Preparing for a gestational diabetes test depends on which of the two tests you’re taking. The first, a one-hour screening, requires no fasting and no special preparation. The second, a three-hour diagnostic test, requires at least eight hours of fasting and a few days of dietary adjustment beforehand. Most pregnant people take the screening test between 24 and 28 weeks, though your provider may test earlier if you have risk factors like a family history of diabetes or a previous pregnancy with gestational diabetes.
The Two Tests and How They Differ
Gestational diabetes screening uses a two-step process. The first step is the glucose challenge test, sometimes called the one-hour test. You drink a sweet glucose solution, sit in the office for an hour, and then have your blood drawn once. That’s it. You can eat normally before this test and go about your day beforehand.
If your blood sugar comes back above the cutoff on that screening, your provider will schedule the oral glucose tolerance test, often called the three-hour test. This one is more involved. You fast for at least eight hours overnight, then have your blood drawn before drinking a higher-concentration glucose solution. After that, your blood is drawn again at the one-hour, two-hour, and three-hour marks. A diagnosis of gestational diabetes requires elevated readings at two or more of those four blood draws (fasting, one hour, two hours, three hours).
Not everyone who fails the one-hour screening ends up with a gestational diabetes diagnosis. The screening is intentionally sensitive, meaning it flags anyone whose blood sugar is even slightly elevated so the more precise three-hour test can sort out who truly has the condition.
Preparing for the One-Hour Screening
There’s very little you need to do. You don’t need to fast, and you can eat and drink normally in the days and hours before the test. Some providers suggest avoiding a very sugary meal right before your appointment, but there’s no formal restriction. The test itself takes about an hour of sitting in the office after you drink the glucose solution, so bring something to read or do while you wait.
Preparing for the Three-Hour Diagnostic Test
This test requires more planning. In the days leading up to it, eat and drink as you usually would. In fact, some providers specifically instruct you to eat at least 150 grams of carbohydrates per day for the three days before the test. That’s roughly the equivalent of eating bread, rice, pasta, fruit, and other carb-containing foods at every meal. The reason: restricting carbs beforehand can actually skew your results and produce a falsely high reading, because your body becomes less efficient at processing a sudden load of sugar when it hasn’t been getting much.
The night before the test, eat a normal dinner, then stop eating and drinking (except water) at least eight hours before your appointment. Most people schedule the test first thing in the morning so the fasting window overlaps with sleep.
What to Eat in the Days Before
You don’t need to follow a special diet, but you do need to make sure you’re not accidentally under-eating carbs. If you’ve been cutting back on bread, pasta, or rice in an effort to “pass” the test, that strategy can backfire. Aim for your normal balanced meals with carbohydrates at each one. A day with oatmeal at breakfast, a sandwich at lunch, and pasta or rice at dinner will easily reach the 150-gram carbohydrate target without any special planning.
What to Expect During the Test
The glucose solution tastes extremely sweet, like flat orange soda or a sugary fruit drink. Most offices offer it chilled, which makes it easier to get down. You’ll typically need to finish it within about five minutes.
Some women feel nauseated, sweaty, or lightheaded after drinking it. This is common and usually passes. If you’re prone to nausea, the morning fasting period can make it worse, so getting to your appointment on time (rather than extending the fast longer than necessary) helps.
During the waiting periods between blood draws, you need to stay seated in the office. You can’t eat, drink anything other than water, chew gum, or smoke. Bring a book, your phone charger, headphones, or anything else to keep yourself comfortable for what will be a roughly three-and-a-half-hour visit. Some people bring a snack to eat immediately after the final blood draw, which is a good idea since you’ll have been fasting for a long stretch by that point.
Things That Can Affect Your Results
A few factors beyond diet can influence how your body processes glucose during the test. Physical activity is one: vigorous exercise the morning of the test can lower blood sugar and potentially mask a real problem, while being sedentary during the test itself is required so your muscles don’t pull extra glucose from the bloodstream and alter the readings.
Certain medications can also affect blood sugar. If you’re taking steroids, beta-blockers, or other medications, let your provider know well before the test so they can advise you on whether to adjust anything. Illness and stress can temporarily raise blood sugar too, so if you’re sick on the day of a scheduled three-hour test, ask your provider about rescheduling.
If You Need the Three-Hour Test After Screening
Getting called back for the diagnostic test can feel stressful, but it’s a routine next step. Many women who screen positive on the one-hour test go on to have normal results on the three-hour test. Your provider will typically schedule the follow-up within a week or two of your screening results.
If the three-hour test does confirm gestational diabetes, the condition is highly manageable. Most people control it with dietary changes and blood sugar monitoring, and it resolves after delivery. Knowing sooner gives you and your provider time to keep your blood sugar in a healthy range for the rest of your pregnancy, which is exactly why the screening exists in the first place.