The glucose test takes anywhere from one to three hours depending on which version your provider orders. Most people searching this question are pregnant and facing their first glucose screening, which is the shorter one-hour version. If that initial screen comes back high, you’ll be asked to return for a longer follow-up test that takes three hours. Here’s what to expect from each version so you can plan your day.
The 1-Hour Glucose Challenge Test
This is the standard screening test for gestational diabetes, typically done between 24 and 28 weeks of pregnancy. If you have risk factors like a family history of diabetes or a higher BMI, your provider may order it earlier.
The test itself is straightforward. You drink a sweet syrup containing 50 grams of sugar, then wait one hour in the office or lab before a single blood draw from your arm. You cannot eat or drink anything other than water after finishing the drink. Most people are in and out within about 90 minutes once you factor in check-in time and the blood draw itself. No fasting beforehand is required for this version, so you can eat normally before your appointment.
The 3-Hour Glucose Tolerance Test
If your one-hour screening comes back elevated, the next step is a three-hour oral glucose tolerance test (OGTT). This one is more involved. You need to fast for at least eight hours beforehand, which is why most clinics 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 higher-sugar solution, this time containing 100 grams of sugar. Blood is drawn again at the one-hour, two-hour, and three-hour marks. That means four total blood draws over the course of the appointment. You stay in the office or lab the entire time, and you still can’t eat or drink anything besides water.
Plan for roughly three and a half to four hours total from the time you walk in to the time you leave. When you add the overnight fast, you’re looking at about 11 to 12 hours without food. Bring a snack to eat immediately after your last blood draw.
The 2-Hour Glucose Tolerance Test
Some providers skip the one-hour screening entirely and go straight to a two-hour OGTT. This version uses 75 grams of sugar and involves three blood draws: one fasting, one at one hour, and one at two hours. It also requires an eight-hour fast. Total time in the clinic is typically two and a half to three hours. This approach is more common outside pregnancy for diagnosing type 2 diabetes or prediabetes, but some prenatal providers use it as well.
What the Waiting Period Feels Like
The sugar drink is very sweet, similar to a flat orange soda but thicker and more concentrated. Some people tolerate it fine. Others feel nauseous, lightheaded, or jittery afterward, especially during the longer fasting tests when your stomach is empty. These side effects are temporary and typically fade within an hour or two after the test is over and you eat something.
During the wait, you’ll be sitting in a waiting room or exam area. Bring a book, your phone charger, or something to pass the time. Most clinics ask you to stay seated and avoid walking around, since physical activity can affect your blood sugar readings. You can sip plain water but nothing else.
How Quickly Results Come Back
For the one-hour screening, many clinics can process results the same day or within one to two business days. Your provider’s office will typically call or message you through a patient portal. For the three-hour test, results also come back within a few days in most cases. If one or more of your blood draws comes back above the threshold, your provider will walk you through what a gestational diabetes diagnosis means and outline next steps.
Planning Your Appointment
For the one-hour test, schedule it at a time when you can sit for about 90 minutes without rushing. You don’t need to fast, but avoid eating a sugar-heavy meal right before, as that can skew results.
For the three-hour test, book the earliest morning slot available so you spend less of your waking hours fasting. Eat a balanced dinner the night before with protein and complex carbs. Have a snack and a full water bottle waiting in your car or bag for after the final blood draw. Some people feel shaky or drained after fasting that long, so if possible, have someone else drive you home or give yourself time to sit and eat before getting behind the wheel.