Glucose Test in Pregnancy: Levels & What to Expect

Somewhere between weeks 24 and 28, your provider will hand you an order for a glucose test in pregnancy (the oral glucose tolerance test, or OGTT). For a lot of expecting mothers it's the most confusing and nerve-wracking lab of the whole pregnancy: you show up fasting, drink a very sweet solution, and then wait around the lab for a couple of hours. Let's walk through it calmly — what the test is for, when it's done, how to prepare, exactly how the procedure works step by step, what the normal levels are, and what to do if your result comes back "high."

What the glucose test is and why it's done

The glucose tolerance test is a way to check how your body handles sugar (glucose) during pregnancy. You give a blood sample while fasting, then drink a solution with a precisely measured amount of glucose, and after a set time your blood is drawn again. From how quickly your blood sugar rises and then comes back down, your provider can see whether your body is making enough of its own insulin to keep glucose in a healthy range.

The main goal of the test is to catch gestational diabetes early — high blood sugar that first appears during pregnancy. We cover the condition itself in detail in a separate article on gestational diabetes; here we're focused on the test that detects it.

Why does it matter? Gestational diabetes most often has no symptoms at all — you can feel perfectly fine while your blood sugar runs high. That's exactly why the test is offered to every pregnant person, not just those with complaints. Sometimes a routine urine test picks up glucose — a little "sugar in the urine" during pregnancy can be normal, but it's one more reason not to skip the glucose test. Catching it early lets you gently adjust your diet and monitoring and lower the risks for both you and your baby.

When in pregnancy the glucose test is done

The best window for routine glucose testing is 24–28 weeks. This is the midpoint of pregnancy: by now the placenta is producing enough hormones that naturally lower your sensitivity to insulin, so any hidden problems with sugar metabolism become visible. The test falls in the second trimester of pregnancy — the season of routine screenings.

In some cases the test, or a sugar check, is ordered earlier — in the first trimester or right at your first prenatal visit. This is usually tied to risk factors:

• being overweight before pregnancy or rapid pregnancy weight gain;
• gestational diabetes in a previous pregnancy or a large baby (over 9 lb / 4 kg);
• diabetes in close relatives;
• age over 30–35, or polycystic ovary syndrome;
• elevated fasting sugar or glucose in the urine on routine prenatal labs.

If the early check is normal, the full glucose tolerance test is still repeated at 24–28 weeks. Later than 28 weeks (usually up to week 32) the test is still possible if for some reason it wasn't done on time.

How to prepare for the test: dos and don'ts

Preparation directly affects your result — mistakes in diet or routine can "spoil" the glucose test in pregnancy and give false numbers. Let's break the prep down step by step.

For 3 days before the test

For the three days before the test, eat as you normally do, without cutting carbs: your diet should include at least 150 g of carbohydrates a day (grains, bread, fruit, vegetables). This matters: if you "go on a diet" the day before and drop carbs, your body reacts more sharply to the sweet solution, and the result can come back falsely high. Keep your physical activity normal too, without exhausting workouts.

The night before and the morning of

How to take the test correctly:

• Your last meal is the evening before, 8–14 hours before the test. Have your usual dinner, with carbs, but don't eat late.
• Come in strictly fasting in the morning. You may drink only plain, still water; tea, coffee, juice, and sweet drinks are off-limits.
• Don't smoke the evening before or during the test (smoking isn't recommended in pregnancy at all).
• Tell your provider in advance about all your medications: some (for example, glucocorticoids and certain blood-pressure drugs) affect blood sugar.
• Set aside at least 2–2.5 hours for the visit and bring something to pass the time.

When the test should be postponed

The glucose tolerance test isn't done in certain situations — in these cases it's rescheduled or replaced with another test:

• severe early-pregnancy nausea and vomiting (you simply won't keep the solution down);
• an acute infection or inflammatory illness, or a flare-up of a chronic condition;
• strict bed rest ordered by your provider;
• past stomach surgery (dumping syndrome);
• if fasting sugar is already high (7.0 mmol/L / 126 mg/dL or above) — then the glucose load isn't needed and is even undesirable; the diagnosis is made from that result.

How the test works, step by step

A lot of the anxiety comes from not knowing, so here's how the glucose test in pregnancy actually goes, step by step. The classic version used internationally is the one-step test with 75 g of glucose and three blood draws:

• Step 1. Fasting blood draw. Blood is taken from a vein and your glucose is measured right away. If it's normal, you move on to the glucose load. (If fasting sugar is already high, the test stops here.)
• Step 2. The glucose drink. You're given a solution of 75 g of glucose dissolved in 250–300 ml of warm water. Drink it calmly, over about 5 minutes. It's very sweet; sometimes you're allowed to add a little lemon juice to make it easier.
• Step 3. The 1-hour draw. Exactly 60 minutes after the drink, your blood is drawn again.
• Step 4. The 2-hour draw. Another hour later comes the third and final blood draw.

The whole time, it's important to sit quietly near the lab: don't walk around, don't run errands, and don't eat or drink anything but water. Activity and food change your blood sugar and skew the result. In the United States the two-step approach is more common: first a 50 g glucose challenge for one hour with no fasting needed, and only if that comes back high do you go on to the longer 3-hour, 100 g test (which does require fasting). The one-step 75 g test is the standard in many other countries and is the version described above.

Normal glucose test levels in pregnancy

Pregnancy uses stricter cutoffs than the rest of the population — because even a small rise in blood sugar matters for the baby. Results are read from venous plasma. Normal glucose tolerance test levels in pregnancy (the values considered healthy) for the one-step 75 g test are:

• Fasting: below 5.1 mmol/L (92 mg/dL);
• 1 hour after the drink: below 10.0 mmol/L (180 mg/dL);
• 2 hours: below 8.5 mmol/L (153 mg/dL).

On the one-step test, gestational diabetes is diagnosed if even one value reaches or exceeds the threshold: fasting ≥ 5.1 mmol/L, 1 hour ≥ 10.0 mmol/L, or 2 hours ≥ 8.5 mmol/L. In other words, a single "out-of-range" value is enough for the diagnosis — you don't need all three to be high. If fasting sugar is 7.0 mmol/L (126 mg/dL) or higher (or a random sugar is ≥ 11.1 mmol/L / 200 mg/dL), that already points to overt (pre-existing) diabetes and calls for separate care.

Units can differ: some labs report in mg/dL and others in mmol/L, but the reference points above are the same either way. In the US two-step pathway, the 50 g screening draw is usually flagged at about 130–140 mg/dL (7.2–7.8 mmol/L), and the follow-up 3-hour 100 g test uses the Carpenter–Coustan cutoffs — fasting 95 mg/dL (5.3 mmol/L), 1 hour 180 mg/dL (10.0 mmol/L), 2 hours 155 mg/dL (8.6 mmol/L), and 3 hours 140 mg/dL (7.8 mmol/L) — with two or more values met or exceeded needed for the diagnosis. Always leave the interpretation to your provider, who reads the numbers alongside the full picture of your pregnancy.

What a high result means and what happens next

First things first — don't panic. A high test result doesn't mean you did something wrong or that something will happen to your baby. Gestational diabetes is a common condition that responds well to management, and for most people blood sugar returns to normal after birth.

If the test finds gestational diabetes, the next steps usually look like this:

• your provider (an OB-GYN, and if needed an endocrinologist) explains the diagnosis and sets up a monitoring plan;
• the foundation of treatment is food: gentle adjustments to your diet and spreading carbohydrates evenly through the day. Our guide to foods to avoid during pregnancy can help here;
• you'll be shown how to check your sugar at home with a glucometer — usually fasting and one to two hours after meals;
• gentle physical activity is added (a walk after meals);
• if diet and activity aren't enough, your provider may prescribe insulin — it's safe in pregnancy. For more on managing the condition, see our article on gestational diabetes.

Sometimes a result is borderline or questionable (for instance, if the prep wasn't followed). In that case your provider may order a repeat test. Don't make treatment decisions on your own based on the numbers on a lab slip — talk them through with your provider.

Common questions: safety, nausea, and opting out

Is the test safe for mom and baby

Yes. The glucose tolerance test is a standard, well-studied screening done for millions of pregnant people around the world. A one-time dose of glucose doesn't harm you or your baby, and the temporary rise in blood sugar during the test comes back down quickly. The only limits are the situations where the test is postponed (above).

Why the test can make you nauseous

Nausea is a common and understandable reaction: a very sweet drink on an empty stomach can bring on queasiness on its own, and in the second trimester a sensitive stomach adds to it. To make it easier:

• sip the solution slowly over a few minutes rather than gulping it;
• if it's allowed, add a little lemon juice;
• sit quietly, breathe deeply, and avoid sudden movements.

If you do vomit after drinking the solution, the test will have to be rescheduled for another day — be sure to tell the lab staff, since there's no point continuing.

Can you decline the glucose test

Every test during pregnancy is voluntary, and technically you can decline. But it's worth understanding: the glucose tolerance test is still the most accurate way to catch gestational diabetes, which often runs silently. Simpler tests (fasting sugar alone or an A1C) are less sensitive and can miss it. If the test worries you, it's better to talk your concerns over with your provider than to skip it — together you can find a comfortable option.

Key takeaways

• The glucose test in pregnancy screens for gestational diabetes, which usually has no symptoms.
• The routine window is 24–28 weeks; with risk factors, sugar is checked earlier.
• Prep: 3 days of your usual carb-containing diet, then 8–14 hours fasting; come in strictly fasting, water only.
• Procedure: fasting draw → 75 g glucose drink → repeat draws at 1 and 2 hours; sit quietly the whole time. (In the US, a 50 g 1-hour screen first, then a 3-hour 100 g test if it's high.)
• Normal levels: fasting < 5.1; 1 hour < 10.0; 2 hours < 8.5 mmol/L. A diagnosis is made if even one value is exceeded.
• A high result is no reason to panic: gestational diabetes is well controlled with diet and monitoring.

This article is general information and does not replace personalized medical advice. Always discuss the timing, preparation, reference levels, and interpretation of your own result with your OB-GYN or endocrinologist.