Glucose tolerance test: what to expect, how to prepare

Most US OBs use the two-step gestational diabetes screening protocol recommended by ACOG. The first step is the one-hour glucose challenge, which almost every pregnant person in the US is offered between weeks 24 and 28. This article covers that screening visit specifically — what it is, what to expect, how to prep, and what the result means.

Why it is done at weeks 24 to 28

Gestational diabetes develops because of placental hormones that increase insulin resistance. Those hormones rise steadily from week 20 onward and peak in the third trimester. By weeks 24 to 28, the insulin demand is high enough to unmask diabetes in people whose pancreas cannot keep up — but early enough that a diagnosis still leaves 10 to 14 weeks for management. About 6 to 9 percent of US pregnancies are diagnosed with gestational diabetes.

If you have risk factors (BMI > 30, previous gestational diabetes, family history of type 2 diabetes, polycystic ovary syndrome, prior baby over 9 lbs), your OB may screen earlier — sometimes at the first prenatal visit.

What the one-hour test is like

1. You do not need to fast. Many people eat a light breakfast before. (See "what to actually eat" below.)
2. You arrive at the lab or OB office and drink a 50-gram glucose solution. 8 oz of an extremely sweet beverage, usually orange or fruit-punch flavored. You have 5 minutes to finish it.
3. You wait one hour. Most labs require you to stay in the waiting area. Bring a book. Avoid heavy activity (walking around, exercise) and avoid sugary snacks — both can affect the result.
4. At one hour, they draw blood. A single venipuncture, takes 30 seconds.
5. Result usually back the same day or next business day.

The drink itself is the part nobody loves. It is more sugar than most people consume in a regular drink — 50g is roughly 12 teaspoons. About 30 percent of people experience nausea, jittery feelings, or a brief headache. About 2 to 3 percent vomit, which requires repeating the test.

The cutoff (and why "failing" is misleading)

• < 140 mg/dL at one hour: pass. No further testing.
• 140 to 199 mg/dL: proceed to the 3-hour confirmatory test.
• ≥ 200 mg/dL: some practices skip the 3-hour and diagnose gestational diabetes directly. Most still confirm.

Some practices use a stricter cutoff (130 mg/dL or 135 mg/dL) which catches more cases but produces more false positives. ACOG considers either threshold acceptable.

About 15 to 20 percent of people "fail" the one-hour test. Of those, only about 10 to 30 percent are diagnosed with gestational diabetes after the 3-hour test. The one-hour test is intentionally over-sensitive — it is a screening test, not a diagnostic one. A failed one-hour does not mean you have gestational diabetes.

What to actually eat that morning

The standard advice is "eat normally" but the realistic guidance is more specific:

• Have a balanced breakfast 1 to 2 hours before — protein, fat, complex carbs. Eggs and whole-grain toast. Greek yogurt and fruit. Oatmeal with nuts. Not pancakes with syrup or sugary cereal.
• Avoid simple sugars and white flour in the meal before the test. The juice drink alone is 50g of sugar — adding another 30g of breakfast sugar can push the one-hour reading falsely high.
• Hydrate well the morning of. Dehydration can affect the reading.
• Drink the glucose solution cold and quickly. Faster consumption is better tolerated. Some labs let you refrigerate it 5 minutes ahead.

What to do if you cannot keep the drink down

If you vomit within 30 minutes of finishing, the test has to be repeated on a different day. If you feel nauseated, ginger candy or a tiny sip of water often helps without affecting the result. Some labs will allow a slower 10-minute consumption window if you ask.

If you have hyperemesis gravidarum or cannot tolerate the glucose drink at all, your OB has alternatives: a 4-week home glucose monitoring window, a hemoglobin A1c test (less commonly used in pregnancy but accepted), or a non-pregnancy oral glucose tolerance test format.

If you fail the one-hour: the 3-hour test

The 3-hour test is the diagnostic test. It is more involved:

• Overnight fast. 8 to 12 hours, water only.
• Fasting blood draw on arrival. Baseline glucose.
• Drink a 100-gram glucose solution. Twice the dose of the screening drink.
• Blood draws at 1, 2, and 3 hours. Four draws total.
• 4 hours total at the lab. Bring food and water for after the final draw.

Gestational diabetes is diagnosed if two or more of the four values are above threshold. The Carpenter-Coustan thresholds (most US labs):

• Fasting: < 95 mg/dL
• 1 hour: < 180 mg/dL
• 2 hour: < 155 mg/dL
• 3 hour: < 140 mg/dL

About 70 to 80 percent of people who fail the one-hour pass the three-hour. Only one elevated value on the three-hour is sometimes called "impaired glucose tolerance" — managed with diet but not a formal diabetes diagnosis.

What gestational diabetes management actually looks like

If you are diagnosed, the standard path is:

1. Referral to a diabetes educator or dietitian. Carb-counting, meal planning, snack timing.
2. Home blood glucose monitoring. Fasting plus 1 hour after each meal, 4 times per day. Finger-prick lancet system.
3. Dietary adjustments. About 75 to 80 percent of gestational diabetes is managed with diet alone — controlled carbohydrates spread evenly across meals.
4. Exercise. Walking 10 to 15 minutes after meals lowers post-prandial glucose substantially. Often the single most effective intervention.
5. Medication if needed. Insulin is the first line if diet and exercise are not enough. Metformin is sometimes used; oral glyburide less commonly.
6. Slightly more frequent monitoring in the third trimester — more ultrasounds, sometimes weekly non-stress tests near the end.
7. Postpartum glucose screening at 6 to 12 weeks to confirm the diabetes resolved (about 90 percent of cases do).

Gestational diabetes is managed; it is not a verdict on your future health. About half of people who have gestational diabetes go on to develop type 2 diabetes within 10 years, but lifestyle factors after delivery are the bigger driver than the diagnosis itself.

Risk factors that lower the bar for early screening

Your OB may screen at the first prenatal visit (instead of week 24 to 28) if you have:

• BMI 30 or above pre-pregnancy
• Previous gestational diabetes
• Family history of type 2 diabetes (first-degree relative)
• PCOS (polycystic ovary syndrome)
• Previous baby ≥ 9 lbs or unexplained stillbirth
• Age > 40
• South Asian, Black, Hispanic/Latino, Native American, or Pacific Islander background (higher baseline risk)

If the early screen is negative, the standard screen at 24 to 28 weeks is still done.

General guidance based on ACOG and ADA recommendations. Your specific screening and management plan is set by your OB based on your full health picture.