Gestational diabetes mellitus (GDM) is a common metabolic disorder characterized by hyperglycemia that is first recognized during pregnancy. Without appropriate screening and timely intervention, GDM can lead to significant short- and long-term complications for both the mother and the offspring, including preeclampsia, macrosomia, neonatal hypoglycemia, and an increased lifetime risk of type 2 diabetes. Over the past two decades, major health organizations have refined their screening guidelines to improve detection rates while balancing the burden on healthcare systems. Understanding these evolving recommendations is essential for clinicians, policymakers, and expectant mothers alike.

Overview of GDM Screening Guidelines

The central objective of GDM screening is to identify women who would benefit from glucose-lowering interventions during pregnancy. However, there is ongoing debate regarding the optimal screening strategy—specifically, whether a one-step or two-step approach is superior, what glucose thresholds should be used, and whether screening should be universal or risk-factor–based. Despite these differences, consensus exists on the critical screening window of 24 to 28 weeks of gestation, when placental hormonal changes most profoundly affect maternal glucose metabolism. Early screening (before 24 weeks) is reserved for women with high-risk characteristics, such as a previous history of GDM, obesity, or a family history of type 2 diabetes. The following sections detail the positions of the most influential health organizations and provide a comparative analysis of their respective recommendations.

Guidelines from Major Health Organizations

American Diabetes Association (ADA)

The American Diabetes Association advocates for a two-step screening approach as the preferred method. Step one involves a non‑fasting 50‑gram oral glucose challenge test (GCT) administered at 24–28 weeks of gestation. A plasma glucose value at one hour of ≥140 mg/dL (7.8 mmol/L) is considered a positive screen and prompts step two: a diagnostic 100‑gram oral glucose tolerance test (OGTT) performed after an overnight fast. The ADA uses the Carpenter/Coustan criteria for interpretation: meeting or exceeding two of the four following thresholds confirms GDM—fasting ≥95 mg/dL (5.3 mmol/L), 1‑hour ≥180 mg/dL (10.0 mmol/L), 2‑hour ≥155 mg/dL (8.6 mmol/L), and 3‑hour ≥140 mg/dL (7.8 mmol/L). The ADA also recommends earlier screening (at the first prenatal visit) for women with risk factors, using standard diagnostic criteria if not previously diagnosed with overt diabetes. The organization regularly updates its Standards of Medical Care in Diabetes with evidence-based guidance.

World Health Organization (WHO)

The World Health Organization endorses universal screening using a one‑step 75‑gram OGTT at 24–28 weeks of gestation. This recommendation aligns with the diagnostic criteria established by the International Association of Diabetes and Pregnancy Study Groups (IADPSG). Diagnosis is made when any single plasma glucose value meets or exceeds the following thresholds: fasting ≥5.1 mmol/L (92 mg/dL), 1‑hour ≥10.0 mmol/L (180 mg/dL), or 2‑hour ≥8.5 mmol/L (153 mg/dL). The WHO emphasizes that even mild hyperglycemia below the threshold for overt diabetes can increase the risk of adverse pregnancy outcomes, and that the one‑step approach provides a uniform, simplified pathway that can be implemented globally, even in resource‑limited settings. Their 2013 diagnostic criteria have been adopted by many countries outside the United States.

International Association of Diabetes and Pregnancy Study Groups (IADPSG)

The IADPSG, following the landmark Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study, proposed the one‑step 75‑gram OGTT for all pregnant women. Their diagnostic thresholds were derived from HAPO data showing a continuous relationship between maternal glucose levels and adverse outcomes. The IADPSG criteria require that only one of the three glucose values (fasting, 1‑hour, or 2‑hour) be met or exceeded to establish a GDM diagnosis. This approach substantially increases the number of women diagnosed compared with the two‑step method, a point of contention among some organizations. The IADPSG argues that earlier and more inclusive detection leads to improved perinatal outcomes through lifestyle intervention and, when necessary, pharmacological therapy. Many professional societies now endorse the IADPSG criteria as the international standard.

American College of Obstetricians and Gynecologists (ACOG)

The American College of Obstetricians and Gynecologists currently supports the two‑step screening approach with the 50‑g GCT followed by the 100‑g OGTT. ACOG recommends using either the Carpenter/Coustan criteria or the National Diabetes Data Group (NDDG) thresholds for the diagnostic test. The NDDG thresholds are slightly higher: fasting ≥105 mg/dL (5.8 mmol/L), 1‑hour ≥190 mg/dL (10.6 mmol/L), 2‑hour ≥165 mg/dL (9.2 mmol/L), and 3‑hour ≥145 mg/dL (8.0 mmol/L). ACOG acknowledges the one‑step IADPSG criteria but notes that its adoption would increase the prevalence of GDM by 2–3 times without clear evidence that the subsequent interventions improve outcomes sufficiently to offset the costs and potential for over‑medicalization. The organization continues to monitor emerging data and may revise its position as evidence evolves.

Comparison of Guidelines

While all major organizations agree that screening between 24–28 weeks is essential, key differences emerge in the testing method and diagnostic thresholds:

  • Testing approach: The ADA and ACOG favor a two‑step strategy (50‑g GCT followed by 100‑g OGTT), whereas the WHO and IADPSG recommend a single‑step 75‑g OGTT. The two‑step method is often more feasible in busy outpatient settings because the initial GCT does not require fasting. However, the one‑step method simplifies the diagnostic process and reduces the number of visits.
  • Diagnostic thresholds: The two‑step approach requires that two or more glucose values during the OGTT exceed the specified cutoffs, while the one‑step approach requires only one abnormal value. This difference results in a higher prevalence of GDM using the IADPSG/WHO criteria.
  • Target population: All four organizations recommend universal screening—that is, testing all pregnant women regardless of risk factors. However, the ADA and ACOG explicitly allow for early risk‑based screening before 24 weeks for high‑risk individuals, whereas the WHO and IADPSG do not routinely recommend first‑trimester GDM screening in the absence of diagnostic criteria for overt diabetes.

Risk Factors and Early Screening

Even with universal screening at 24–28 weeks, early identification of women at elevated risk allows for prevention and early management. Recognized risk factors for GDM include maternal age over 25 years, body mass index ≥30 kg/m², a history of GDM in a prior pregnancy, a family history of type 2 diabetes, belonging to certain ethnic groups (e.g., South Asian, Hispanic, African American, or Indigenous), a previous delivery of an infant weighing ≥4,000 g, and polycystic ovary syndrome. The ADA recommends that women with one or more of these risk factors undergo glucose testing at the first prenatal visit using standard diagnostic criteria. If no diabetes is found, re‑testing at 24–28 weeks is still warranted. This early screening approach helps distinguish pre‑existing type 2 diabetes from GDM, which carries different management implications.

Diagnostic Criteria at a Glance

The following list summarizes the glucose thresholds used by the major organizations for diagnosing GDM after a 75‑g or 100‑g OGTT:

  • IADPSG / WHO (75‑g OGTT, one abnormal value): Fasting ≥92 mg/dL (5.1 mmol/L), 1‑hour ≥180 mg/dL (10.0 mmol/L), 2‑hour ≥153 mg/dL (8.5 mmol/L).
  • ADA / ACOG (100‑g OGTT, two or more abnormal values, Carpenter/Coustan): Fasting ≥95 mg/dL (5.3 mmol/L), 1‑hour ≥180 mg/dL (10.0 mmol/L), 2‑hour ≥155 mg/dL (8.6 mmol/L), 3‑hour ≥140 mg/dL (7.8 mmol/L).
  • ACOG alternative (NDDG thresholds): Fasting ≥105 mg/dL (5.8 mmol/L), 1‑hour ≥190 mg/dL (10.6 mmol/L), 2‑hour ≥165 mg/dL (9.2 mmol/L), 3‑hour ≥145 mg/dL (8.0 mmol/L).

Clinicians should be aware of which criteria their institution uses and ensure consistent application to avoid diagnostic confusion.

Implications of GDM Diagnosis

A diagnosis of GDM triggers a multidisciplinary care plan that includes medical nutrition therapy, self‑monitoring of blood glucose, physical activity counseling, and, if targets are not met within two weeks, pharmacological therapy with insulin or metformin. Adequate glycemic control reduces the incidence of macrosomia, shoulder dystocia, neonatal intensive care unit admissions, and preeclampsia. Beyond the immediate pregnancy, women diagnosed with GDM have a seven‑fold higher lifetime risk of developing type 2 diabetes; therefore, postpartum glucose testing at 4–12 weeks and ongoing surveillance are critical. The choice of screening strategy directly affects how many women receive these interventions. Large observational studies suggest that the one‑step IADPSG criteria identify a population that benefits from intervention, but randomized controlled trials comparing the two‑step and one‑step methods in terms of patient‑important outcomes remain limited. The ongoing debate underscores the need for individualized shared decision‑making and consistent updates from professional bodies.

Conclusion

GDM screening guidelines from major health organizations continue to evolve as new evidence emerges. While the ADA, ACOG, WHO, and IADPSG agree on the importance of universal screening at 24–28 weeks, their differences in testing protocols and diagnostic thresholds reflect divergent interpretations of the available data. Healthcare systems should select a strategy that balances feasibility, cost, and the goal of improving maternal and neonatal outcomes. Regardless of the chosen approach, timely detection coupled with effective management remains the cornerstone of reducing the adverse effects of GDM. Clinicians are encouraged to stay abreast of updates from the ADA, WHO, IADPSG, and ACOG to ensure their practice aligns with the best available evidence.