Somewhere around week 24 your prenatal schedule grows a line that says "glucose test," and with it a small orange bottle. Do you fast? Does a week of cutting sugar help? There are two different tests with two different sets of rules, and most of the confusion comes from mixing them up.

This post is part of a series on what to eat before a specific event, following the one on what to eat before a blood test. One rule sits above everything below: the sheet from your OB, midwife, or lab overrides anything on this page, because practices genuinely differ on the one-hour test.

The short version

Which test: the 1-hour glucose challenge test is a screen, 50 grams of glucose, usually no fasting. The 3-hour glucose tolerance test is the diagnostic follow-up, 100 grams, 8 to 14 hours of fasting, four blood draws.

The three days before: eat the way you normally eat. For the fasting tests, the ADA says at least 150 grams of carbohydrate a day, because cutting carbs beforehand can produce a false positive. Keep taking your prenatal vitamin.

The night before: a normal dinner for the 1-hour test. For the 3-hour test, a normal dinner, then the fast starts, water only.

The morning of: 1-hour test, follow your clinic's sheet, since some say eat breakfast and some say skip it. 3-hour test, nothing but water, no gum, no coffee, no prenatal until after, and a snack in your bag.

After: eat right away. A high 1-hour result is a referral to the 3-hour test and nothing more.

Which test is on your calendar

The food rules depend on which test you have, so settle that first. The MedlinePlus page on glucose screening in pregnancy lays out the two-step version most US practices use. Step one is the glucose challenge test: you drink a glucose solution and your blood is drawn one hour later, with no preparation or diet change needed. If that result is high, you come back for the 3-hour glucose tolerance test, where you drink 100 grams of glucose and have blood drawn before the drink and three more times, once an hour.

Some practices skip the screen and use a one-step test: a fasting 75-gram drink with blood drawn before, at one hour, and at two hours. The American Diabetes Association's 2026 Standards of Care lists both strategies in its Table 2.8 and places them at 24 to 28 weeks. ACOG's patient FAQ gives the same window.

Why cutting sugar before the test backfires

The instinct before a glucose test is to eat clean for a few days so the number comes back low. The guidelines say to do the opposite.

The ADA's 2026 Standards of Care put it in one sentence: carbohydrate restriction in the days before an OGTT can falsely elevate post-challenge glucose, potentially resulting in a false-positive test. Their recommendation 2.14 says carbohydrate intake of at least 150 grams a day should be assured for the three days before an OGTT.

A 2021 study in the SPRING cohort followed 95 pregnant women with risk factors for gestational diabetes and recorded what they ate in the 24 hours before a 75-gram OGTT. Median intake was about 237 grams of carbohydrate, and for every 50-gram reduction, the one-hour glucose on the test was 8.9 mg/dL higher. The study is observational, so it shows an association rather than a cause, but the direction matches the ADA's warning.

The clearest patient handout I found says it in plainer words. A 3-hour GTT sheet from Rushmore OB/GYN ends with: it is very important not to diet before the test, as dieting may cause false results. I find this reassuring. The test wants your normal life. Give it your normal life.

The three days before: eat normally, carbs included

For the 1-hour test there is no preparation. MedlinePlus says to eat your normal food in the days before, and the Weill Cornell patient sheet for the 1-hour test does not mention diet at all.

For the 3-hour test, the three days before have a rule, and it runs the opposite way from what most people expect. The University of Florida's lab instructions say that for three days prior, patients should eat an unrestricted diet of 150 or more grams of carbohydrate. The University of Washington's Maternal and Infant Care Clinic and Northwestern Medicine both say to eat your normal diet in the three days before. For nearly everyone eating three meals a day, normal already clears 150 grams.

What 150 grams looks like, using the Rushmore sheet's examples:

Two slices of bread is about 30 grams. So is a cup of cooked rice or pasta, a large banana, or a glass of juice.

Keep the pasta, the potatoes, the fruit, and the cereal in. This is the one week where "eat the bread" is the medical advice.

Keep taking your prenatal vitamin and any prescribed medication. MedlinePlus says to ask your provider whether any of your medicines could affect the results.

The night before

For the 1-hour test, dinner is ordinary. Eat what you would eat on a Tuesday.

For the 3-hour test, the night before is where the countdown turns strict:

Have a full dinner. The University of Washington sheet says it will be a long time before your next meal, so have a good snack before you stop eating for the night.

Count back from the appointment. MedlinePlus and the UF lab give the fasting window as at least 8 hours and no more than 14. Many sheets, including Rushmore and UW, simply say nothing after midnight.

Water only once the fast starts. UW is specific: plain water, no tea, no coffee, no other beverages.

No gum, mints, cough drops, candy, or smoking. Rushmore adds no iron or vitamin pills during the fast, so the prenatal waits until the test is done.

Pack the bag now: a water bottle, a snack for after, and something to read.

The bag for the 3-hour test: a sandwich, an apple, a yogurt, and water, all of it for after the last draw.

The morning of the 1-hour test

This is the one place where practices disagree, so the sheet in your hand wins. Weill Cornell's instructions say fasting is not required, and the ADA notes that screening with the 50-gram drink does not require fasting. So at many clinics the morning of the 1-hour test is a normal breakfast and a drive to the appointment. Other practices ask you to skip breakfast or eat something light. Neither practice is wrong. The number they compare you to assumes you followed their rule.

Once you are there, the Cornell sheet is a good picture of the hour:

Drink the whole 50-gram bottle within five minutes, and note the exact time you finish. The blood draw is timed from that minute.

After the drink, only sips of plain water. No mints, gum, cough drops, or smoking.

Stay at the office. If you cannot be back at the one-hour mark, the test gets rescheduled.

The morning of the 3-hour test

The 3-hour test runs first thing in the morning, which is on purpose. The ADA says the fasting OGTT should be performed in the morning after an overnight fast of at least 8 hours.

Nothing but plain water when you wake up. No coffee, no tea, no gum, and no prenatal yet.

Drink the 100-gram bottle within five minutes when they hand it to you. The UW sheet warns that it is twice the volume of the 1-hour drink, about 10 ounces, and carbonated.

Stay seated between draws. The UF lab says the subject should remain seated and not smoke throughout, and Northwestern says to stay within the lab area since activity can affect the results.

Tell the staff if you feel nauseated, sweaty, or lightheaded. MedlinePlus lists all three as common reactions to the drink. If you vomit, say so immediately. The staff decide whether the test can continue.

After the test: eat, then read the numbers calmly

Eat as soon as the last draw is done. Northwestern says you will be very hungry and should eat something before you leave the clinic or drive, and suggests crackers, cheese, or a granola bar. Then go home and take the prenatal you skipped.

For the 1-hour screen, MedlinePlus calls a result of 140 mg/dL or below normal. The ADA's Table 2.8 notes that ACOG accepts a cutoff of 130, 135, or 140 mg/dL. A high screen is a referral to the 3-hour test. Northwestern's sheet says it directly: a positive result on the 1-hour test does not mean that you have gestational diabetes.

For the 3-hour test, the ADA lists the Carpenter-Coustan thresholds: fasting 95, one hour 180, two hours 155, three hours 140 mg/dL. The diagnosis requires at least two of the four values at or above the line, though the ADA notes that ACOG allows a practice to use one elevated value. ACOG also accepts a slightly higher set called the NDDG thresholds, so check which set your lab report names. For the 2-hour, 75-gram one-step test, any one value at or above fasting 92, one hour 180, or two hours 153 mg/dL is a diagnosis.

If only one value is high on the 3-hour test, MedlinePlus says your provider may suggest some diet changes and retest you later. If you are diagnosed, the ACOG FAQ describes what management usually means: checking your blood sugar with a meter, regular meals and snacks, 30 minutes of moderate exercise most days, and a 10 to 15 minute walk after each meal.

How BodyBuddy helps

A glucose test is three or four separate instructions spread across two sheets and a portal message, and the one that matters most ("eat normally, do not cut carbs") is the one nobody reads twice. BodyBuddy is an AI coach that texts you every day over iMessage, built for the stretch between an appointment on the calendar and doing what the sheet says.

Upload a photo or PDF of your clinic's test instructions and your prenatal visit plan in the BodyBuddy app and it turns them into a daily plan: the three days of normal eating, the hour the fast starts, the appointment time, the "water only, no gum, no prenatal until after" line. Then it texts you each day so the two of you get it done together. Ask it whether your sheet says to eat breakfast before the 1-hour test and it answers from your own sheet.

If the result is a diagnosis, the same thing works for what comes next. Upload the meal plan from your dietitian or diabetes educator and BodyBuddy texts you through it, meal by meal. Text it a photo of what you ate and it tells you the carbs, protein, and calories on the plate. There is a short guide on importing a doctor's or dietitian's plan if you want to see how the upload works.

BodyBuddy is not medical care and does not replace your OB, midwife, or dietitian. It does not interpret your results; your practice does. It helps you follow their instructions.

FAQ

Can I eat before the 1-hour glucose test in pregnancy?

At most practices, yes. Weill Cornell's patient sheet says fasting is not required, and MedlinePlus says no preparation is needed for the first step. Some practices ask you to skip breakfast or keep it light, so check your own sheet.

What should I not eat before a glucose test in pregnancy?

For the fasting tests, the thing to avoid is eating too little carbohydrate. The ADA says cutting carbs in the days before can cause a false positive and recommends at least 150 grams a day for three days. Once the fast starts, everything except plain water is off, including gum, mints, coffee, and your prenatal vitamin.

Can I drink water before the glucose test?

Yes. The Cornell sheet allows sips of plain water after the 1-hour drink, and the UW and Northwestern sheets allow plain water through the fast and the 3-hour test. Coffee, tea, and flavored drinks are out.

What happens if I fail the 1-hour glucose test?

You get scheduled for the 3-hour test. Northwestern's sheet says a positive 1-hour result does not mean you have gestational diabetes, and under the Carpenter-Coustan thresholds the 3-hour test needs two of four values over the line for a diagnosis.

What happens if I throw up the glucose drink?

Tell the staff right away. The result depends on keeping the full dose down for the full window, so the test usually has to be stopped and repeated another day.

What to do this week

Find out today which test you have and what time it is. Put two things on the calendar: the appointment and, for the 3-hour test, the hour the fast starts. Keep eating the way you normally eat, bread and all. Leave the prenatal on the counter the morning of the fasting test with a note to take it after, and pack the snack the night before.

If you want the sheet turned into daily texts, with the fasting start and the "eat normally" line in front of you on the right days, upload it to BodyBuddy and let the two of you carry it to the appointment.

What to eat before a glucose test in pregnancy: the 1-hour and 3-hour tests explained