test for gestational diabetes” and 01777 ”oGTT to rule out/confirm gestational diabetes” (25). For the study period it was assessed whether a GOP relevant for the respective test was billed at least once (screening implementation) and whether one of the two GDM diagnoses (O24.4 and O24.9) according to ICD-10-GM (28) was coded (prevalence estimation). In addition, it was assessed how frequently manifest diabetes (ICD-10-GM code E10–E14 as well as O24.0–O24.3) was diagnosed alone or in combination with GDM. Consistently, only confirmed diagnoses were included.

Statistical analyses

To calculate the screening implementation and prevalence rates, the percentage of persons insured with billed GOP/coded diagnosis in the study cohort was identified and described in an age-stratified way. The MicroStrategy Developer Version 10.5.0 (29) software was used for all analyses.

Results

Pregnancy cohort and study cohort

Figure 1 shows the flowchart for the formation of the pregnancy cohort and the study cohort. In 3.4% of the 38.5 million women, the pregnancy flat fee-per-case was billed at least once during the study period. For more than half of these women, pregnancies lasting at least three quarters were identified. An incidental pregnancy was found in 78.1% of these women; thus, the pregnancy cohort defined here consists of 572 554 women. This pregnancy cohort includes approximately 80% of the annual deliveries (live births) recorded in the official birth statistics (eBox).

Figure 1

Flowchart for the formation of the pregnancy cohort and the study cohort

eBox

Pregnancy cohort and official birth statistics

Altogether 0.9% (n = 5363) of the women in the pregnancy cohort were excluded from further analyses because they had already been diagnosed with manifest diabetes prior to pregnancy. Thus, the population on which the further analyses were based consisted of the 567 191 pregnant women remaining in the study cohort (Figure 1).

The mean age of these women at the estimated start of pregnancy (age in the index quarter) was 30 years, with a standard deviation of 5 years. The youngest pregnant woman in our study cohort was 13 years, the oldest 58 years old. Half of these women were aged between 27 and 34 years at the start of pregnancy (median: 30 years).

Implementation rates for the two-step screening

In 80.8% of all pregnant women, either the pre-test alone or the diagnostic test alone or both of the two test procedures were used (Figure 2). None of the two tests was performed in 19.2% of the pregnant women. Almost all tested women underwent at least the pre-test (94%); in 83.3% of these women no further oGTT-based testing was performed. In relation to the total study cohort, 12.7% underwent oGTT in addition to the pre-test. Another 4.8% underwent only oGTT. The distribution of the test methods is depicted in Figure 2.

Figure 2

Screening implementation and GDM prevalence according to method

Estimation of GDM prevalence

In our cohort, 13.2% of the pregnant women were diagnosed with GDM. Figure 2 shows the prevalence of GDM in relation to the test method used. Of the women who underwent at least one of the two tests, 11.4% were diagnosed with GDM. In the remaining 1.8% of women with GDM, the diagnosis was established, but neither of the two test methods was billed during the study period.

Figure 3 and Figure 4 depict the age-stratified distributions of the test methods and the GDM prevalence. The use of the pre-test alone declines with advancing age, while the use of the diagnostic test increases. With increasing age, the prevalence of GDM rises from less than 8% in the youngest age groups to over 26% among women ≥45 years of age. In older pregnant women, the diagnosis of GDM is more frequently based on the combination of the two test methods.

Figure 3

Age-stratified distribution of the test methods

Figure 4

Age-stratified distribution of GDM prevalence

Besides the diagnosis of GDM, the diagnosis of manifest diabetes was reported for the first time during the study period in 5956 women. This equals 1.0% of the study cohort. It can be assumed that these were pregnant women diagnosed for the first time with manifest diabetes (potentially as the result of GDM screening). If only women with GDM alone are included in the analysis, the overall prevalence of GDM is reduced to 12.2%.

Another 1831 pregnant women—equivalent to 0.3% of the study cohort—were first diagnosed with manifest diabetes in the pregnancy, but without additional GDM diagnosis.

If one looks at the pregnancy cohort regardless of any already diagnosed manifest diabetes or GDM, altogether 12 340 (2.2%) of 572 554 pregnant women had manifest diabetes during pregnancy (T1D: n = 4809; T2D and other manifest diabetes: n = 7531).

Discussion

This study is the first to provide outpatient care data on the nationwide implementation of GDM screening in Germany and the GDM prevalence among all statutory health insurance members. With a rate of more than 80%, GDM screening is comprehensively implemented. The remaining number of untested women reflects the voluntary nature of the screening program which is to be offered in a shared decision making process. More than two thirds of all pregnant women received the pre-test and in only 13% of women further testing based on the oral glucose tolerance test was required. This is in line with international data on the implementation of two-step test procedures for GDM (30). It can thus be assumed that the two-step screening strategy spares many women from having to undergo the more complex and demanding oGTT.

Our study found a 1-year prevalence of GDM of 13.2%. In 1% of the pregnant women, both GDM and manifest diabetes was diagnosed, indicating that it was possible to identify manifest diabetes in the process of the further evaluation of screening results. Altogether 13.5% were first diagnosed with a diabetogenic metabolic state (GDM and/or manifest diabetes) during pregnancy. With 13.2%, the GDM prevalence in our study is considerably above the 5% estimated in the Quality Report in Obstetrics for 2015 (22). The GDM data collected in maternity hospitals from the Mutterpass may underestimate the actual prevalence of GDM, because GDM testing is performed late in pregnancy and thus this diagnosis may not have been added to the Mutterpass in all cases. Furthermore, transfer errors may occur when the information from the Mutterpass is entered into the hospital documentation system after childbirth.

A recent study assessing GDM prevalence in the North Rhine region based on outpatient billing data—as we have used in our analysis—found a rate of 6.8% (23). This is considerably lower than our results presented here. This difference is explained, among other factors, by their definition of the pregnancy cohort which used the inclusion criterion of at least one billed flat fee (GOP 01770). Using this approach, the population also includes pregnant women who do not reach the screening period (6 to 7 months’ gestation), for example due to spontaneous or induced abortion; in these women a diagnosis according to the Maternity Directive cannot be established. By contrast, a study based on service data from the AOK Berlin statutory health insurance showed a considerably higher prevalence between 14% and 18%, because it used a stricter definition for the population (21). International studies estimate a prevalence of 13% for Europe (15).

Age is an important risk factor for GDM (31). Our analyses also show a considerable rise in GDM diagnoses with increasing age. This is also reflected in the implementation of screening: While younger women frequently underwent only the pre-test, which usually is sufficient to rule out GDM, a combination of the two test methods was increasingly used with rising age of the pregnant woman. This is indicative of an age-sensitive benefit of a two-step screening strategy.

Strengths and limitations

The data source of this study is the population of all pregnant women with statuary health insurance making use of outpatient care; thus, it represents the reality of care across health insurances and regions with regard to actual service provision and coding. Earlier studies evaluating GDM prevalence or screening implementation in Germany were limited to specific regions and/or health insurances; consequently, their representativeness of the total population was also limited (19, 21, 23).

Furthermore, the analyses relate to the period two years after the introduction of the screening program and one year after the introduction of the corresponding GOP. Thus, it can be assumed that there was enough time allowed for the nationwide roll-out of a new screening program to be able to capture its implementation.

A limitation of this study is the restricted validity of service data, as these are not collected for research purposes and suffer from several shortcomings. For example, diagnoses appearing without corresponding testing may have been established in a hospital setting or transferred from previous pregnancies where GDM was diagnosed. The prevalence estimates presented in our study are based on administrative data. Subject to the data source, this administrative prevalence represents a more or less accurate approximation to the actual prevalence.

Furthermore, the results depend on the definition and selection of the studied population and the operationalization of the care item. In our study, we aimed at accomplishing a content/medically-based and transparently presented cohort formation to achieve a valid estimate of the population of all pregnant women. By requiring billing of the pregnancy flat fee-per-case across three quarters, we wanted to ensure that the pregnancy had actually reached the period of the recommended screening examinations.

The comparison with official statistics on the number of live births shows a correspondence of approximately 80%. The difference of 20% is partly explained by the fact that pregnant women with private health insurance were not included in the analyzed service data. In addition, our pregnancy cohort did not account for women who changed their names during pregnancy. An “ongoing” pregnancy, as defined here, was found in 56% of all pregnant women. Pregnancy flat fees-per-case which were billed less frequently than in 3 consecutive quarters (44%) can be explained by method-related reasons (exclusion of pregnant women because of the selected cohort design, name change) and by medical reasons (for example, spontaneous or induced abortion, preterm birth). Finally, using service data it is not possible to continuously follow pregnant women who do not at all times receive services by the German outpatient care system.

Conclusion

GDM screening was widely implemented two years after its introduction by the Federal Joint Committee (G-BA). This indicates its increasing acceptance by pregnant women and their doctors. Especially the relatively low rate of pregnant women requiring oGTT is in line with the aim of the stepped screening strategy, as it reflects the ability of the pre-test to exclude a large proportion of pregnant women from further testing. The 1-year prevalence of GDM is with 13.2% in the range of current international prevalence estimates. Furthermore, the validation of the cohort we have used here and the age-based results are indicative of a reliable approximation to the actual prevalence. Prompt detection and treatment of GDM is necessary to prevent complications for both mother and child. However, based on the available data it is not possible to determine whether the high prevalence found is due to an increase in GDM cases or an increase in detection rates.

Data protection
This study complied with the provisions on data protection and conduct of analyses of the “Good Practice of Secondary Data Analysis” (GPS) guideline.

Conflict of interest statement

The authors declare that no conflict of interest exists.

Manuscript received on 13 January 2017, revised version accepted on 31 March 2017

Translated from the original German by Ralf Thoene, MD.

Corresponding author
Dr. phil. Hanne Melchior
Kassenärztliche Bundesvereinigung
Dezernat 7 – Sektorenübergreifende Qualitäts- und Versorgungskonzepte
Abteilung Indikationsbezogene Versorgungskonzepte
Herbert-Lewin-Platz 2,
10623 Berlin, Germany

Supplementary material
eBox, eTables:
www.aerzteblatt-international.de/17m0412

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