Preparing for a gestational diabetes test depends on which test you’re having. The initial one-hour screening requires no fasting, while the follow-up three-hour diagnostic test requires an overnight fast and a few days of eating enough carbohydrates beforehand. Most pregnant people are tested 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 typically happens in two steps. The first is a one-hour glucose challenge test, which is a quick screening to flag anyone who needs further evaluation. You drink a bottle containing 50 grams of glucose and have your blood drawn one hour later. If your blood sugar comes back above the screening threshold (usually 130 or 140 mg/dL, depending on your provider’s cutoff), you move on to the second test.
The second test is a three-hour oral glucose tolerance test. This one is diagnostic, meaning it gives a definitive yes or no. You drink a stronger solution containing 100 grams of glucose, and your blood is drawn four times: once fasting, then at one, two, and three hours after the drink. A diagnosis of gestational diabetes requires two or more elevated values out of the four draws. The commonly used thresholds are 95 mg/dL fasting, 180 at one hour, 155 at two hours, and 140 at three hours.
Preparing for the One-Hour Screening
You do not need to fast before the one-hour test. You can eat normally that day, and many providers will schedule it as a regular part of a prenatal visit. That said, what you eat in the hours before the test can affect how you feel during it. A meal high in refined sugar or simple carbohydrates right before drinking the glucose solution can make nausea worse and may push your blood sugar higher than it would otherwise be. Eating a balanced meal with protein, healthy fat, and complex carbohydrates two to three hours before the test is a practical approach.
Once you drink the glucose solution, you’ll need to finish the entire bottle within five minutes. After that, you cannot eat or drink anything except sips of plain water until your blood is drawn. You’ll also need to stay in the office or lab during the waiting period rather than walking around or running errands.
Preparing for the Three-Hour Diagnostic Test
The three-hour test requires more planning. You need to fast for 8 to 14 hours beforehand, so most people schedule it first thing in the morning and stop eating after dinner the night before. Water is fine during the fasting period.
In the three days leading up to the test, you should eat at least 150 grams of carbohydrates per day. This is sometimes called “carb loading,” though it’s really just eating a normal, unrestricted diet. If you’ve been cutting carbs or eating low-carb in the weeks before, your body can respond abnormally to the glucose load and produce a falsely elevated result. For reference, 150 grams of carbohydrates is roughly what you’d get from a few servings of grains, fruit, and starchy vegetables spread across the day. It’s not an extreme amount, but it does mean you shouldn’t be actively restricting carbs before this test.
Plan to be at the lab for about three and a half hours. Bring something to read or do, because you’ll need to stay seated in the waiting area between blood draws. You cannot eat or drink anything other than water during the entire test.
What the Glucose Drink Tastes Like and How to Handle It
The glucose drink is a thick, very sweet liquid. It comes in a few flavors (orange, lemon-lime, and fruit punch are common), though “flavor” is a generous word for what is essentially sugar water. The one-hour version contains 50 grams of sugar, and the three-hour version contains 75 to 100 grams.
Common side effects include nausea, upset stomach, headache, and lightheadedness. These are temporary and resolve after the test. Drinking it cold helps. Some people find it easier to drink quickly rather than sipping slowly, since you need to finish it within five minutes anyway. If you’ve felt queasy during previous blood draws, let the staff know so they can have you lie down if needed.
Up to 30% of pregnant women have trouble tolerating the standard glucose drink. If you’ve vomited during a previous attempt or have serious concerns, talk to your provider before your appointment. Some research has explored candy-based alternatives like jelly beans, though these haven’t been widely adopted because they can be less accurate. Your provider may have options, but this is a conversation to have in advance rather than on test day.
Stress, Sleep, and Accuracy
Psychological stress can genuinely raise your blood sugar. Research has found that major life events, anxiety, depression, and short sleep duration are all associated with higher fasting glucose levels during pregnancy. The mechanism is straightforward: stress triggers your body to release cortisol, which raises blood sugar.
This doesn’t mean you need to be perfectly zen on test day, but it’s worth being aware of. Getting a reasonable night’s sleep before the test and arriving without rushing can make a small difference. If you’re going through a particularly stressful period, that context is worth mentioning to your provider when you discuss results.
What to Eat the Night Before
For the one-hour screening, no special meal planning is needed the night before. Eat what you normally would.
For the three-hour test, your last meal before fasting matters more. A dinner that includes protein, fat, and complex carbohydrates (think chicken with rice and vegetables, or pasta with a protein source) gives your body steady fuel through the overnight fast without spiking your blood sugar right before the fasting window. Avoid a heavy, sugar-laden dessert as your last meal, since it can affect your fasting glucose level the next morning.
What to Bring to the Appointment
For the one-hour test, you won’t need much since the whole process takes about an hour and a half including check-in. For the three-hour test, bring a snack to eat immediately after your last blood draw. After fasting all night and sitting through three hours of blood draws on a stomach full of sugar water, you will be hungry and possibly shaky. Crackers with peanut butter, a granola bar, cheese and fruit, or anything substantial enough to stabilize your blood sugar is a good idea. A book, your phone charger, or headphones will also help pass the time.
Wear a short-sleeved shirt or one with sleeves that roll up easily, since you’ll be getting multiple blood draws from your arm. If you tend to get cold in medical offices, a blanket or cardigan that’s easy to move out of the way works well.
If Your Results Come Back High
Failing the one-hour screening does not mean you have gestational diabetes. It means your blood sugar was above the screening cutoff and you need the three-hour test to find out for sure. A significant number of people who fail the screening pass the diagnostic test.
If the three-hour test does confirm gestational diabetes, the condition is manageable. Most people control it through dietary changes and blood sugar monitoring. The key adjustments involve balancing carbohydrates with protein and fat at every meal, eating smaller and more frequent meals, and checking blood sugar levels several times a day with a finger-stick monitor. Some people do need medication, but diet and lifestyle changes are the first line of management and work well for many.