Introduction
Prevalence of GDM is influenced by (1) population characteristics, for example Asian or Middle Eastern ethnicity and obesity;44,117–120 (2) criteria used for GDM diagnosis, because lower glucose level thresholds will identify greater numbers of women with GDM;121–123 and (3) screening and testing strategy, because the application of universal – rather than selective – glucose tolerance testing leads to greater numbers of women tested, leading to increased numbers identified.124
Prevalence of GDM is increasing alongside rising levels of obesity and inactivity, which can increase insulin resistance,125 mirroring the increasing rate of type 2 diabetes in the non-pregnant population.
The shift from identifying women at future risk of type 2 diabetes, to trying to predict risk of perinatal and longer-term ill-health outcomes in the infants of women who have had GDM, has prompted changes to diagnostic criteria. Criteria with lower thresholds will identify more women at risk, thus increasing prevalence and if treatment strategies remain unchanged, costs will increase. However, providing treatment to more women may reduce the risk of perinatal and longer-term ill health, potentially saving money for the UK NHS (and the individual). Chapter 7 details a cost-effectiveness analysis that examines alternative identification and treatment strategies.
We have estimated the prevalence of GDM using different criteria for WB and SA women in the BiB cohort,22 described in Chapter 2 of this report. In this chapter, however, we report a systematic review to determine the prevalence of GDM in the UK and Irish obstetric population, using identified and eligible published reports. We also derive and compare estimates from three IPD cohorts (including that of the BiB study22). This section is reported in accordance with PRISMA guidelines.56
Methods
Search strategy
Searches were undertaken in July 2014 in MEDLINE, MEDLINE In-Process & Other Non-Indexed Citations, EMBASE, the Maternity and Infant Care database and CENTRAL. No date restrictions were applied to the searches; citations were restricted to English language only (see Appendix 7, Table 84).
Title and abstract screening and full-text screening were performed in duplicate by two reviewers with disagreements resolved by consensus or by a third reviewer.
Three cohort studies were eligible and provided data at the individual participant level:
the
BiB study
22 (John Wright, Bradford Institute for Health Research, September 2013)
the
Atlantic DIP study
59 from the Irish Atlantic seaboard (Fidelma Dunne, Department of Medicine, National University of Ireland, September 2013)
the Warwick/Coventry cohort,
60 unpublished data from Warwick Hospital, George Eliot Hospital, Nuneaton and University Hospital Coventry (Ponnusamy Saravanan, Warwick Medical School, September 2013).
Because IPD were available from an Irish cohort (Atlantic DIP59), we have considered prevalence of GDM in the UK and Ireland together.
Inclusion/exclusion criteria
This review sought to identify all cohorts of pregnant women in whole, or in part, in the UK or Republic of Ireland who were assessed for GDM.
The included studies had to have the following characteristics.
Population
Pregnant women from the UK or Republic of Ireland without pre-existing diabetes.
Diagnostic test
All women had to receive an OGTT (75 g or 100 g) in pregnancy to diagnose GDM using recognised diagnostic criteria, or with criteria reported in the paper.
Outcomes
Studies had to report numbers of women, with and without GDM, according to the diagnostic test used or the prevalence of GDM.
Study design
All published, unpublished and ongoing observational cohort studies, or cross-sectional studies reporting data for women resident in the UK or Republic of Ireland. Only studies published in English were included.
When multiple publications reported prevalence estimates for the same cohort of women only the most recent and comprehensive publication was included.
Quality assessment
We assessed the characteristics of all of the publication/study criteria (including the population, location and publication year) that were used to diagnose GDM and derive prevalence estimates.
Data extraction
The following data were extracted from each publication:
year of publication
location of the study
details of the population characteristics, for example ethnicity, age,
BMI distribution (if reported)
details of the
OGTT methods and diagnostic criteria used
total number of women with and without
GDM, or the prevalence of GDM
prevalence of
GDM in participant subgroups, such as ethnic group or
BMI group.
For the IPD, the prevalence of GDM was calculated, based on the reported OGTT glucose measurements, with GDM diagnosed according to a range of diagnostic criteria as described earlier in Table 1. Prevalence was also calculated by ethnic group (white, SA or ‘Other’) and by age categories using the modified WHO 1999 criteria11 (fasting glucose level of ≥ 6.1 mmol/l and 2-hour post-load glucose level of ≥ 7.8 mmol/l).
Synthesis methods
Prevalences of GDM, along with their 95% CI, were estimated from the data for each study. These prevalence estimates are shown on forest plots. Studies were categorised by GDM diagnostic criteria and year of publication, in order to investigate the effect of these factors (see and ).
Prevalence of GDM by year the study was undertaken and GDM criteria used. DPSG (EASD), Diabetic Pregnancy Study Group (of the European Association for the Study of Diabetes).
Estimated prevalence according to different GDM criteria in the IPD cohorts. See Table 1 for criteria thresholds.
Meta-analyses of the prevalence data were considered, but not performed because of the heterogeneity across the studies, particularly the diversity of diagnostic criteria used to diagnose GDM.
Results
The database searches identified 1591 references for checking (1196 following deduplication). After title and abstract screening, 92 publications were retrieved for full-text screening (17 of which were potentially relevant for the systematic review on risk factors and so kept for that review). The main reasons for exclusion were that the study was published only as a conference abstract and data reported were insufficient, or the study did not include a UK or Irish population. The full list of excluded citations with reasons is contained in Appendix 3, Table 64.
Of the 92 publications, 12 were potentially eligible for inclusion. We also identified three cohorts with IPD (the Atlantic DIP study,59 Warwick/Coventry60 and the BiB study22), reporting GDM prevalence for a UK or Irish cohort.42,44,118,126–134 After data extraction, two publications128,134 were excluded because they reported data from the same cohort. One additional paper (on the HAPO cohort6) was included, having been identified for another review undertaken as part of this project (see Chapter 3).131 One publication135 was excluded because it reported prevalence for the Atlantic DIP cohort59 for which IPD were available. After including the IPD cohorts, a total of 13 studies with 16 cohorts of women (see ) defined either by criteria used to define GDM or by location (for multisite studies) were included. Full details of the identification process are presented in .
Summary of included studies and cohorts
Quality assessment and included studies
A summary of GDM diagnostic criteria are presented in the introduction to this report (see Table 1). summarises the 10 published studies42,44,118,127–129,131–133 and the three IPD cohorts included in this review.
Prevalence of gestational diabetes mellitus by year the study was undertaken and gestational diabetes mellitus criteria used
shows prevalence by year and GDM criteria used by each study. Using data from the three IPD cohorts we calculated GDM prevalence according to the most commonly used GDM diagnostic criteria presented in Table 1; 1-hour post-load glucose levels (75-g OGTT) were not available for the BiB,22 Atlantic DIP59 and Warwick/Coventry cohorts,60 therefore prevalences may be underestimated for criteria that include a 1-hour glucose level [American Diabetes Association (ADA), IADPSG, NDDG (National Diabetes Data Group)]. These prevalence estimates are shown in . The Atlantic DIP study59 has higher prevalence estimates for all diagnostic criteria. NDDG criteria are the most conservative, having the highest glucose thresholds. The WHO 1980, WHO 1999, ADA and Australasian Diabetes in Pregnancy Society (ADIPS) criteria produce similar prevalence estimates, despite their different glucose threshold criteria. The IADPSG criteria give the highest prevalence estimates for the IPD cohorts, similarly to published estimates, as a result of the lower fasting glucose threshold.
Prevalence of gestational diabetes mellitus by ethnicity
Two published studies44,118 report prevalence of GDM by ethnicity (). Both of these studies44,118 were undertaken when recommended criteria thresholds were higher (1992 and 1995) than those now suggested by the IADPSG and consequently report lower GDM prevalence than would be expected today. Both studies,44,118 however, report differing GDM prevalence by ethnicity, with women of Asian and SA origin having the highest rates. Koukkou et al.118 do not provide more information on the origin of the Asian women in their study (they were recruited from an inner-city London hospital), so they could be of any number of Asian ethnicities.
Prevalence of GDM reported in published studies by ethnicity
Prevalence of GDM by ethnicity was calculated using the three IPD cohorts. These data are summarised in .
Prevalence of GDM by ethnicity, as a percentage (95% CI) [no. with GDM/total no.], in the IPD cohorts
Prevalence of gestational diabetes mellitus by age
The published studies provided insufficient data to estimate prevalence by age, but we have been able to calculate estimates using the IPD cohorts. The results are summarised in . GDM prevalence appears to increase as age category increases in all three cohorts. A logistic regression confirmed this, with a statistically significant increase in odds of GDM of 1.08, 95% CI 1.08 to 1.10 per year.
Prevalence of GDM by age, as a percentage (95% CI) [no. with GDM/total no.], in the IPD cohorts
Prevalence of gestational diabetes mellitus by timing of oral glucose tolerance test
The BiB22 IPD included information on the timing of the OGTT, in terms of gestational age.
We examined results (numbers in parenthesis) by the following gestational age categories (in weeks plus days): < 25 (438), 25–25 plus 6 days (1733), 26–26 plus 6 days (5695), 27–27 plus 6 days (1133), 28–28 plus 6 days (529), 29–29 plus 6 days (276), 30–30 plus 6 days (263) and ≥ 31 (364). A logistic regression analysis found no evidence that the prevalence of GDM changed according to the timing of the test (OR 1.00, 95% CI 0.96 to 1.04).
Discussion
Studies in this review demonstrate a wide range of GDM prevalences. The differences in prevalence are partly explained by the differing criteria and thresholds used to diagnose GDM. Prior to 2010, the WHO criteria11 were used widely in the UK and Ireland, and GDM prevalence was consistently estimated at between 1% and 3% across cohorts. Since 2010, however, variation in estimates are wider (8–24%). The IADPSG criteria8 (published in 2010 and used in several later studies) produced the highest prevalences because of their lower (than previous criteria) fasting glucose threshold. Given the linear monotonic association across the whole spectrum of glucose levels and adverse outcomes, using lower thresholds (as recommended by the IADPSG) will increase the number of women identified who are at increased risk of an adverse outcome. Treatment aims to reduce glucose levels with the goal of reducing the associated increased risks. Treatment trials,51,52 however, have used diagnostic criteria with higher glucose level thresholds than those recommended by the IADPSG and now endorsed by the WHO (or those derived using the BiB data,22 detailed in Chapter 2), therefore the degree to which treatments will improve outcomes for women identified by these criteria using lower glucose level thresholds is unknown.
Several criteria recommend that women have their risk of GDM evaluated either by assessment of maternal characteristics/risk factors (including ethnicity and weight) or by administration of the 50-g OGCT, those that are classified as ‘high risk’ are offered diagnostic testing usually using the OGTT. Some criteria (including the IADPSG), however, recommend universal testing. Criteria recommending that all women are offered testing, rather than only ‘high-risk’ women, will increase the prevalence of GDM irrespective of glucose level thresholds used.124
Differing population characteristics explain some of the diversity in prevalence estimates. In the BiB study,22 GDM prevalence in SA women was two- to threefold greater than in WB women (see Tables 4 and ). Other characteristics also influence prevalence, including advanced maternal age or increasing maternal weight. We have shown that timing of OGTT does not seem to influence prevalence of GDM, however we had few women undergoing OGTT below 25 or above 30 weeks’ gestation. Women who are tested outside the usual 26–28 week range may have specific high-risk status, including previous GDM or symptoms/clinical indications such as polyhydramnios or ultrasound indication of a LGA fetus, therefore the population characteristics of studies with a wider range of OGTT timings should be examined carefully.
Strengths and limitations
We identified 13 studies22,42,44,59,60,118,126–129,131–133 undertaken over 25 years in varied areas of England and Ireland. We were able to demonstrate how prevalence changed over these 25 years and how participant characteristics and criteria influence prevalence. The studies were large, all included > 1000 women and all reported their inclusion and GDM criteria. Our IPD provided valuable information that was not available from published estimates, and showed that, even in contemporary cohorts, GDM prevalence can vary considerably between groups with varying maternal characteristics, including ethnicity.
Few published studies included populations at high risk of GDM because of their ethnicity therefore the inclusion of the BiB cohort22 is extremely valuable. Estimates of prevalence for SA women in the Atlantic DIP cohort59 are uncertain because there were few women of SA ethnicity in that cohort, and even fewer with diagnosed GDM. We undertook several subgroup comparisons; however, these results should be interpreted cautiously given that the studies were not designed or powered to detect differences in prevalence across subgroups. The prevalence of GDM in WB women in the BiB study22 is lower (5%) than that in the Atlantic DIP59 (9%) and the Warwick/Coventry60 (8%) cohorts, even although all used the same diagnostic criteria. The Atlantic DIP study59 (like the BiB study22) universally offered an OGTT, whereas the Warwick/Coventry studies60 selectively tested their population, but both cohorts59,60 had similar and higher GDM prevalence in their white populations than the BiB study,22 and it is unclear why this is so.
We did not identify any eligible studies that included Scottish or Welsh cohorts, therefore, although we intended to present data on UK prevalence of GDM, our data represent England, Northern Ireland and the Republic of Ireland (as we were able to include the Atlantic DIP cohort59).
Conclusions
The prevalence of GDM is increasing in the UK; the offer of an OGTT to all women, the lowering of diagnostic thresholds, and increases in the proportion of women at risk, either because of their ethnicity or increasing weight or age, are all contributing factors (which is examined in the Chapter 5). Within a narrow gestational time frame we have demonstrated that timing of OGTT does not seem to influence prevalence; however, we had few women tested at < 25 or > 31 weeks of gestation and therefore caution should be taken when interpreting these findings. We showed that populations of older women or women whose ethnicity conveys a high risk of diabetes will have higher GDM prevalence.
