The one-hour glucose test is a screening for gestational diabetes done during pregnancy, typically between 24 and 28 weeks. You drink a sweet liquid containing 50 grams of sugar, wait one hour, then have your blood drawn. The whole process takes a little over an hour, and most people don’t need to fast beforehand.
What Happens During the Test
When you arrive at your provider’s office or lab, you’ll be given a small bottle of a sugary syrup, sometimes called glucola. It contains 50 grams of glucose in a few ounces of liquid, roughly the sugar equivalent of drinking a large soda in a few minutes. Most offices offer it chilled, and it comes in a few flavors like orange, lemon-lime, or fruit punch. You’ll need to finish it quickly.
Once you’ve finished the drink, the clock starts. You stay in the office or lab for the full hour. During that time, you can’t eat or drink anything other than water. After exactly one hour, a blood sample is taken from a vein in your arm. That sample measures how your blood sugar responded to the large dose of glucose.
Why the Test Works
The logic behind the test is straightforward. When you consume a concentrated dose of sugar, your pancreas releases insulin to move that sugar out of your bloodstream and into your cells. In a healthy pregnancy, this system keeps blood sugar within a predictable range even after a big glucose hit. During pregnancy, though, hormones from the placenta can interfere with insulin’s ability to do its job. If your body can’t compensate by producing extra insulin, blood sugar stays elevated longer than it should. The one-hour blood draw catches that elevated level.
Do You Need to Fast?
For the one-hour screening, most providers do not require fasting. You can eat normally before the test. Some providers suggest avoiding a heavy, sugary meal right before your appointment since that could affect your baseline blood sugar, but there’s no universal rule about it. If your provider has specific instructions, follow those. The three-hour follow-up test, by contrast, does require fasting for eight to 12 hours beforehand.
What the Results Mean
Your result is a single blood sugar number measured in milligrams per deciliter (mg/dL). The cutoff your provider uses to flag an abnormal result varies slightly by practice. Screening thresholds range from 130 to 140 mg/dL, and the American College of Obstetricians and Gynecologists notes there isn’t clear evidence that one threshold is better than another. Many practices use 135 or 140 mg/dL as their cutoff.
If your blood sugar comes back below that threshold, the screening is considered normal and no further testing is needed. If it comes back above the cutoff, that does not mean you have gestational diabetes. It means you need the longer, more definitive follow-up test to find out.
What Happens If You Screen High
The follow-up is a three-hour glucose tolerance test. This one requires overnight fasting for at least eight to 12 hours. When you arrive, your blood is drawn first to get a fasting blood sugar level. Then you drink a larger glucose solution, this time containing 100 grams of sugar. Your blood is drawn at three separate points: one hour, two hours, and three hours after you finish the drink.
Each of those four blood draws (fasting plus three timed draws) has its own threshold:
- Fasting: lower than 95 mg/dL
- One hour: lower than 180 mg/dL
- Two hours: lower than 155 mg/dL
- Three hours: lower than 140 mg/dL
If two or more of those values come back elevated, you’ll typically be diagnosed with gestational diabetes. If only one value is elevated, your provider may retest you in a few weeks rather than making a diagnosis right away.
How It Feels
The glucose drink is very sweet, and some people find it unpleasant. Nausea is the most commonly reported side effect, especially in the first trimester or if you’re already prone to pregnancy-related nausea. Some people feel lightheaded, jittery, or a bit “off” during the waiting period as their blood sugar spikes and then drops. These sensations are temporary and typically pass within an hour or two after the test.
Drinking the solution slowly or on a very full stomach can make nausea worse, so sipping it steadily over a few minutes (rather than sip by tiny sip over 15 minutes) is usually better. Bringing something to eat for right after the blood draw can help you feel normal again quickly. You can resume all your usual activities as soon as the test is done.
How Long Results Take
Most providers have one-hour screening results back within one to three days, though some labs process them the same day. If you need the three-hour follow-up, that test is usually scheduled within a week or two of getting your screening result. The full process from initial screening to a final answer rarely takes longer than a few weeks.