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The IDF has been measuring the prevalence of diabetes nationally, regionally, and globally since the year 2000 through systematic review of published literature on diabetes prevalence. Their most recent estimates (2017) suggest that GDM accounts for 86.4% of all hyperglycemia in pregnancy [13].

Unfortunately, attempts to obtain accurate prevalence data are severely affected by the variable approach to screening and diagnosis as described above. This is highlighted in a recent survey by the European Board and College of Obstetrics and Gynaecology, which received responses from 28 countries [46]. The most commonly used diagnostic criteria were the IADPSG criteria (67.9%) [21], the 1999 WHO criteria (10.7%) [16], the European Association for the Study of Diabetes criteria (7.1%) [47], and the Carpenter and Coustan criteria in 7.1% [28]. Approximately two-thirds of responders reported risk factor-based screening (64.3%), and the remainder recommended universal screening (35.7%) [46].

From a European perspective, a 2012 publication aimed to identify and review the best European evidence relating to GDM prevalence [48]. This was achieved through systematic literature review and narrative synthesis. Overall, prevalence was most often reported as 2–6% pregnancies with lower levels toward the Northern Atlantic Seaboard of Europe and higher in the Southern Mediterranean seaboard. It is evident, however, that pockets of higher prevalence do exist. This is highlighted by a population-based study along the Irish Atlantic seaboard where researchers performed universal screening for GDM in 5,500 women with an OGTT at 24–28 weeks and diagnosed 12.4% with GDM using IADPSG criteria and 9.4% using older 1999 WHO criteria [49]. In the United States, it is estimated that the prevalence of GDM in 2010 was between 4.6 and 9.2% depending on the data source analyzed [50]. However, using IADPSG criteria, the overall frequency of GDM in the Hyperglycemia and Neonatal Outcomes study was 17.8% [51].

The overall prevalence of GDM has undoubtedly increased over time, and a number of factors have contributed to this situation – aside from the effect of the newer diagnostic criteria with lower glucose cutoffs. Classical risk factors for GDM are outlined in Table 2. In recent years, the number of women affected by one or more of these factors has increased. In particular, pregnancy is now occurring at an increasing age. In the United States between 1980 and 2004, the proportion of first births increased by a factor of 3 in women aged ≥30 years (8.6–25.4%), by a factor of 6 in women aged ≥35 years (1.3–8.3%), and a factor of 15 (0.1–1.5%) in women aged ≥40 years [52]. Similar to type 2 diabetes in pregnancy, the prevalence of GDM has also followed the increase in rates of obesity observed in the background population [35].

An additional risk factor for GDM is non-European ethnicity [31], and in particular, immigrant women are at an increased risk of GDM across all ethnic groups [53]. The reasons behind this are not entirely clear, and although the literature suggests that living in a neighborhood of shared ethnic ancestry (ethnic enclave) has a positive influence on immigrant health [54], a large study of migrant women in New York City found no effect of such residence on GDM in most immigrant groups [55].

Timing of screening for GDM is an area of intense debate and has epidemiological implications. While the majority of guidelines recommend screening for pregestational diabetes (using standard diagnostic criteria described above) in all women at the first antenatal visit, the diagnostic pathway is not so clear in the case of GDM. Although the standard time for screening is considered to be 24–28 weeks gestation, the original IADPSG guidelines recommended that fasting plasma glucose between 5.1 and 6.9 mmol/L could be considered diagnostic of GDM at any time in pregnancy [21]. However, in 2016, IADPSG representatives stated that this is no longer justified [56]. This clarification was partially based on data from China and Italy reporting that glucose levels ≥5.1 mmol/L in early pregnancy are poorly predictive of later GDM [57, 58]. In the Vitamin D and lifestyle intervention for GDM prevention (DALI) study, women with a BMI ≥29.0 kg/m² were evaluated for GDM in early pregnancy using IADPSG criteria [59]. Approximately 20% were diagnosed, and these women were more insulin resistant than their counterparts with normal glucose tolerance. Subsequently, a further 20% developed GDM when tested at the standard time of 24–28 weeks, but a further 10% were diagnosed at 35–37 weeks gestation [60]. Urgent research is needed to clarify the best approach and timing to diagnosis and management of GDM. In the interim, practices are varying dramatically and likely contributing significantly to differences in reported rates of GDM.