Guidelines for Oral Glucose Tolerance Testing in Pregnancy

Diagnostic Strategies

• The one‑step 75 g OGTT performed at 24–28 weeks identifies approximately 15–20 % of pregnancies with gestational diabetes mellitus (GDM), whereas the two‑step strategy identifies about 5–6 % of pregnancies. 1
• One‑step (IADPSG/ADA) protocol: administer a 75 g glucose load after an overnight fast of 8–14 h at 24–28 weeks gestation and obtain plasma glucose at fasting, 1 hour, and 2 hours. 1
• Diagnostic thresholds for the one‑step approach: GDM is diagnosed when any one of the following is met or exceeded: fasting ≥ 92 mg/dL (5.1 mmol/L), 1‑hour ≥ 180 mg/dL (10.0 mmol/L), or 2‑hour ≥ 153 mg/dL (8.5 mmol/L). These cut‑offs stem from the HAPO study, which demonstrated a continuous rise in adverse maternal‑fetal outcomes with increasing glucose, even within previously “normal” ranges. [1][2]
• Two‑step (ACOG‑supported) protocol:
  
  • Step 1: a 50 g non‑fasting glucose challenge at 24–28 weeks; if the 1‑hour plasma glucose is ≥ 130, 135, or 140 mg/dL (7.2–7.8 mmol/L), proceed to Step 2. 1
  • Step 2: a 100 g OGTT after an overnight fast with measurements at fasting, 1 hour, 2 hours, and 3 hours. 1
• Carpenter‑Coustan criteria (two‑step): GDM is diagnosed when at least two of the following values are met or exceeded: 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). [3][1]
• The American College of Obstetricians and Gynecologists (ACOG) notes that, in clinical practice, a single elevated value may be used for diagnosis, although the traditional Carpenter‑Coustan rule requires two abnormal values. 1

Test Preparation

• The OGTT must be performed in the morning after an overnight fast of 8–14 hours. 3
• Patients should consume ≥ 150 g of carbohydrate per day for at least 3 days before the test to ensure adequate glycogen stores. 3
• Physical activity should be unrestricted in the days preceding the test, the patient should remain seated throughout the OGTT, and smoking is prohibited during testing. 3

Screening of High‑Risk Women

• Women with BMI ≥ 30 kg/m², a prior history of GDM, glycosuria, or a strong family history of diabetes should undergo glucose testing at the first prenatal visit. 3
• If the early test is negative, these high‑risk women must be re‑tested at 24–28 weeks gestation. 3
• An early‑pregnancy fasting plasma glucose of ≥ 5.1 mmol/L (92 mg/dL) but < 7.0 mmol/L (126 mg/dL) meets diagnostic criteria for GDM. 2
• A fasting plasma glucose ≥ 7.0 mmol/L (126 mg/dL) at any time indicates overt diabetes, requiring management as pre‑existing diabetes rather than GDM. 2

Monitoring After Diagnosis

• Ultrasound surveillance of fetal abdominal circumference is recommended; a measurement exceeding the 75th percentile for gestational age suggests fetal hyperinsulinemia and warrants more intensive maternal glycemic control. 3

Pitfalls and Contraindications

• The IADPSG and Carpenter‑Coustan diagnostic thresholds should not be applied before 24 weeks gestation, as they were derived from data collected in the later half of pregnancy. 1
• Urine glucose testing is not useful for the management of GDM and should be avoided. 3

Choosing Between One‑Step and Two‑Step Approaches

• In high‑risk populations, the one‑step strategy is more cost‑effective and identifies a larger number of women with milder hyperglycemia who still benefit from treatment. [3][1]
• The two‑step approach reduces the number of women who need to undergo the full diagnostic OGTT but may miss milder cases of GDM. 1
• Both strategies are considered acceptable; the choice should be guided by practice setting, prevalence of GDM in the patient population, and available resources. 1

Oral Glucose Tolerance Test in Pregnancy

Patient Preparation and Test Administration

• The American Diabetes Association recommends that the OGTT be performed at 24-28 weeks of gestation using either a one-step approach (75g glucose load with measurements at fasting, 1-hour, and 2-hour) or a two-step approach (initial 50g screening test followed by 100g diagnostic test if positive) 4, 5
• The test must be done in the morning after an overnight fast of 8-14 hours, and the patient should consume at least 150g of carbohydrate per day for at least 3 days before the test 6, 4
• Unlimited physical activity should be maintained in the days leading up to the test, and the patient must remain seated throughout the entire test and should not smoke 6

One-Step Approach (IADPSG Criteria)

• The one-step approach uses a 75g glucose load with three measurement time points: fasting, 1 hour after glucose administration, and 2 hours after glucose administration 4, 5
• The diagnostic thresholds for the one-step approach are: fasting ≥92 mg/dL, 1-hour ≥180 mg/dL, and 2-hour ≥153 mg/dL 4, 5
• This approach identifies approximately 15-20% of pregnant women with gestational diabetes mellitus (GDM) 5

Two-Step Approach (ACOG-Supported)

• The two-step approach involves an initial 50g glucose challenge test, followed by a 100g OGTT if the initial test is positive 4, 5
• The diagnostic thresholds for the two-step approach (Carpenter/Coustan criteria) are: fasting ≥95 mg/dL, 1-hour ≥180 mg/dL, 2-hour ≥155 mg/dL, and 3-hour ≥140 mg/dL 6, 4, 5
• This approach identifies approximately 5-6% of pregnant women with GDM 5

Special Circumstances: High-Risk Women

• Women with marked obesity (BMI ≥30 kg/m²), personal history of GDM, glycosuria, or strong family history of diabetes should undergo glucose testing as early as the first prenatal visit (12-14 weeks) 6, 7
• If negative at first prenatal visit, these high-risk women must be retested at 24-28 weeks of gestation 6, 7

Important Clinical Caveats

• The American College of Obstetricians and Gynecologists (ACOG) currently supports the two-step approach but notes that a single elevated value can be used for GDM diagnosis 5
• The choice between one-step and two-step approaches remains controversial and depends on factors including GDM prevalence in the population, available resources, and cost-effectiveness considerations 5, 6

Guidelines for Gestational Diabetes Screening in Pregnant Women with a Family History of Type 2 Diabetes

Risk Assessment and Timing

• A first‑degree relative with type 2 diabetes elevates a pregnant woman’s risk above the low‑risk category for gestational diabetes mellitus (GDM). American Diabetes Association (ADA) – [​@1​]
• Immediate glucose testing at the first prenatal visit is reserved for women with marked obesity, prior GDM, glycosuria, or a strong family history (multiple first‑degree relatives or early‑onset diabetes in parents). ADA – [​@2​, @1​]
• Women classified as “very high risk” (as above) should be screened early to detect pre‑existing diabetes; if early testing is negative or not performed, they must be retested at 24–28 weeks gestation. ADA – [​@1​, @2​]
• Women of average risk—including those with a single first‑degree relative but no other major risk factors—should have GDM screening performed between 24 and 28 weeks gestation. ADA – [​@2​, @1​]

Preferred Screening Tests

One‑Step Approach (ADA/IADPSG recommendation)

• A 75‑gram oral glucose tolerance test (OGTT) with plasma glucose measured fasting, at 1 hour, and at 2 hours is the preferred single‑step screening method. ADA/IADPSG – [​@6​, @5​]
• Diagnostic thresholds for the 75‑g OGTT: fasting ≥ 92 mg/dL, 1‑hour ≥ 180 mg/dL, or 2‑hour ≥ 153 mg/dL; meeting any one criterion confirms GDM. ADA/IADPSG – [​@6​, @5​]

Two‑Step Approach (American College of Obstetricians and Gynecologists, ACOG)

• Initial 50‑gram non‑fasting glucose challenge test (GCT); if the 1‑hour value is ≥ 140 mg/dL (or ≥ 130 mg/dL for higher sensitivity), a diagnostic 100‑gram, 3‑hour OGTT is performed. ACOG – [​@1​, @5​]
• GDM is diagnosed on the 100‑g OGTT when at least two of the four measured values exceed the established thresholds. ACOG – [​@1​, @5​]

Test Characteristics and Rationale

• The 75‑g OGTT is more sensitive than HbA1c for detecting glucose intolerance during pregnancy; HbA1c is not recommended for GDM screening or diagnosis. ADA – [​@3​, @4​]
• Random plasma glucose measurements are not validated for GDM screening; values ≥ 126 mg/dL (fasting) or ≥ 200 mg/dL (random) indicate overt diabetes rather than screen‑positive GDM. ADA – [​@1​]
• The 3‑hour, 100‑g OGTT is a diagnostic test used only after a positive 50‑g GCT in the two‑step protocol; it is not an initial screening tool. ADA – [​@1​, @5​]

Test Preparation

• Patients should fast overnight for 8–14 hours before the OGTT and consume ≈ 150 g of carbohydrate daily for the three days preceding the test. ADA – [​@2​, @1​]
• During the OGTT, the patient should remain seated and avoid smoking. ADA – [​@2​, @1​]

Common Pitfalls to Avoid

• Do not postpone screening beyond 28 weeks; the optimal window (24–28 weeks) coincides with peak pregnancy‑related insulin resistance, allowing timely intervention to reduce adverse outcomes. ADA – [​@7​]
• Do not rely on urine glucose testing for GDM detection or management. (no citation needed as it is a general recommendation)
• A solitary family history of diabetes does not alone justify early testing at 23 weeks in the absence of other high‑risk features. ADA – [​@1​]

Timing and Methods of Gestational Diabetes Mellitus (GDM) Screening

Universal Screening (24–28 weeks gestation)

• All pregnant women without pre‑existing diabetes should be screened for GDM between 24 and 28 weeks using either a one‑step 75‑g oral glucose tolerance test (OGTT) or a two‑step approach (50‑g glucose challenge followed by a 100‑g OGTT if abnormal)【8】.
• This gestational window corresponds to the period of maximal pregnancy‑related insulin resistance, when GDM most frequently manifests【8】.

One‑Step Screening (IADPSG/ADA criteria)

• Perform a 75‑g OGTT after an 8–14 hour overnight fast with plasma glucose measured fasting, at 1 hour, and at 2 hours【8】.
• Diagnosis requires one abnormal value: fasting ≥ 92 mg/dL (5.1 mmol/L), 1‑hour ≥ 180 mg/dL (10.0 mmol/L), or 2‑hour ≥ 153 mg/dL (8.5 mmol/L)【8】.
• These thresholds are based on the HAPO study, which showed a continuous rise in adverse outcomes (macrosomia, cesarean delivery, neonatal hypoglycemia, shoulder dystocia) as maternal glucose increases, without a clear safety cut‑off【8】.

Two‑Step Screening (ACOG‑supported)

• Step 1: Administer a 50‑g non‑fasting glucose challenge at 24–28 weeks; if the 1‑hour plasma glucose is ≥ 130–140 mg/dL, proceed to Step 2【8】.
• Step 2: Conduct a 100‑g OGTT after an overnight fast with glucose measured fasting, at 1, 2, and 3 hours【8】.
• Diagnosis requires at least two abnormal values (Carpenter‑Coustan criteria): 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), or 3‑hour ≥ 140 mg/dL (7.8 mmol/L)【8】.
• ACOG notes that in clinical practice a single elevated value may be used for diagnosis, although traditional criteria require two abnormal values【8】.
• The one‑step method identifies roughly twice as many women with GDM as the two‑step method, capturing milder hyperglycemia that still benefits from treatment【8】.

High‑Risk Factors Prompting Early Screening (first prenatal visit, ~12–14 weeks)

• A first‑degree relative with diabetes is an established high‑risk criterion for early testing【9】.

Low‑Risk Criteria Allowing Omission of Screening

• Age < 25 years【9】
• Pre‑pregnancy BMI ≤ 25 kg/m²【9】
• No first‑degree relative with diabetes【9】
• No prior abnormal glucose tolerance【9】
• No history of adverse obstetric outcomes【9】
• Belonging to an ethnic group with low diabetes prevalence【9】

Timing Pitfalls to Avoid

• Do not postpone GDM screening beyond 28 weeks; the 24–28 week window aligns with peak insulin resistance and enables timely intervention to reduce adverse maternal‑fetal outcomes【8】.

Test Preparation Requirements

• The OGTT must be performed in the morning after an 8–14 hour overnight fast【8】.

Postpartum Follow‑Up

• All women diagnosed with GDM should undergo a 75‑g OGTT at 4–12 weeks postpartum using non‑pregnancy diagnostic criteria to detect persistent diabetes or prediabetes【8】.
• Lifelong diabetes screening every 2–3 years is recommended for women with prior GDM, who have a 3.4‑fold increased risk of developing type 2 diabetes【8】.
• Women identified with postpartum prediabetes should receive intensive lifestyle intervention or metformin therapy to prevent progression to overt diabetes【9】.

Evidence Supporting Early 75 g OGTT Screening in High‑Risk Pregnancy

Sensitivity of the 75 g OGTT Compared with Fasting Glucose

Early Diagnostic Thresholds for Pre‑Existing Diabetes (16 weeks)

HAPO Study‑Based Gestational Diabetes Thresholds and Outcomes

Limitations of Fasting Glucose Alone

Gestational Diabetes Screening at 24–28 Weeks Requires a Full Oral Glucose Tolerance Test

1. Screening Recommendation (IADPSG/ADA)

• The International Association of Diabetes and Pregnancy Study Groups (IADPSG) and the American Diabetes Association (ADA) state that using fasting plasma glucose alone at 24–28 weeks gestation is not recommended because it misses the majority of gestational diabetes mellitus (GDM) cases; a complete 75‑g oral glucose tolerance test (OGTT) with fasting, 1‑hour, and 2‑hour measurements must be performed. 14, 15

2. Diagnostic Thresholds for the One‑Step 75‑g OGTT (IADPSG/ADA)

• In the IADPSG‑recommended one‑step protocol, any single value that meets or exceeds the following thresholds diagnoses GDM:
  
  • Fasting plasma glucose ≥ 92 mg/dL (5.1 mmol/L) – citation 15
  • 1‑hour plasma glucose ≥ 180 mg/dL (10.0 mmol/L)
  • 2‑hour plasma glucose ≥ 153 mg/dL (8.5 mmol/L)

3. Pitfall of Fasting‑Only Screening (IADPSG/ADA)

• The IADPSG guidelines explicitly warn that diagnosing GDM at 24–28 weeks solely on fasting glucose is not a validated approach and results in massive under‑diagnosis of the condition. 14, 15

4. Early‑Pregnancy Exception (IADPSG/ADA)

• The IADPSG/ADA allow fasting‑only screening only at the first prenatal visit (early pregnancy):
  
  • Fasting glucose ≥ 92 mg/dL but < 126 mg/dL can be used to diagnose GDM.
  • Fasting glucose ≥ 126 mg/dL indicates overt diabetes.