The 3-hour glucose test is a diagnostic blood test used during pregnancy to determine whether you have gestational diabetes. It’s typically ordered after you’ve had an elevated result on the shorter 1-hour glucose screening, and it involves fasting overnight, drinking a sugary solution, and having your blood drawn four times over three hours. Most pregnant people take this test between 24 and 28 weeks of gestation.
Why You’re Taking This Test
Most prenatal care follows a two-step approach to checking for gestational diabetes. The first step is a quick screening: you drink a smaller sugar solution (50 grams) without fasting, and your blood is drawn once an hour later. If your blood sugar comes back above a certain threshold, that doesn’t mean you have gestational diabetes. It means you need the second step, the 3-hour test, to find out for sure.
The 1-hour screening casts a wide net on purpose. Many people who flag on the screening go on to pass the 3-hour test. The 3-hour version is the actual diagnostic test, meaning its results are what your provider uses to make or rule out a diagnosis.
How the Test Works
You’ll need to fast for at least 8 hours before the test, so most labs schedule it first thing in the morning. When you arrive, a nurse draws your blood to measure your fasting blood sugar. Then you drink a glucose solution containing 100 grams of sugar, which is twice the amount in the 1-hour screening drink. You have about five minutes to finish it.
After that, your blood is drawn three more times: at the 1-hour, 2-hour, and 3-hour marks. That means four total blood draws over the course of the appointment. You’ll need to stay at the lab for the full three hours. During the wait, you should remain seated and avoid eating, smoking, or strenuous activity, as all of these can affect your blood sugar readings. Drinking water is fine.
What the Results Mean
Your provider compares each of your four blood sugar readings against specific cutoff values:
- Fasting: 95 mg/dL
- 1 hour: 180 mg/dL
- 2 hours: 155 mg/dL
- 3 hours: 140 mg/dL
If two or more of your four readings meet or exceed these thresholds, you receive a gestational diabetes diagnosis. If only one value is elevated, your provider may recommend dietary changes and retest you later rather than diagnosing gestational diabetes right away. If all four values come in below the cutoffs, you’ve passed the test and no further glucose testing is needed for that pregnancy.
How to Prepare
In the days before the test, eat normally. Some older guidelines recommended eating at least 150 grams of carbohydrates per day for three days before the test to ensure accurate results. Current guidelines from major medical organizations no longer include this specific instruction, but the underlying logic still holds: restricting carbs before the test can artificially raise your results. So the days leading up to the test are not the time to start a low-carb diet. Eat your usual meals, including bread, rice, pasta, fruit, and other carbohydrate-containing foods.
The night before, stop eating after dinner to begin your fast. Water is fine overnight and the morning of the test.
What the Drink Feels Like
The 100-gram glucose drink is intensely sweet, more so than the 1-hour version. It’s mixed into about 8 to 10 ounces of water and comes in flavors like orange or lemon-lime, though “flavor” is generous. The most commonly reported side effect is nausea, which makes sense given how concentrated the sugar is on an empty stomach. Some people also experience dizziness, weakness, or sweating during the waiting period. These reactions are temporary and typically pass once the test is over and you eat something.
Vomiting can be an issue. If you throw up before the test is complete, the results may not be valid and you might need to reschedule. Sipping the drink slowly (within the five-minute window) and keeping still afterward can help.
If You’re Diagnosed With Gestational Diabetes
A diagnosis means your body isn’t managing blood sugar efficiently during pregnancy, which can affect both your health and your baby’s growth. The good news is that gestational diabetes is one of the most manageable pregnancy complications. Treatment focuses on keeping blood sugar within a target range through a combination of dietary changes, physical activity, and regular blood sugar monitoring at home.
For many people, adjusting what and when they eat is enough to keep levels in check. Your provider or a dietitian will help you figure out which carbohydrates to prioritize, how to pair them with protein and fat, and how to space meals throughout the day. If diet and activity alone don’t bring your numbers down, medication (usually insulin or an oral medication) is added.
Gestational diabetes typically resolves after delivery. However, about half of people who have it go on to develop type 2 diabetes later in life. Because of this, your provider will check your blood sugar again after pregnancy, usually at 6 to 12 weeks postpartum, and recommend periodic testing in the years that follow.
If Only One Value Is High
This is a common result that can feel confusing. One abnormal reading out of four doesn’t meet the diagnostic threshold for gestational diabetes, but it does suggest your body is working harder than usual to process sugar. Your provider will likely recommend changes to your diet and may retest you later in pregnancy. Some research suggests that even one elevated value is associated with slightly higher risks for larger birth weight and other outcomes, so it’s worth taking the dietary guidance seriously even without a formal diagnosis.