For the 1-hour 50g glucose challenge test (GCT) during pregnancy, no fasting is required — eat your usual diet, including normal carbohydrate intake. For the diagnostic 3-hour 100g OGTT (or 2-hour 75g OGTT in some practices), fast 8 to 14 hours overnight after a normal carbohydrate day. Avoid low-carb diets in the days before either test, since carbohydrate restriction causes false positives.
The Two Pregnancy Glucose Tests
Most US obstetric practices use a two-step screening approach for gestational diabetes mellitus (GDM):
- Step 1 — 1-hour 50g glucose challenge test (GCT): a screening test typically done at 24 to 28 weeks of pregnancy. No fasting required.
- Step 2 — 3-hour 100g oral glucose tolerance test (OGTT): a diagnostic test done if the 1-hour GCT is abnormal. Requires overnight fasting.
A growing number of practices use the one-step alternative endorsed by the WHO and IADPSG: a single 2-hour 75g OGTT after overnight fasting, with GDM diagnosed if any one of three values is elevated. Both approaches are accepted by ACOG, with the two-step approach remaining most common in the United States.
What to Eat Before the 1-Hour GCT
The 1-hour 50g glucose challenge test does not require fasting in the standard ACOG protocol. The most important guidance for the days before is:
- Eat your normal diet, including your usual carbohydrate intake — at least 150 grams of carbohydrate per day for the 3 days before the test.
- Do NOT cut carbohydrates in the days before, even with good intentions. Low-carb intake causes physiologic insulin resistance and false-positive screens.
- Eat a balanced meal 2 to 3 hours before the test if your appointment is mid-morning or afternoon. Include protein, complex carbohydrates, and healthy fats.
- Avoid extra sugar right before the test; do not try to “warm up” with juice, candy, or sugary coffee drinks.
- Drink water normally. Stay hydrated.
What to Eat Before the 3-Hour OGTT
The 3-hour 100g OGTT requires more preparation. According to ACOG Practice Bulletin 190 on gestational diabetes, the recommended preparation is:
- 3 days of unrestricted diet with at least 150 grams of carbohydrate per day before the test.
- Fast 8 to 14 hours overnight before the test (water only).
- No food, smoking, or chewing gum during the test itself.
- Sit quietly between draws — no walking around or exercise.
You will have a fasting blood draw, then drink the 100g glucose solution within 5 minutes, then have additional draws at 1, 2, and 3 hours after the drink.
Sample 24-Hour Pre-Test Plan
| Time | 1-Hour GCT (no fast) | 3-Hour OGTT (fast) |
|---|---|---|
| Day before — breakfast | Eggs, whole-grain toast, fruit | Eggs, whole-grain toast, fruit |
| Day before — lunch | Grilled chicken sandwich, side salad | Grilled chicken sandwich, side salad |
| Day before — dinner | Pasta with vegetables and protein | Pasta with vegetables and protein (finish by usual time) |
| Evening | Snack OK; avoid alcohol | Stop eating; water only after dinner |
| Test morning | Light breakfast 2 to 3 hours before, no extra sugar | Continue fast; water only |
Cutoffs for the 1-Hour GCT
The 1-hour value after the 50g glucose drink is interpreted against a single cutoff. Most US practices use:
- Under 140 mg/dL: normal screen.
- 140 to 199 mg/dL: abnormal screen — proceed to 3-hour OGTT.
- 200 mg/dL or higher: presumed gestational diabetes; some practices skip the 3-hour test and start treatment.
Some practices use a lower cutoff of 130 or 135 mg/dL for higher sensitivity, which catches more cases of GDM but increases the number of women referred for the 3-hour test.
Cutoffs for the 3-Hour OGTT (Carpenter-Coustan)
| Time Point | Carpenter-Coustan Cutoff | NDDG Cutoff |
|---|---|---|
| Fasting | 95 mg/dL | 105 mg/dL |
| 1 hour | 180 mg/dL | 190 mg/dL |
| 2 hours | 155 mg/dL | 165 mg/dL |
| 3 hours | 140 mg/dL | 145 mg/dL |
Gestational diabetes is diagnosed when 2 or more of the 4 values meet or exceed the chosen cutoffs. Carpenter-Coustan is more commonly used than NDDG in current US practice.
Why Carbohydrate Loading Beforehand Matters
If you have been eating a low-carb diet — even unintentionally because of pregnancy nausea or food aversions — the OGTT can produce a false-positive result. Carbohydrate restriction lowers insulin secretion and reduces tissue glucose uptake efficiency. When you suddenly take in 100 grams of glucose, the response looks abnormal even though your usual eating pattern would not have caused glucose problems. The 3-day, 150-grams-per-day rule is designed to normalize your insulin and glucose handling before testing.
What Happens If You Are Diagnosed With GDM
Gestational diabetes is managed with dietary changes, blood glucose monitoring, and medication if lifestyle alone does not achieve targets. According to the CDC, GDM affects roughly 5 to 9 percent of US pregnancies. Most women with GDM return to normal glucose after delivery, but the diagnosis raises long-term risk of type 2 diabetes — up to 50 percent in the decade after pregnancy. Postpartum screening at 6 to 12 weeks and ongoing surveillance are important. For broader context, see our prediabetes 101 hub and the diet and nutrition hub.
Tips for Tolerating the Glucose Drink
- Ask for the chilled or carbonated version if available — many find it easier to keep down.
- Sip slowly within the 5-minute window rather than chugging.
- Sit upright for the first 15 to 20 minutes.
- Bring water, snacks for after the test, and something to do during the wait.
- Tell the nurse early if you feel light-headed, sweaty, or nauseated.
The Bottom Line
For the 1-hour pregnancy glucose challenge, eat your normal diet — including usual carbohydrates — and do not fast. For the 3-hour OGTT, eat at least 150 grams of carbohydrate per day for 3 days, then fast 8 to 14 hours overnight. The biggest preparation pitfall is cutting carbohydrates beforehand, which causes false-positive results. Talk with your obstetric provider if you have severe nausea, food aversions, or other concerns about completing the test.
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before starting, stopping, or changing any medication or treatment plan.
