Screening for Gestational Diabetes in Pregnant Women
All pregnant women without previously diagnosed diabetes should be screened for gestational diabetes at 24-28 weeks of gestation using either a one-step or two-step approach, with both methods considered acceptable based on regional preferences and resources. 1, 2
Universal Screening at 24-28 Weeks
Universal screening between 24-28 weeks of gestation is the standard of care for all pregnant women, supported by the U.S. Preventive Services Task Force (B recommendation), American Diabetes Association, and American College of Obstetricians and Gynecologists. 1
This timing is based on the HAPO study, which demonstrated strong, graded associations between maternal glycemia at 24-28 weeks and adverse outcomes including macrosomia, cesarean delivery, neonatal hypoglycemia, and preeclampsia. 1, 2
The moderate net benefit of screening at this gestational age includes significant reductions in preeclampsia, fetal macrosomia, and shoulder dystocia when gestational diabetes is detected and treated. 1
Two Screening Approaches
Two-Step Approach (Commonly Used in U.S.)
Initial screening: 50-g glucose challenge test (GCT) performed in non-fasting state, with threshold of 130-140 mg/dL. 1, 2
Diagnostic testing: If GCT is positive, proceed to 100-g oral glucose tolerance test (OGTT) in fasting state with measurements at fasting, 1-hour, 2-hour, and 3-hour. 1, 2
Diagnosis requires 2 or more abnormal values: fasting ≥95 mg/dL, 1-hour ≥180 mg/dL, 2-hour ≥155 mg/dL, or 3-hour ≥140 mg/dL (Carpenter-Coustan criteria). 1, 2, 3
One-Step Approach (International Consensus)
Direct diagnostic testing: 75-g OGTT performed in fasting state with measurements at fasting, 1-hour, and 2-hour. 1, 2, 3
Diagnosis requires only 1 abnormal value: fasting ≥92 mg/dL, 1-hour ≥180 mg/dL, or 2-hour ≥153 mg/dL (IADPSG criteria). 1, 2, 3
This approach diagnoses approximately twice as many women with gestational diabetes compared to the two-step method, though a large pragmatic trial found no differences in perinatal outcomes between the two approaches despite the higher diagnosis rate. 4
Early Screening for High-Risk Women
Women with significant risk factors should be screened at their first prenatal visit (12-14 weeks) to detect pre-existing undiagnosed type 2 diabetes, not gestational diabetes. 1, 2, 5
High-Risk Criteria Requiring Early Screening:
BMI ≥30 kg/m² - the most significant modifiable risk factor. 2
History of previous gestational diabetes - confers 4.14 times higher risk. 2
Family history of diabetes in first-degree relatives. 2
High-risk ethnicity: Hispanic, Native American, South or East Asian, African American, or Pacific Island descent. 1, 2
History of delivering macrosomic infant (>4.05 kg or 9 lb). 2, 5
History of polycystic ovary syndrome (PCOS). 2
Early Screening Method:
Use standard diabetes diagnostic criteria (not gestational diabetes criteria): fasting plasma glucose ≥126 mg/dL or random glucose ≥200 mg/dL with symptoms indicates overt diabetes. 1, 2
Critical: If early screening is negative, these high-risk women must be rescreened at 24-28 weeks using gestational diabetes criteria, as gestational diabetes typically develops later in pregnancy. 1, 2, 5
Women Who May Not Require Screening
Women with all of the following low-risk characteristics may potentially skip screening: age <25 years, BMI <25 kg/m², normal pre-pregnancy weight, no family history of diabetes in first-degree relatives, no history of glucose intolerance, and no history of adverse pregnancy outcomes. 1, 2
However, given that the vast majority of pregnant women have at least one risk factor, universal screening at 24-28 weeks is now considered standard practice in the United States. 1
Evidence Regarding Screening Before 24 Weeks
There is insufficient evidence to recommend routine screening before 24 weeks in average-risk women, as stated by the U.S. Preventive Services Task Force. 1, 2
Early screening in obese women does not necessarily improve perinatal outcomes compared to routine screening at 24-28 weeks, based on recent randomized controlled trial data. 2
Common Pitfalls to Avoid
Failing to rescreen high-risk women at 24-28 weeks after negative early screening - this is the most critical error, as gestational diabetes develops later in pregnancy and early screening only detects pre-existing diabetes. 1, 2
Using gestational diabetes criteria for early screening - early screening should use standard diabetes diagnostic criteria to identify pre-existing type 2 diabetes. 1, 2
Not screening women with BMI 25-29.9 kg/m² who have additional risk factors - these women warrant consideration for early screening. 2
Postpartum Follow-Up
All women diagnosed with gestational diabetes must be screened 4-12 weeks postpartum with a 75-g OGTT using non-pregnant diagnostic criteria to assess for persistent diabetes or prediabetes. 1, 2, 3, 6
Lifelong screening for diabetes should continue at least every 2-3 years, as women with gestational diabetes have a 3.4-fold increased risk of developing type 2 diabetes. 2, 3, 6