Between 24 and 28 weeks of pregnancy, all expectant mothers are screened for Gestational Diabetes Mellitus (GDM). In the United States, obstetricians use the two-step approach: an initial 50g 1-hour non-fasting screening test (normal < 130–140 mg/dL), followed by a 100g 3-hour fasting diagnostic OGTT if the screen is elevated. Meeting or exceeding two or more Carpenter-Coustan thresholds confirms gestational diabetes.
Between the 24th and 28th week of pregnancy, one of the most important milestones is the routine pregnancy glucose test.
During the second and third trimesters, the placenta produces high levels of hormones designed to shunt nutrients to the growing fetus. In some mothers, however, these hormones overwhelm the pancreas, triggering Gestational Diabetes Mellitus (GDM).
What is the difference between the 1-hour screening test and the 3-hour diagnostic test, what are the Carpenter-Coustan cutoff numbers, and what should you eat before the test?
Why Is the Glucose Test Done at 24 to 28 Weeks?#
During pregnancy, the placenta secretes significant amounts of diabetogenic hormones:
- Human Placental Lactogen (hPL)
- Progesterone & Estrogen
- Placental Cortisol & Prolactin
These hormones naturally reduce maternal insulin sensitivity so that glucose remains in the maternal bloodstream longer, allowing it to cross the placenta and nourish the baby. Between weeks 24 and 28, placental hormone secretion reaches peak intensity. If the mother’s pancreas cannot produce 2 to 3 times more insulin to compensate, blood sugar rises into the gestational diabetes range.
Step 1: The 50-Gram 1-Hour Glucose Challenge Test (GCT)#
The first step is a non-fasting screening test:
- The Protocol: You drink a 50-gram glucose drink (Glucola) within 5 minutes. Exactly 60 minutes later, blood is drawn to measure plasma glucose.
- Fasting: Fasting is not required. You can eat a normal, light meal earlier in the day.
- The Cutoff:
- Normal Result: < 130 to 140 mg/dL (Pass).
- Abnormal Result: ≥ 130 to 140 mg/dL (roughly 15% to 20% of women test above this threshold and are referred for the 3-hour diagnostic test).
- Automatic Diagnosis: If the 1-hour screen exceeds 200 mg/dL, gestational diabetes is diagnosed immediately without needing a 3-hour test.
Step 2: The 100-Gram 3-Hour Diagnostic OGTT#
If you test above the 1-hour screening threshold, you will take the definitive 3-hour diagnostic test:
- The Protocol: Requires an overnight 8-to-12-hour fast. Blood is drawn at fasting (Hour 0), you drink a 100-gram Glucola solution, and blood is drawn at 1, 2, and 3 hours.
- Carpenter-Coustan Diagnostic Criteria:
| Measurement Point | Carpenter-Coustan Diagnostic Cutoff | National Diabetes Data Group (NDDG) Cutoff |
|---|---|---|
| Fasting (Hour 0) | ≥ 95 mg/dL (5.3 mmol/L) | ≥ 105 mg/dL (5.8 mmol/L) |
| 1 Hour Post-Load | ≥ 180 mg/dL (10.0 mmol/L) | ≥ 190 mg/dL (10.6 mmol/L) |
| 2 Hours Post-Load | ≥ 155 mg/dL (8.6 mmol/L) | ≥ 165 mg/dL (9.2 mmol/L) |
| 3 Hours Post-Load | ≥ 140 mg/dL (7.8 mmol/L) | ≥ 145 mg/dL (8.0 mmol/L) |
- Official Diagnosis: Under ACOG guidelines, meeting or exceeding two or more abnormal values confirms a clinical diagnosis of Gestational Diabetes Mellitus.
What Should You Eat Before the Pregnancy Glucose Test?#
A common mistake pregnant mothers make is trying to "trick" the test by strictly cutting out all carbohydrates for several days prior:
- The Carb-Starvation Backfire: If you eat zero carbs for 3 days before the test, your liver upregulates gluconeogenesis and your pancreas downshifts insulin production. When you suddenly drink 50g or 100g of pure sugar, your blood sugar will skyrocket, causing a false-positive failure.
- The Proper Preparation:
- Eat normal, balanced meals containing complex carbohydrates (oatmeal, sweet potatoes, brown rice) for 3 days before the test.
- On the morning of a 1-hour non-fasting test, eat a balanced protein-and-fat breakfast (e.g., scrambled eggs with avocado and whole-grain toast) and avoid sugary cereals, pastries, or fruit juices.
Uncontrolled gestational diabetes can cause excessive fetal growth (macrosomia > 4,000g), birth trauma, neonatal hypoglycemia after delivery, and increased lifetime risk of maternal Type 2 diabetes. Most cases are successfully managed with dietary nutrition and blood sugar tracking.
To explore how high morning blood sugar happens in non-pregnant adults, read [Impaired Fasting Glucose & The Dawn Phenomenon Explained](/blog/impaired-fasting-glucose-dawn-phenomenon-somogyi-effect).
Track Your Health & Pregnancy Biomarkers with Meridian#
Monitoring your laboratory blood biomarkers over time gives you objective validation that your diet, exercise, and lifestyle habits are keeping your metabolic health and glucose tolerance in optimal ranges.
Meridian is an offline personal health vault for iPhone designed to give you complete ownership of your medical diagnostic data.
- Instant Lab Report Extraction: Take a photo or upload a PDF of your pregnancy glucose screen, Complete Blood Count, and Comprehensive Metabolic Panel from Quest, Labcorp, or your clinic. Meridian extracts your biomarkers on-device using Apple VisionKit.
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