Gestational Diabetes Mellitus: Screening, Diagnosis, and Management
Screening Strategy
All pregnant women without pre-existing diabetes should undergo universal screening for gestational diabetes at 24–28 weeks gestation using either a one-step or two-step approach. 1, 2
Risk-Based Early Screening (First Prenatal Visit, ~12–14 weeks)
High-risk women require immediate glucose testing at their first prenatal visit and must be retested at 24–28 weeks if initially negative. 1, 2
High-risk criteria include:
- BMI ≥30 kg/m² (the single most important risk factor) 2, 3
- Personal history of prior GDM 1, 4
- Glycosuria on routine urinalysis 1
- First-degree relative with diabetes 1, 2, 3
- High-risk ethnicity (Hispanic, Native American, South/East Asian, African American, Pacific Islander) 2, 3
Early screening uses standard diabetes diagnostic thresholds:
- Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) indicates overt pre-existing diabetes 1, 2
- Random plasma glucose ≥200 mg/dL (11.1 mmol/L) with hyperglycemia symptoms indicates overt diabetes 1, 2
Low-Risk Women Who May Skip Screening
Women meeting all of the following criteria may forgo GDM screening entirely: 1, 2
- Age <25 years
- Pre-pregnancy BMI ≤25 kg/m²
- No first-degree relative with diabetes
- No history of abnormal glucose metabolism
- No history of poor obstetric outcomes
- Not a member of high-risk ethnic group
Critical pitfall: These criteria are extremely stringent; most pregnant women require screening because they possess at least one risk factor. 2
Diagnostic Approaches
One-Step Approach (IADPSG/ADA Criteria)
Perform a 75-g oral glucose tolerance test at 24–28 weeks with plasma glucose measured fasting, at 1 hour, and at 2 hours. 1, 2
Diagnostic thresholds—GDM is diagnosed if ANY ONE value is met or exceeded:
- Fasting ≥92 mg/dL (5.1 mmol/L) 1, 2
- 1-hour ≥180 mg/dL (10.0 mmol/L) 1, 2
- 2-hour ≥153 mg/dL (8.5 mmol/L) 1, 2
These thresholds are derived from the HAPO study, which demonstrated a continuous relationship between maternal glucose levels and adverse outcomes (macrosomia, cesarean delivery, neonatal hypoglycemia, shoulder dystocia) without a clear safety threshold. 2, 5, 6
The one-step approach identifies approximately 15–20% of pregnancies with GDM—roughly twice the rate of the two-step method—capturing milder hyperglycemia that still benefits from treatment. 2, 5
Two-Step Approach (ACOG-Supported)
Step 1: Administer a 50-g non-fasting glucose challenge test at 24–28 weeks; if 1-hour plasma glucose is ≥130–140 mg/dL (threshold set by local consensus), proceed to Step 2. 1, 2
Step 2: Perform a 100-g oral glucose tolerance test after an overnight fast with measurements at fasting, 1,2, and 3 hours. 1, 2
Carpenter-Coustan diagnostic criteria—GDM is diagnosed when AT LEAST TWO of the following values are met or exceeded:
- Fasting ≥95 mg/dL (5.3 mmol/L) 1, 2
- 1-hour ≥180 mg/dL (10.0 mmol/L) 1, 2
- 2-hour ≥155 mg/dL (8.6 mmol/L) 1, 2
- 3-hour ≥140 mg/dL (7.8 mmol/L) 1, 2
ACOG notes that in clinical practice a single elevated value may be used for diagnosis, although traditional criteria require two abnormal values. 1, 2
The two-step approach identifies approximately 5–6% of pregnancies with GDM but may miss milder cases. 2
Test Preparation Requirements
The oral glucose tolerance test must be performed in the morning after an overnight fast of 8–14 hours. 1, 2
Patients must consume at least 150 g of carbohydrate per day for at least 3 days before the test to ensure adequate glycogen stores. 1, 2
The patient should remain seated throughout the test and must not smoke during testing. 1, 2
Physical activity should be unrestricted in the days preceding the test. 1
Management After Diagnosis
Glycemic Targets
While the provided evidence does not specify exact target glucose values for management, treatment aims to achieve normoglycemia through diet, exercise, and pharmacologic therapy when necessary. 7, 8
Fetal Surveillance
Ultrasound monitoring 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. 2
Treatment Approach
Initial management consists of medical nutrition therapy and regular physical activity; if glycemic targets are not achieved, insulin therapy or certain oral hypoglycemic agents should be initiated. 7, 8
Treating GDM reduces serious perinatal complications, with a number needed to treat of 34 to prevent outcomes such as macrosomia and shoulder dystocia. 2
Postpartum and Long-Term Follow-Up
All women diagnosed with GDM must undergo a 75-g oral glucose tolerance test at 4–12 weeks postpartum using non-pregnancy diagnostic criteria to detect persistent diabetes or prediabetes. 2, 4
Lifelong diabetes screening is required at least every 3 years thereafter, as women with prior GDM have a 3.4-fold increased risk of developing type 2 diabetes. 2, 4
Women identified with postpartum prediabetes should receive intensive lifestyle intervention or metformin therapy to prevent progression to overt diabetes. 2, 4
Breastfeeding should be encouraged, and preconception screening is essential before future pregnancies. 9
Critical Pitfalls to Avoid
Do not postpone screening beyond 28 weeks; the 24–28 week window aligns with peak pregnancy-related insulin resistance and enables timely intervention to reduce adverse maternal-fetal outcomes. 2, 7
Do not apply gestational diabetes thresholds (92/180/153 mg/dL) to early-pregnancy screening; early testing should use standard non-pregnancy diabetes diagnostic criteria (fasting ≥126 mg/dL or random ≥200 mg/dL). 2
The most common error is failing to retest high-risk women at 24–28 weeks after a negative early screen, leading to missed GDM diagnoses. 2, 4
Do not rely on urine glucose testing for GDM detection or management. 2
Do not use HbA1c for routine GDM screening or diagnosis; it has limited sensitivity for detecting glucose intolerance during pregnancy. 2
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. 2