Can I Drink Water During the 1-Hour Glucose Test?

Plain water in small amounts is generally permitted during the one-hour glucose challenge test (GCT), the screening most pregnant women take around 24 to 28 weeks. The test involves drinking a 50-gram glucose solution and having your blood drawn one hour later, and sipping a modest amount of water during that waiting period will not meaningfully change your blood sugar reading. That said, the details matter more than most people realize, and what your provider tells you to do before the test can vary in ways that actually affect your results.

Why Water Gets a Pass

The one-hour glucose screen is designed to measure how your body handles a specific sugar load. Water contains no calories, no sugar, and no protein, so it does not provoke an insulin response or add glucose to your bloodstream. When researchers study what influences oral glucose tolerance test results, the variables that consistently move the needle are caloric intake, macronutrient composition, and hormonal responses to food. Water does not fit any of those categories.

There is also a practical reason providers tend to allow water. The glucose drink is intensely sweet, and many women feel nauseated or develop a headache during the one-hour wait. Sipping water can ease those symptoms without introducing anything that would interfere with the measurement. The key word, though, is “sipping.” Chugging a large volume of water right before the blood draw is a different story, since it could theoretically dilute blood plasma slightly. A few small sips over the hour is what most labs and OB offices have in mind when they say water is fine.

How Dehydration Can Actually Skew Your Results

If anything, avoiding water entirely may be the bigger risk to test accuracy. Research on people with type 2 diabetes found that even mild dehydration, amounting to roughly 1.6 percent of body weight lost through water restriction over three days, led to meaningfully higher blood glucose readings during an oral glucose tolerance test. The dehydrated participants had higher glucose at every time point, and the effect appeared to be driven by the stress hormone cortisol rather than by insulin problems.

This study looked at people who already had diabetes, so the magnitude of the effect would likely be smaller in someone with normal glucose metabolism. But the direction of the finding is clear: being under-hydrated tends to push blood sugar readings up, not down. If you are worried about a false-positive result on your glucose screen, showing up dehydrated is working against you.

What You Should Avoid During the Test

While plain water is generally acceptable, other beverages and foods are not. Anything with calories, sugar, or caffeine can affect your blood glucose or your body’s insulin response in ways that muddy the results. The list of things to skip includes:

Juice, soda, or sweetened drinks: These add sugar directly to your bloodstream and will raise your reading.
Coffee or tea: Even black coffee can affect glucose metabolism and cortisol levels, and adding cream or sugar compounds the problem.
Food of any kind: Eating during the one-hour window introduces additional glucose that the test cannot separate from the glucose drink.
Flavored water or sports drinks: Many contain small amounts of sugar or sugar substitutes that could influence the reading.

Gum is another thing people wonder about. A study on gum chewing in fasted men found that chewing gum did not significantly change blood glucose or insulin levels, though it did raise levels of a gut hormone called GLP-1.¹ Since GLP-1 influences how the body handles sugar after a meal, most labs recommend playing it safe and skipping the gum during the test.

The Fasting Question

One of the most confusing aspects of the one-hour glucose screen is that different providers give different instructions about eating beforehand. Some tell you to fast for a set number of hours. Others say you can eat normally. Both camps feel strongly about their approach, and the research shows why there is disagreement: your pre-test eating status substantially changes your result.

A randomized controlled trial directly compared fasting and fed states before the one-hour glucose screen. Women who fasted had a significantly higher screen-positive rate than those who ate normally beforehand: about 32 percent versus 13 percent. Their average glucose readings were also considerably higher, around 128 mg/dL for the fasting group compared with 113 mg/dL for the fed group.² That is a large enough gap to push a lot of women over the screening threshold who would have passed if they had eaten.

The standard two-step screening protocol used by ACOG in the United States was actually designed to be administered without fasting. The idea is that the 50-gram glucose load is big enough to produce a reliable signal regardless of what you ate a few hours earlier.³ When a provider tells you to fast, they may be applying the rules for the three-hour diagnostic test (which does require fasting) to the one-hour screen, or they may have a clinical reason. Either way, the evidence suggests that fasting before the one-hour screen increases false positives, so if you received conflicting advice, it is worth clarifying directly with your provider which protocol they intended.

When You Last Ate and What Time of Day You Test

Even if you do eat before the test, the timing of that meal relative to the glucose drink matters. A prospective cohort study found that having your last meal less than two hours before the one-hour glucose challenge test significantly increased the false-positive rate.⁴ So eating right before heading to your appointment is not the same as eating two or three hours beforehand. The glucose from your recent meal has not fully cleared, and it stacks on top of the test drink.

Time of day also plays a role. Research examining over a thousand pregnant women found that afternoon testing generally produced lower glucose readings than morning testing, particularly when the fasting interval was short. Women tested in the afternoon with a fasting period of two hours or less had roughly half the odds of a positive screening result compared to women tested in the morning with a longer fast.⁵ This is partly because insulin sensitivity follows a circadian rhythm, tending to be higher earlier in the day, which means your body clears glucose differently at 9 a.m. than at 3 p.m.

The practical takeaway is that the combination of when you last ate and what time you test can shift your result by enough to cross a diagnostic threshold. If you have flexibility in scheduling, a mid-morning appointment with a light meal eaten two to three hours earlier roughly matches what the test was designed around.

Dealing with Nausea from the Glucose Drink

The glucose solution tastes like flat soda concentrate, and many women experience nausea, headache, or bloating during the waiting period. Vomiting before the hour is up usually means the test has to be repeated, so anything that reduces nausea without affecting glucose levels is worth considering.

A randomized trial tested whether altering the glucose solution itself could help. Researchers tried different water temperatures and additives mixed into the glucose drink. While the formula affected how the drink tasted and reduced the degree of nausea women experienced, it did not significantly change the incidence of gestational diabetes diagnoses, and it did not eliminate vomiting or bloating.⁶ This is encouraging because it suggests that small changes in how the drink is prepared or consumed do not throw off the screening accuracy.

Some strategies that women and providers commonly use during the waiting period include sitting still rather than walking around, taking slow deep breaths, and sipping small amounts of plain water. Since the research confirms that modest differences in the solution’s preparation do not change diagnostic outcomes, a few careful sips of water to settle your stomach should be fine.

Does the Temperature of Your Water Matter?

This is one of those questions that sounds trivial but has a surprisingly detailed answer. Water temperature affects how quickly liquids leave your stomach, and stomach emptying rate is part of what determines how fast glucose reaches your bloodstream.

A study in healthy young men found that cold water (around 2°C) slowed gastric contractions significantly compared to warm water (around 60°C). The frequency of stomach contractions was lower after drinking cold water, and the effect persisted for up to an hour.⁷ Separately, an older study found that both cold (4°C) and warm (50°C) drinks emptied from the stomach more slowly than body-temperature drinks, with cold drinks showing the most pronounced initial delay.⁸

What does this mean in practice? If you drink ice-cold water during the test, it could slightly slow the rate at which any remaining glucose solution in your stomach moves into your small intestine, potentially shifting the timing of your glucose peak by a few minutes. The effect is small and probably clinically irrelevant for a screening test, but if you want to remove any possible variable, room-temperature water is the most neutral choice.

Why False Positives Are So Common

The one-hour glucose screen is intentionally designed to cast a wide net. It is a screening test, not a diagnostic one, so its job is to catch everyone who might have gestational diabetes, at the cost of flagging many women who do not. This means false positives are built into the system. Research has found that women who screen positive but ultimately test negative on the follow-up three-hour test tend to be older, have higher BMI, and are more likely to have certain underlying conditions like chronic hypertension.⁹

Understanding this context helps put the water question in perspective. Whether you sipped water during the test is unlikely to be the thing that tipped your result one way or the other. The factors that move the needle most are your underlying metabolic profile, whether you fasted or ate beforehand, how recently you ate, and what time of day you tested. Water is a minor variable in a test that has many larger ones.

The One-Step Versus Two-Step Testing Difference

If you have heard conflicting advice about the glucose test from friends or online forums, part of the confusion may come from the fact that two entirely different testing protocols exist in widespread use. The two-step approach, which is the standard recommended by ACOG in the United States, starts with the one-hour, 50-gram glucose challenge test. This screen does not require fasting, and if you pass, you are done. If you fail, you come back for a fasting three-hour test with a 100-gram glucose load.³

The one-step approach, recommended by the International Association of Diabetes and Pregnancy Study Groups and used more commonly outside the United States, skips the screening and goes directly to a fasting 75-gram, two-hour oral glucose tolerance test. This version does require fasting, and the rules about water intake before and during the test are stricter because fasting status is part of the diagnostic criteria.

When someone online says “you absolutely cannot eat or drink anything,” they may be describing the one-step fasting test or the three-hour diagnostic follow-up, not the one-hour screening test. Knowing which test you are taking clears up most of the conflicting information you will encounter.

Blood Draw Logistics That Can Affect Results

Something that gets far less attention than diet and water intake is how your blood is actually collected and processed. The one-hour glucose screen uses venous blood drawn from your arm, which is analyzed by a laboratory. But if you have ever checked your own blood sugar at home with a finger-stick meter, you may have noticed that the numbers do not always match up perfectly with lab results.

Research comparing capillary blood (from a finger stick) to venous blood (from an arm draw) has found consistent differences in readings. The mean bias between the two sampling methods is generally within about 10 mg/dL, but it can be larger depending on the equipment used.¹⁰ ¹¹ This is why the formal glucose screen uses a venous draw analyzed by a lab rather than a portable meter. If your provider ever offers to do a “quick check” with a finger-stick glucometer instead of a formal lab draw, understand that the result may not be directly comparable to the standard screening thresholds.

Timing of the blood draw also matters. The test is calibrated to a one-hour window. If the lab is running behind and your blood is drawn at 70 or 80 minutes instead of 60, your glucose may have already started to drop, potentially giving you a falsely reassuring number. If the draw happens early, you might catch a reading that is still rising. Keeping track of when you finished the drink and flagging it if the draw is noticeably late is a small but worthwhile habit.

When Providers Restrict Water Anyway

Despite the general consensus that small amounts of plain water are fine, some providers and labs have blanket “nothing by mouth” policies that include water. This is not because water has been shown to affect the results. It is usually an institutional simplification: telling patients “nothing at all” is easier to enforce and less prone to misinterpretation than “water is fine but nothing else.” Patients hear “you can drink” and sometimes show up with orange juice or sweetened iced tea. A strict no-intake policy eliminates that risk.

If your provider says no water and you are concerned about dehydration, especially in the summer or if you tend to get lightheaded during blood draws, it is reasonable to ask them specifically whether a few sips of plain water would be acceptable. Most will say yes once they understand you mean water and nothing else. If they still say no, the one-hour window is short enough that healthy individuals can generally tolerate it without meaningful dehydration, provided you hydrated well in the hours leading up to the appointment.

Hydrating strategically the night before and the morning of the test, then stopping about 30 minutes before you drink the glucose solution, is a practical compromise that keeps you comfortable without introducing any variable during the test itself. Given what the dehydration research shows about elevated cortisol pushing glucose readings upward, arriving well-hydrated is arguably more important to test accuracy than whether you take a sip during the waiting hour.¹²