The glucose tolerance test — also called the oral glucose tolerance test (OGTT) — is a blood test that shows how your body handles sugar. It's the most important tool for detecting gestational diabetes early enough to do something about it.
Many pregnant women dread the test. Some find the sugar drink unpleasant; others worry about the results. Here's exactly what happens — step by step — and what you do next.
What Is a Glucose Tolerance Test?
The oral glucose tolerance test (OGTT) measures how your body responds to a specific amount of sugar. You drink a glucose solution, and healthcare providers take blood samples to see how quickly your body lowers your blood sugar back to normal.
During pregnancy, the placenta produces hormones that make your cells less sensitive to insulin. For most pregnant women, the pancreas compensates by producing more insulin. But in some women, production can't keep up, and blood sugar stays too high.
According to the WHO and the American Diabetes Association, gestational diabetes affects around 6–9% of pregnancies worldwide. It is typically detected between weeks 24 and 28, and usually resolves after delivery.
Who Should Be Tested?
Not all pregnant women are automatically offered the OGTT. Most international guidelines recommend testing based on risk factors, or universal screening depending on regional protocols.
Risk Factors for Gestational Diabetes
- BMI over 25 at the start of pregnancy
- Age over 25 with ethnic background from South Asia, the Middle East, or sub-Saharan Africa
- Age over 35
- Previous gestational diabetes
- Family history of gestational diabetes (parents, siblings)
- Previous baby weighing over 4.5 kg (9.9 lbs)
- Polycystic ovary syndrome (PCOS)
- Multiple pregnancy (twins or more)
If you have none of these risk factors, you may not be routinely offered the test. Discuss with your midwife or doctor if you're unsure — it's always better to test than to miss it.
When Is the Test Done?
The timing depends on your risk profile:
| Risk Factor | Test Timing |
|---|---|
| Previous gestational diabetes | Weeks 12–16, repeat at weeks 24–28 |
| One or more other risk factors | Weeks 24–28 |
| Sugar in urine at a routine check | As soon as discovered |
| No risk factors | Not routinely offered |
Most women are tested between weeks 24 and 28. At this point, insulin resistance is high enough for the test to give reliable results, but early enough for interventions to make a real difference.
Had gestational diabetes in a previous pregnancy? Let your midwife or OB know at your very first appointment so that early testing can be arranged. Learn more about what to expect from prenatal check-ups.
How to Prepare
Preparation is simple but important for reliable results.
In the Days Before the Test
- Eat normally in the days beforehand. Don't go low-carb or drastically change your diet — it can produce artificially low results.
- Maintain your usual level of physical activity. Don't exercise significantly more or less than usual.
- Tell your midwife or doctor about any medications you take. Some medications (like corticosteroids) can affect blood sugar.
The Evening and Night Before
- Eat a normal evening meal, ideally including complex carbohydrates (whole grains, vegetables).
- Fast from midnight (or at least 8–10 hours before the test). You must arrive fasting.
- You can drink water, but no coffee, tea, juice, or other beverages.
On Test Day
- Arrive at your scheduled time — ideally early in the morning.
- Bring something to read, listen to, or keep you occupied — you'll be there for 2–3 hours.
- Pack a snack to eat afterward (fruit, nuts, a sandwich).
Don't drive alone to the test if you feel unwell. Some women feel dizzy, nauseous, or exhausted during the test. Have a plan for getting home, or bring someone with you.
How the Test Works — Step by Step
Step 1: Fasting Blood Draw
The first blood sample is taken when you arrive, before you drink the glucose solution. This measures your fasting blood sugar — your baseline.
Step 2: The Glucose Drink
You drink 75 grams of glucose dissolved in 250–300 ml of water. This is usually served as a sweet, slightly syrupy drink. Some women find the taste unpleasant, but it isn't harmful.
Tips for getting it down:
- Drink it quickly rather than sipping — it's over faster
- Some women find it easier when the drink is cold
- It's fine to take a brief pause between sips, but try to finish it within 5 minutes
Step 3: Wait and Second Blood Draw
After drinking the glucose solution, you wait for 2 hours. During this time:
- Stay seated — don't take walks or do any physical activity
- Don't eat or drink anything other than water
- Don't smoke (for the entire testing period)
After exactly 2 hours, a second blood sample is taken. This shows how well your body has lowered your blood sugar.
Some clinics also take a blood sample at the 1-hour mark, but this varies between providers.
What Might You Feel?
Most women tolerate the test well, but some common reactions include:
- Nausea — the sweet drink on an empty stomach can cause discomfort
- Dizziness — normal after fasting and a sugar load
- Tiredness — your body is working hard to process the glucose
- Headache — can come from fasting, dehydration, or the test itself
These are harmless and pass quickly once the test is over. Eat your snack as soon as you're done.
What Do the Results Mean?
Results are measured in mmol/L (millimoles per liter of blood). The WHO and IADPSG use the following diagnostic thresholds:
Diagnostic Thresholds
| Timing | Normal Value | Gestational Diabetes |
|---|---|---|
| Fasting | Below 5.1 mmol/L | 5.1 mmol/L or higher |
| After 1 hour | Below 10.0 mmol/L | 10.0 mmol/L or higher |
| After 2 hours | Below 8.5 mmol/L | 8.5 mmol/L or higher |
Just one elevated value is sufficient to diagnose gestational diabetes.
(Note: Some countries use slightly different thresholds. Your healthcare provider will use the guidelines that apply locally.)
What If Values Are Just Over the Threshold?
A value just slightly above the limit still means a gestational diabetes diagnosis. But it also means you can likely manage it with diet and activity alone, without medication.
What If Values Are Very High?
Fasting blood sugar above 7.0 mmol/L or a random blood sugar above 11.1 mmol/L may suggest you had diabetes before pregnancy (pre-existing diabetes), not just gestational diabetes. This requires prompt follow-up and possibly insulin therapy.
Results usually come back within 1–3 business days. At some clinics you get same-day results. Contact your midwife or doctor if you haven't received results within a week.
What Happens If You're Diagnosed?
First of all: don't panic. Gestational diabetes can be managed well, and the vast majority of women have a completely normal pregnancy and delivery with the right care.
Dietary Changes
Diet is the most important intervention. You don't need to cut out carbohydrates, but you do need to be more mindful about what and when you eat.
The key principles are:
- Eat regularly — 3 main meals and 2–3 snacks
- Choose complex carbohydrates (whole grains, vegetables, legumes) over simple ones (white bread, sweets)
- Combine carbohydrates with protein and fat for more stable blood sugar
- Spread carbohydrate intake evenly throughout the day
Physical Activity
Moderate physical activity helps your body use sugar more efficiently. 30 minutes of daily walking is a good starting point. Exercise after meals is particularly effective for lowering blood sugar.
Blood Sugar Monitoring
You'll learn to measure your own blood sugar with a small device that pricks your finger. Measurements are typically taken:
- Fasting in the morning
- 1–2 hours after each main meal
Target values for self-monitoring:
- Fasting: Below 5.3 mmol/L
- 1 hour after a meal: Below 7.8 mmol/L
- 2 hours after a meal: Below 6.7 mmol/L
Insulin Therapy
Around 10–20% of pregnant women with gestational diabetes need insulin in addition to diet and activity. This is most common when fasting blood sugar is difficult to control.
Insulin is safe to use during pregnancy. It does not cross the placenta and does not directly affect the baby.
Additional Monitoring
With gestational diabetes, you'll receive closer care:
- More frequent prenatal appointments
- Extra ultrasounds to monitor the baby's growth
- Assessment of delivery method and timing (some women are recommended induction around weeks 38–40)
What Does This Mean for the Baby?
Untreated gestational diabetes can lead to:
- Large baby (macrosomia) — birth weight over 4.5 kg (9.9 lbs)
- Increased risk of shoulder dystocia at delivery
- Low blood sugar in the baby after birth
- Increased risk of cesarean section
With good blood sugar control, all of these risks are significantly reduced. Most babies born to mothers with well-managed gestational diabetes have completely normal birth weights.
Use our due date calculator to track your pregnancy week by week and stay on top of important appointments and milestones throughout your pregnancy.
After Delivery
Does Gestational Diabetes Go Away?
Yes, in the vast majority of cases. Blood sugar typically returns to normal within days or weeks after delivery. You'll usually be offered a repeat glucose tolerance test 6–12 weeks postpartum to confirm that everything has returned to normal.
Increased Risk of Type 2 Diabetes
Women who have had gestational diabetes have up to a 50% risk of developing type 2 diabetes later in life. You can reduce this risk by:
- Maintaining a healthy weight
- Regular physical activity
- A balanced diet
- Blood sugar monitoring every 1–3 years
In a Future Pregnancy
If you've had gestational diabetes once, the risk is higher in a future pregnancy. Let your care provider know early so testing can be scheduled from weeks 12–16.
Frequently Asked Questions
Is the glucose tolerance test painful?
The blood draws are like normal blood tests — a small needle prick in the arm. The glucose drink tastes very sweet and can cause nausea, but is not painful. Most women find the test tedious (long wait) rather than uncomfortable.
Can I decline the glucose tolerance test?
Yes, all testing in pregnancy is voluntary. But your midwife and doctor will strongly recommend it if you have risk factors. Undetected gestational diabetes can cause complications for both you and your baby that are easily preventable.
Can I eat something if I feel very nauseous during the test?
No, unfortunately. Eating during the test makes the results unreliable, and you'd need to repeat the test on another day. Tell the healthcare staff about your nausea — they can let you lie down or give you some cold water.
What if I vomit the glucose drink?
The test must be stopped and repeated on another day. This is not uncommon. Some clinics may offer an alternative method (such as HbA1c measurement combined with fasting blood sugar), but the oral glucose tolerance test is considered the gold standard.
Can my blood sugar be elevated without me noticing?
Absolutely. Gestational diabetes rarely causes obvious symptoms. Some women notice increased thirst, more frequent urination, or fatigue — but these are common pregnancy complaints regardless. That's exactly why the test is so important: it catches what you can't feel yourself.
Summary
| Topic | Details |
|---|---|
| What | Test measuring the body's ability to handle sugar |
| Who | Pregnant women with risk factors (or universal screening depending on your country) |
| When | Weeks 24–28 (or weeks 12–16 for high-risk women) |
| Preparation | Fast from midnight, drink water only |
| Duration | Approx. 2–3 hours |
| Diagnostic thresholds | Fasting: 5.1 mmol/L, after 2 hours: 8.5 mmol/L (WHO) |
| Treatment | Diet, activity, possibly insulin |
| Prognosis | Usually resolves after delivery |
Read More
- Pregnancy diet and nutrition guide
- Pregnancy symptoms guide
- Flying during pregnancy safely
- Traveling during pregnancy tips
Sources
- WHO. "Diagnostic criteria and classification of hyperglycaemia first detected in pregnancy." who.int
- American Diabetes Association. "Standards of Medical Care in Diabetes — Gestational Diabetes." diabetes.org
- IADPSG. "International Association of Diabetes and Pregnancy Study Groups Recommendations on the Diagnosis and Classification of Hyperglycemia in Pregnancy." iadpsg.org
- ACOG. "Gestational Diabetes Mellitus." Practice Bulletin No. 190. acog.org