You do not need to fast for the one-hour glucose screening test, which is the test most pregnant women get first. Eat your normal meals, drink water, and show up at your appointment on time. You will drink a sweet glucose solution and have your blood drawn exactly one hour later. The two-hour and three-hour tests are different — those do require fasting, and your clinician will give you specific instructions.
Most of the confusion about this test comes from mixing up the different versions. Once you know which test you are having, preparation becomes simple.
What Is the Glucose Test in Pregnancy Actually Checking?
The test measures how well your body moves glucose out of your bloodstream after a sugar load. Pregnancy changes how insulin works for everyone, and the test checks whether those normal changes have gone further than they should.
During pregnancy, the placenta releases hormones that make cells less responsive to insulin. This is normal and usually mild. When insulin resistance becomes pronounced enough that blood sugar rises beyond a certain range, the condition is called gestational diabetes. It develops in a minority of pregnancies, and it often causes no symptoms at all — which is exactly why screening exists.
There are two steps to screening in the United States. The first is a one-hour screening test, sometimes called the glucose challenge test. It does not diagnose anything. It sorts women into a lower-risk group and a group that needs a closer look. The second step is a longer fasting test that can actually diagnose gestational diabetes.
One clarification that surprises many women: a high result on the one-hour screen does not mean you have gestational diabetes. It means your blood sugar rose more than the screening cutoff, and you need the diagnostic test to know for sure. A meaningful share of women who fail the screen pass the longer test.
How To Prepare For Glucose Test Pregnancy: Step by Step
Preparation depends entirely on which test you are scheduled for. Confirm that first — it changes what you do the night before.
If you are having the one-hour screening test
- Do not fast. Eat your regular meals. Arriving very hungry does not improve the test and may make you feel unwell after the glucose drink.
- Avoid a very high-sugar meal right before. There is no official rule against it, but a heavy sugary breakfast immediately before the drink is not a fair test of your baseline. Eat something normal.
- Drink water normally. Staying hydrated makes the blood draw easier.
- Skip strenuous exercise that morning. Vigorous activity affects blood glucose. A normal walk is fine.
- Ask about medications. Some medications, including certain steroids, can raise blood sugar. Do not stop anything on your own — call your clinician’s office and ask whether anything you take matters for this test.
- Plan for the full hour. You drink the solution at the start, then wait. You usually cannot leave the office or eat during that hour. Bring something to read.
- Bring a snack for afterward. Some women feel shaky or nauseated after the test. A snack in your bag is a small kindness to yourself.
If you are having the two-hour or three-hour diagnostic test
- You will be asked to fast. Typically this means nothing to eat or drink except water for 8 to 12 hours before the test. Follow the exact number your clinician gives you — it varies by practice.
- Schedule it for the morning. An early appointment means most of your fasting hours happen while you sleep.
- Water is generally allowed. Confirm this with your clinician, but plain water during the fast is usually fine and helps with the blood draws.
- Expect several blood draws over two or three hours. You will have blood taken before the drink and again at intervals afterward. Plan to stay at the lab the whole time.
- Arrange a ride if you feel faint. Most women drive themselves home without trouble, but low blood sugar after a long fast can leave you lightheaded.
What Happens During the Test, Minute by Minute?
The one-hour test is straightforward. A lab technician draws a small blood sample to establish your starting point in some practices, or skips straight to the drink in others. You are given a measured bottle of glucose solution — usually chilled and flavored — and asked to finish it within about five minutes.
Then you wait. Exactly one hour after you finish the drink, your blood is drawn again. The timing matters, so do not wander far from the lab.
The glucose drink contains 50 grams of glucose for the one-hour screen. The diagnostic test uses a larger load, typically 75 grams for the two-hour version or 100 grams for the three-hour version. It is very sweet. Some women find it hard to finish. Sipping steadily rather than gulping helps, and if you feel nauseated, tell the staff — vomiting the drink means the test usually has to be repeated another day.
For the longer test, you will have your fasting blood drawn first, then drink the solution, then have blood drawn at one, two, and sometimes three hours afterward. You stay seated or close to the lab between draws.
Can I Eat or Drink Anything Before the One-Hour Test?
Yes. Food and water are both fine before the one-hour screening. This is the single most common point of confusion, and it leads some women to fast unnecessarily.
The one-hour test is not designed to be done fasting. It is a challenge test — it deliberately stresses your system with sugar and watches the response. Your baseline meal does not invalidate it.
The diagnostic tests are the opposite. Those are compared against fasting reference values, so eating beforehand would make the results uninterpretable. That is why the fasting instruction exists there and not for the screen.
If you are unsure which test you are booked for, call the office before your appointment. Guessing wrong in either direction creates problems — fasting for a test that does not require it, or eating before one that does.
What Do the Results Mean?
Cutoffs vary somewhat between practices and laboratories. Rather than memorizing numbers, understand what each result type means.
| Result | What it usually means |
|---|---|
| Below the screening cutoff on the one-hour test | No further testing needed at this time. Screening is typically repeated later in pregnancy if your clinician recommends it. |
| At or above the screening cutoff on the one-hour test | You are referred for the longer diagnostic test. This is not a diagnosis. |
| Diagnostic test values within normal range | Gestational diabetes is not present by the criteria used. |
| Diagnostic test values above the threshold | Gestational diabetes is diagnosed, and your care team will discuss management. |
If gestational diabetes is diagnosed, it is treatable. Management typically involves monitoring blood sugar, adjusting diet, and in some cases medication. Most women with gestational diabetes have healthy pregnancies and healthy babies with appropriate care.
When Is the Test Done, and Who Gets It?
Routine screening for gestational diabetes is generally offered to all pregnant women, usually between 24 and 28 weeks of pregnancy. That window is when the placental hormones that affect insulin resistance are most active, making the test most informative.
Women with risk factors — such as previous gestational diabetes, certain pre-pregnancy weight ranges, or a strong family history — may be tested earlier, sometimes in the first trimester, and again later if the early result is normal. Your clinician decides based on your history.
If you are not sure whether you have been scheduled for screening, ask at a routine prenatal visit. It is a standard part of prenatal care in the United States, but scheduling varies by practice.
What If I Feel Sick During the Test?
Tell the staff immediately. Nausea is common, and vomiting the glucose drink usually means the test cannot be completed that day. You will likely be rescheduled.
Feeling shaky, sweaty, or lightheaded after the test is also worth mentioning. These symptoms can happen as blood sugar settles, and the staff can check you before you leave. Do not drive home if you feel unwell — sit for a few minutes and let it pass, or ask someone to come get you.
For most women, the test is uncomfortable rather than difficult. Knowing which test you are having, and preparing for that specific version, removes most of the stress.
Frequently Asked Questions
Do I need to fast for the one-hour glucose test in pregnancy?
No. The one-hour screening test does not require fasting, and you should eat your normal meals beforehand. Only the longer two-hour or three-hour diagnostic tests require fasting.
Can I drink water before the glucose test?
Yes, plain water is fine before both the screening and diagnostic tests. Confirm with your clinician for the fasting test, but water is generally allowed during the fasting period.
What happens if I throw up the glucose drink?
The test usually cannot be completed and will need to be rescheduled for another day. Tell the lab staff right away if you feel nauseated.
Does a failed one-hour glucose test mean I have gestational diabetes?
No. A result above the screening cutoff means you need the longer diagnostic test to find out. Many women who exceed the screening threshold pass the diagnostic test.
