Gestational diabetes mellitus (GDM) remains one of the most common metabolic complications of pregnancy, affecting approximately 7% to 20% of pregnant individuals globally, depending on the population and diagnostic criteria used. Without timely detection and management, GDM can lead to adverse maternal and neonatal outcomes, including preeclampsia, macrosomia, shoulder dystocia, and an increased risk of type 2 diabetes later in life. Screening tests have therefore become a cornerstone of prenatal care, designed to identify women who need further diagnostic evaluation and intervention. Yet the landscape of GDM screening is far from uniform. Different countries, professional organizations, and healthcare systems employ a variety of tests and protocols, each with its own evidence base, logistical considerations, and diagnostic thresholds. Understanding these differences is essential for clinicians, policymakers, and patients navigating prenatal care across borders.

Principles of GDM Screening

GDM screening aims to identify women who have hyperglycemia first recognized during pregnancy, typically between 24 and 28 weeks of gestation. The underlying strategy falls into two broad categories: universal screening, where all pregnant women are tested regardless of risk factors, and selective screening, which uses clinical risk factors (e.g., obesity, family history of diabetes, previous GDM, ethnicity) to determine who receives testing. The choice between these approaches often reflects the prevalence of GDM, available healthcare resources, and cost-effectiveness analyses in each region.

The screening tests themselves can be categorized as either one-step or two-step procedures. The one-step approach uses a single oral glucose tolerance test (OGTT) with a 75‑g glucose load, applying the diagnostic thresholds recommended by the International Association of Diabetes and Pregnancy Study Groups (IADPSG) and the World Health Organization (WHO). The two-step approach, widely used in the United States, begins with a non‑fasting glucose challenge test (GCT) and proceeds to a diagnostic 100‑g OGTT only if the initial screen is positive. Both methods have passionate advocates and detractors, and the debate over which is superior continues to shape international guidelines.

The Glucose Challenge Test (GCT)

The GCT, also known as the O’Sullivan test, is a simple, convenient screening tool that does not require the patient to fast. The pregnant woman drinks a 50‑g glucose solution, and a venous blood sample is drawn one hour later to measure plasma glucose concentration. If the result meets or exceeds a locally defined threshold—commonly 130, 135, or 140 mg/dL (7.2–7.8 mmol/L)—the test is considered positive, and the patient is referred for a diagnostic OGTT.

Procedure and Thresholds

Because the GCT is non‑fasting and requires only a single blood draw, it is easy to administer in busy outpatient settings. The threshold used to define a positive screen varies. The American College of Obstetricians and Gynecologists (ACOG) recommends a threshold of 130–140 mg/dL; many institutions in the United States have adopted 140 mg/dL to minimize false positives while maintaining acceptable sensitivity. Lowering the threshold to 130 mg/dL increases detection rates but also raises the number of women who must undergo the more burdensome diagnostic OGTT. It is important to note that the GCT is not diagnostic—a positive result does not establish GDM, but rather identifies the need for further evaluation.

Advantages and Limitations

The main advantage of the GCT is its convenience and low cost. Because it does not require fasting, it fits easily into routine prenatal visits. However, the test has a false‑positive rate of approximately 15% to 25%, depending on the threshold used, and its sensitivity for detecting GDM is only moderate (around 70–80%). Moreover, the GCT was originally validated for a 100‑g diagnostic OGTT; when paired with the modern one‑step 75‑g protocol, its performance characteristics may differ. Another limitation is the lack of standardization: thresholds and glucose beverage preparations vary, making cross‑center comparisons difficult.

The Oral Glucose Tolerance Test (OGTT)

The OGTT is the gold standard diagnostic test for GDM. It measures the body’s ability to metabolize a standard glucose load over time and provides multiple blood glucose measurements. Two main protocols exist: the 75‑g OGTT (favored by WHO, IADPSG, and many international guidelines) and the 100‑g OGTT (used primarily in the United States as part of the Carpenter‑Coustan or National Diabetes Data Group criteria).

75‑g OGTT (One‑Step)

In the 75‑g OGTT, the patient fasts for at least 8 hours overnight, then drinks a solution containing 75 g of glucose. Blood samples are drawn at fasting, 1 hour, and 2 hours after the glucose load. GDM is diagnosed if any one of the following plasma glucose thresholds (as per IADPSG/WHO) is met or exceeded:

  • 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)

These thresholds were derived from the Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study, which demonstrated a continuous relationship between maternal glucose levels and adverse outcomes. The one‑step approach is simpler for patients (only one visit) and is associated with higher detection rates of mild hyperglycemia, but it also leads to a higher prevalence of GDM diagnoses compared to the two‑step method.

100‑g OGTT (Two‑Step Diagnostic)

The 100‑g OGTT is performed after a positive GCT. Following an overnight fast, the woman drinks a solution with 100 g of glucose. Blood samples are taken at fasting, 1 hour, 2 hours, and 3 hours. Two sets of diagnostic thresholds are widely used: the Carpenter‑Coustan criteria and the older National Diabetes Data Group (NDDG) criteria. Carpenter‑Coustan thresholds are lower and thus more sensitive:

  • 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)

The 100‑g OGTT requires a longer test duration (3‑4 hours) and involves four blood draws, which can be burdensome for pregnant women. However, advocates argue that the two‑step approach reduces overdiagnosis and aligns with U.S. clinical traditions. The ongoing debate between one‑step and two‑step methods reflects differing priorities: sensitivity and early detection versus specificity and resource conservation.

Country‑Specific Screening Protocols

The diversity of GDM screening around the world stems from variations in healthcare systems, epidemiological data, and expert consensus. Below is an expanded review of protocols in several key regions.

United States

The U.S. remains the strongest advocate of the two‑step approach. ACOG and the National Institutes of Health (NIH) endorse universal screening with a 50‑g GCT at 24–28 weeks, followed by a 100‑g diagnostic OGTT if the GCT is positive. Many practices have adopted the Carpenter‑Coustan threshold for the OGTT. However, in 2024, the U.S. Preventive Services Task Force (USPSTF) reaffirmed that evidence is insufficient to recommend one screening approach over another, leaving clinicians to choose based on patient population and local resources. Recent studies, including the large GEM trial, have fueled debate about the optimal strategy.

Canada

Canadian guidelines, issued by Diabetes Canada, recommend a one‑step 75‑g OGTT for all pregnant women at 24–28 weeks, using IADPSG/WHO thresholds. This universal approach was adopted to simplify care and align with international standards. Some provinces have transitioned successfully, though challenges remain related to fasting compliance and laboratory capacity in rural areas.

United Kingdom

The National Institute for Health and Care Excellence (NICE) recommends a risk‑factor‑based screening strategy. Women with one or more risk factors (e.g., BMI >30, previous GDM, family history of diabetes, ethnic background) are offered a 75‑g OGTT at 24–28 weeks. NICE uses a fasting threshold of ≥5.6 mmol/L (101 mg/dL) and a 2‑hour threshold of ≥7.8 mmol/L (140 mg/dL)—slightly different from IADPSG. This selective approach aims to avoid unnecessary testing in low‑risk populations, though critics argue it may miss a significant proportion of GDM cases. NICE Guideline NG3 provides detailed implementation guidance.

Australia

Australia has moved decisively toward universal screening with a one‑step 75‑g OGTT. The Australasian Diabetes in Pregnancy Society (ADIPS) endorses IADPSG criteria, and the test is offered to all pregnant women between 24 and 28 weeks. Early pregnancy testing (first trimester) is recommended for those with high risk factors. This standardization has improved consistency of care across states and territories.

India

India, with its high prevalence of type 2 diabetes and GDM, has adopted a pragmatic approach. The Government of India and the Indian Council of Medical Research (ICMR) recommend universal screening using a one‑step 75‑g OGTT with IADPSG thresholds. However, due to resource constraints and high patient volumes, some centers use a non‑fasting 75‑g OGTT known as the Diabetes in Pregnancy Study Group India (DIPSI) method, which measures a single 2‑hour plasma glucose after a 75‑g glucose load (without fasting). The DIPSI method is simpler but has lower sensitivity compared to the standard OGTT. Ongoing efforts aim to strengthen laboratory capacity and raise awareness among healthcare providers.

China

China has implemented national guidelines that recommend universal screening with a one‑step 75‑g OGTT at 24–28 weeks, using IADPSG criteria. Given the large population and rapid urbanization, efforts focus on training community health workers and ensuring access to testing in both urban and rural settings. Early pregnancy screening (HbA1c or fasting glucose) is also frequently performed to detect pre‑existing diabetes.

Brazil

Brazil adopts a two‑step protocol similar to the U.S., but with some modifications. The Ministry of Health recommends a non‑fasting 50‑g GCT followed by a 75‑g OGTT (using WHO thresholds) for women with a positive screen. Alternative strategies using the risk‑based approach have been proposed in regions with limited resources. The unified health system (SUS) provides free prenatal care, making universal screening feasible but logistically challenging in remote areas.

While the GCT and OGTT remain central to GDM screening, several alternative approaches are gaining attention as researchers seek simpler, more patient‑friendly methods.

HbA1c

Glycated hemoglobin (HbA1c) is widely used for diagnosing diabetes in non‑pregnant individuals, but its role in pregnancy is limited. Pregnancy‑related changes in red cell turnover and iron status can affect HbA1c levels, and the test is less sensitive than the OGTT for detecting mild hyperglycemia. Some guidelines suggest using HbA1c ≥5.7–6.4% in early pregnancy to identify women at risk for GDM, but it is not recommended as a stand‑alone diagnostic test at 24–28 weeks.

Fetal Ultrasound Markers

Fetal measurements such as abdominal circumference (AC) and estimated fetal weight (EFW) have been investigated as indirect markers of hyperglycemia. An AC ≥75th percentile or an EFW >90th percentile between 20–24 weeks can prompt targeted screening. While ultrasound markers do not replace glucose testing, they may help identify a subset of women who would benefit from earlier or more intensive evaluation.

Continuous Glucose Monitoring (CGM)

CGM devices provide real‑time glucose readings over several days and offer a more detailed picture of glycemic variability. Several pilot studies have explored the use of CGM as a screening tool, either as a replacement or adjunct to the OGTT. In 2023, the CGM‑GDM trial began evaluating its feasibility. However, cost, device availability, and the need for interpretation software currently limit widespread adoption.

Importance of Early Screening and Timing

The standard timing for GDM screening is between 24 and 28 weeks of gestation, when placental hormones induce significant insulin resistance. However, women with high risk factors may benefit from early screening at the first prenatal visit (or before 20 weeks) to detect pre‑existing diabetes or early‑onset GDM. Early diagnosis allows for prompt lifestyle modifications, referral to diabetes‑in‑pregnancy clinics, and closer fetal surveillance. For the majority of women, a negative screening result in mid‑pregnancy does not rule out the possibility of developing GDM later; therefore, ongoing vigilance and repeat testing (if symptoms or risk factors emerge) remain important.

Regardless of the screening protocol used, the overarching goal is to identify hyperglycemia before it can cause significant harm. Clinicians should be aware of the local guidelines in their practice setting, while also considering the individual patient’s risk profile, cultural preferences, and ability to complete the required test. International collaborations, such as those led by the World Health Organization, continue to promote harmonized diagnostic criteria to improve global comparability and outcomes.

Conclusion

GDM screening tests are not one‑size‑fits-all. The two‑step protocol (GCT followed by 100‑g OGTT) remains dominant in the United States, while the one‑step 75‑g OGTT has gained widespread acceptance in Canada, Australia, China, and many other countries. Each approach offers distinct trade‑offs in sensitivity, convenience, and cost. Selective screening based on risk factors, as practiced in the UK and parts of India, can reduce testing burden but may miss cases. Emerging methods such as CGM and ultrasound markers hold promise but require further validation. Ultimately, the best test is the one that is accessible, performed correctly, and interpreted using validated thresholds. As the global burden of GDM continues to rise, a nuanced understanding of these screening options empowers healthcare providers to deliver optimal, evidence‑based care for every pregnancy.