Most people searching this question are pregnant and want to know when that glucose test happens: between 24 and 28 weeks of pregnancy. But glucose testing isn’t only for pregnancy. Adults 35 and older with overweight or obesity are also recommended to get screened for prediabetes and type 2 diabetes, and anyone experiencing symptoms like unusual thirst or frequent urination may need testing right away.
Glucose Testing During Pregnancy
If you’re at average risk for gestational diabetes, your provider will schedule a screening test during your second trimester, typically between weeks 24 and 28. If you have higher risk factors (a family history of diabetes, a previous pregnancy with gestational diabetes, or a BMI over 30), you may be tested at your very first prenatal visit.
The standard approach uses a two-step process. The first step is a one-hour screening: you drink a sweet liquid containing 50 grams of sugar, and your blood is drawn one hour later. You don’t need to fast beforehand. If your blood sugar comes back high on that initial screen, it doesn’t mean you have gestational diabetes. It means you move on to the second step.
The follow-up is a three-hour test, and this one does require fasting for eight hours. Your fasting blood is drawn first, then you drink a more concentrated solution with 100 grams of sugar. Blood is drawn again at one, two, and three hours after you finish the drink. Some providers use an alternative single-step approach: a two-hour fasting test with 75 grams of sugar. Your provider will tell you which version they use.
Routine Screening for Adults
Outside of pregnancy, both the U.S. Preventive Services Task Force and the American Diabetes Association recommend screening for prediabetes and type 2 diabetes starting at age 35 for adults with overweight or obesity. This was lowered from age 40 (USPSTF) and age 45 (ADA) in recent guideline updates. The ADA also recommends screening for any adult with overweight or obesity who has at least one additional risk factor, regardless of age.
If your results come back normal, re-screening every three years is the standard recommendation. If your results fall in the prediabetes range, both major guidelines recommend yearly testing to catch any progression early.
Types of Glucose Tests and What the Numbers Mean
There are several ways to measure blood sugar, and each has its own thresholds for normal, prediabetes, and diabetes.
Fasting Plasma Glucose
You fast for at least eight hours (water is fine), then have your blood drawn. A result below 100 mg/dL is normal. Between 100 and 125 mg/dL falls into the prediabetes range. A reading of 126 mg/dL or higher, confirmed on a second test, indicates diabetes.
Oral Glucose Tolerance Test
For non-pregnant adults, this is a two-hour test. After fasting, you drink a solution with 75 grams of sugar, then have blood drawn at the one- and two-hour marks. A two-hour result below 140 mg/dL is normal. Between 140 and 199 mg/dL suggests prediabetes. A reading of 200 mg/dL or higher points to diabetes.
A1C Test
This blood test measures your average blood sugar over roughly the past two to three months. No fasting required. An A1C below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or higher means diabetes. Because it reflects a longer window of time, A1C is often the go-to screening test at routine checkups.
Random Plasma Glucose
This test can be done at any time, no fasting needed. A reading of 200 mg/dL or higher, combined with classic symptoms, is enough to diagnose diabetes on its own. Even readings at lower levels can signal risk: research published in PLOS One found that having two or more random readings at or above 130 mg/dL predicted a diabetes diagnosis within one year with 93% specificity, similar to a fasting test at 126 mg/dL.
Symptoms That Warrant Immediate Testing
You don’t need to wait for a scheduled screening if something feels off. Persistent, unexplained thirst is one of the hallmark early signs of high blood sugar. So is urinating much more frequently than usual, especially at night. Increased hunger despite eating normally, unintentional weight loss, blurred vision, and slow-healing cuts or bruises can all point to elevated blood sugar. These symptoms overlap between type 1 and type 2 diabetes. If you’re experiencing them, a simple blood draw or even a random glucose check can provide answers quickly.
How to Prepare for Your Test
Preparation depends on which test you’re getting. For a fasting glucose test or an oral glucose tolerance test, you’ll need to avoid food and drinks other than water for eight hours beforehand. Most people schedule these for first thing in the morning to make the fasting period easier. For the one-hour pregnancy screening and the A1C test, no fasting is required.
During any version of the glucose tolerance test, you’ll drink the sugar solution within about five minutes. The taste is very sweet, similar to flat soda. Some people feel slightly nauseated afterward. You’ll sit in the office or lab between blood draws, so bringing something to read or do can help pass the time, especially during the three-hour pregnancy version.
What Happens After an Abnormal Result
A single elevated reading typically isn’t enough for a diagnosis on its own (except a random glucose of 200 mg/dL or higher with symptoms). Your provider will usually confirm with a second test, either the same type or a different one. If you’re diagnosed with prediabetes, yearly follow-up testing becomes the norm. If you’re diagnosed with gestational diabetes, your care team will discuss blood sugar monitoring, dietary changes, and whether you need additional treatment for the remainder of your pregnancy. Most gestational diabetes resolves after delivery, but it does increase your long-term risk for type 2 diabetes, so follow-up screening after pregnancy is important too.