Preparation depends on which glucose test you’re taking. The 1-hour glucose challenge test requires no fasting and no special preparation. The 3-hour glucose tolerance test requires an overnight fast of at least 8 hours and, ideally, three days of eating at least 150 grams of carbohydrates per day beforehand. Most pregnant people take the 1-hour screening first, between 24 and 28 weeks of pregnancy, and only move on to the 3-hour test if the first result comes back elevated.
The 1-Hour Screening: No Fasting Needed
The 1-hour glucose challenge test is a screening, not a diagnosis. You drink a sugary solution containing 50 grams of glucose, wait one hour, and have your blood drawn. That’s it.
You can eat and drink normally before this test. No special diet, no fasting, no restrictions. Mayo Clinic’s guidance is straightforward: eat and drink as usual. Some providers may give you slightly different advice, like avoiding a sugar-heavy breakfast right before, but the standard protocol has no preparation requirements. If your blood sugar comes back below 130 to 140 mg/dL (the cutoff varies by provider), you’re done. If it’s above that threshold, you’ll be asked to come back for the longer diagnostic test.
The 3-Hour Test: How to Prepare
The 3-hour glucose tolerance test is more involved, and the preparation matters because it can affect your results. There are two parts to getting ready: what you eat in the days before, and fasting the night before.
Eat Enough Carbs for 3 Days Before
For at least three days leading up to the test, you should eat a minimum of 150 grams of carbohydrates per day, spread across three meals with at least 50 grams per meal. This is called an “unrestricted diet” in clinical terms, but what it really means is: don’t cut carbs before this test.
This step trips people up. If you’ve been eating low-carb to “pass” the test, or if you’ve been following a lower-carb diet in general, your body may respond abnormally to the glucose drink. When you haven’t been eating many carbs, your body becomes less efficient at processing a sudden large dose of sugar, which can lead to a falsely high reading. Eating adequate carbs in the days before essentially primes your body to respond the way it normally would.
A dinner of eggs, turkey bacon, avocado on toast, and a glass of orange juice, for example, comes in under 50 grams of carbohydrates, which isn’t enough for your evening meal the night before the test. Your last meal before fasting is especially important and should include at least 50 grams of carbs. Think: a plate of pasta, a rice bowl, a sandwich with a piece of fruit, or oatmeal with toast. If you’re unsure, tracking your meals with a free app for those three days can give you confidence you’ve hit the target.
Fast for 8 to 14 Hours Overnight
The night before the 3-hour test, stop eating after your evening meal. You need at least 8 hours of fasting but no more than 14 hours. Water is fine and encouraged throughout the fast, but skip everything else: no coffee, no juice, no gum, nothing with calories or sweeteners. Most people schedule the test first thing in the morning and stop eating after dinner, which naturally puts them in the 10- to 12-hour fasting range.
What Happens During the 3-Hour Test
You’ll have your blood drawn when you arrive (the fasting baseline), then drink a glucose solution containing 100 grams of sugar. After that, your blood is drawn at the one-hour, two-hour, and three-hour marks. You’ll stay at the office, clinic, or lab the entire time.
Plan to be there for about three and a half hours total. Bring something to read, your phone charger, or work you can do while sitting. You’ll need to stay relatively still during the waiting periods, so this isn’t the morning to squeeze in errands between draws. Some people feel nauseous or lightheaded after drinking the glucose solution on an empty stomach, which is normal. Eating a solid meal the night before (with those 50-plus grams of carbs) can help reduce nausea.
Gestational diabetes is diagnosed when two or more of the four blood draws come back above the threshold values. If only one value is elevated, you typically won’t receive a diagnosis, though your provider may want to monitor you more closely.
When the Test Is Scheduled
Standard screening happens between 24 and 28 weeks of pregnancy. The U.S. Preventive Services Task Force recommends one-time screening at 24 weeks or after for all pregnant people without symptoms. If you enter prenatal care later than 28 weeks, you’ll still be screened at your first opportunity.
Some people are screened earlier, sometimes in the first trimester. This happens when risk factors for type 2 diabetes are present, such as obesity, a family history of type 2 diabetes, or having had a particularly large baby in a previous pregnancy. Early screening uses your provider’s judgment about which test is appropriate for your situation.
Medications and Supplements to Mention
Certain medications can interfere with blood sugar readings. Corticosteroids (sometimes prescribed during pregnancy for conditions like asthma or to help with fetal lung development) are well known to raise blood sugar. If you’re taking steroids in any form, your provider needs to know before interpreting your results.
Other commonly used medications that may affect glucose readings include albuterol (used in asthma inhalers), acetaminophen (Tylenol), and certain blood pressure medications. You don’t need to stop taking anything prescribed to you. Just make sure your provider has a complete, current list of everything you’re taking, including supplements like high-dose vitamin C, so they can factor that into interpreting your results.
Common Mistakes That Affect Results
The biggest preparation error is restricting carbohydrates before the 3-hour test. It’s counterintuitive, but eating fewer carbs makes it harder for your body to handle the glucose drink efficiently, potentially pushing your numbers higher than they would be on your normal diet. Eat normally, or slightly increase your carb intake, for those three days.
Fasting too long is another common issue. If you skip dinner the night before and then have a morning appointment, you could easily exceed 14 hours without food. This extended fast can also distort your results. Set a reminder to eat a good, carb-rich dinner by 8 or 9 p.m. if your test is early the next morning.
Finally, don’t try to exercise heavily the morning of the test or during the waiting periods between blood draws. Physical activity lowers blood sugar acutely, which could mask a genuine issue. Normal movement is fine, but skip the workout.