Whether you need to fast depends on which gestational diabetes test you’re taking. The first screening test does not require fasting. The follow-up diagnostic test requires 8 hours of fasting beforehand. Most people only take the first test, so if your provider hasn’t specified, you likely don’t need to fast.
The 1-Hour Screening: No Fasting Required
The initial gestational diabetes screening is a one-hour glucose challenge test, typically done between 24 and 28 weeks of pregnancy. You drink a liquid containing 50 grams of glucose, then have your blood drawn one hour later. You can eat and drink normally before this test. The American College of Obstetricians and Gynecologists (ACOG) confirms that a fasting blood sample is not required for this screening.
That said, some people find that eating a heavy, sugary meal right before the test makes them feel more nauseous after drinking the glucose solution. Your provider may suggest avoiding a carb-heavy breakfast that morning, but this is about comfort, not accuracy. There’s no medical requirement to skip meals.
The 3-Hour Diagnostic Test: Fasting Is Required
If your one-hour screening comes back elevated, your provider will schedule a three-hour glucose tolerance test. This one does require fasting. You’ll need to avoid all food and beverages (except water) for at least 8 hours but no more than 14 hours before the test. That means no coffee, juice, gum, or snacks during the fasting window. You should also avoid smoking and vigorous exercise during those 8 hours.
For this test, your blood is drawn four times: once fasting, then at one, two, and three hours after drinking a larger glucose solution containing 100 grams. The fasting blood draw establishes your baseline, which is why eating beforehand would compromise the results.
Water is fine throughout the entire fasting period and during the test itself. Most people schedule the three-hour test first thing in the morning so the fasting window overlaps with sleep.
What to Eat in the Days Before Testing
This is a detail many people miss. For the three-hour test especially, what you eat in the three days leading up to it matters. Cutting carbs before the test, whether intentionally or by coincidence, can actually cause a false positive result. Your body needs regular carbohydrate exposure to process glucose normally.
The World Health Organization recommends eating at least 150 grams of carbohydrates per day for three days before the test. That works out to roughly 50 grams per meal, which is about two slices of bread, a cup of pasta, or a medium baked potato at each sitting. The meal the evening before your overnight fast is particularly important. Research published in the Journal of the Endocrine Society found that patients who ate a low-carbohydrate dinner before their test and then repeated it after adequate carb intake had normal results the second time.
In short: eat your regular meals with plenty of carbs in the days before testing. This isn’t the time to try a low-carb diet.
What Happens If You Fail the First Test
Failing the one-hour screening doesn’t mean you have gestational diabetes. It means your blood sugar was high enough to warrant the more detailed three-hour test. The majority of people who fail the screening go on to pass the diagnostic test. Research in the American Journal of Perinatology found that when the one-hour result is moderately elevated, roughly two out of three people will not be diagnosed with gestational diabetes after completing the three-hour follow-up. The higher your one-hour number, the more likely the diagnosis, but even at the upper range, more than a third of people still pass.
If You’re Tested Before 24 Weeks
Some people are screened earlier in pregnancy, sometimes at the very first prenatal visit. This happens if you have risk factors like being overweight or obese before pregnancy, having a parent or sibling with diabetes, or having had gestational diabetes in a previous pregnancy. The test itself works the same way, and the fasting rules are identical. Your provider will tell you which version of the test you’re getting and whether fasting applies.
Quick Reference
- 1-hour glucose challenge (screening): No fasting. Eat and drink normally. Done between 24 and 28 weeks for most pregnancies.
- 3-hour glucose tolerance test (diagnostic): Fast for 8 to 14 hours. Water only. Eat at least 150 grams of carbs daily for the three days before.