3-Hour Glucose Test in Pregnancy: What to Expect and How Results Work

A 3-hour oral glucose tolerance test (OGTT) is commonly used to check for gestational diabetes mellitus (GDM) after an abnormal pregnancy glucose screen. It usually includes a fasting blood draw, a 100-gram glucose drink, and blood draws one, two, and three hours later. Follow your own prenatal clinic’s instructions, since laboratory procedures and result cutoffs can differ. 1 2

What is a 3-hour glucose test?

The 3-hour glucose tolerance test is a diagnostic blood test for gestational diabetes. Gestational diabetes is high blood glucose first recognized during pregnancy. A common U.S. approach starts with a 50-gram, one-hour screening test and uses a 100-gram, three-hour OGTT when diagnostic testing is indicated. 1 2

This is different from routine home glucose checks, an A1C test, or a fasting glucose test. Those tests can be useful in other settings, but pregnancy testing has its own timing and criteria. For background on glucose measurements outside pregnancy, see our guide to normal blood sugar ranges and our explanation of the A1C blood sugar test.

Why might my prenatal clinician order this test?

Your prenatal clinician may order this test when a screening result suggests that a closer look at glucose handling is needed. The 24-to-28-week window is commonly used for gestational diabetes screening because pregnancy hormones can increase insulin resistance as pregnancy progresses. 2 6

Insulin is a hormone that helps move glucose from blood into cells. During pregnancy, placental hormones can make insulin work less effectively. This pregnancy-related insulin resistance does not mean you did anything wrong. The OGTT measures how your body handles a defined amount of glucose over time. 2

What does a 3-hour glucose test look like?

The test is structured and timed. You will usually have a fasting blood sample taken first, drink the glucose solution, then have three more blood samples collected at set intervals. Four blood draws are typical for the 100-gram, three-hour test. 1

What happens before the glucose drink?

After check-in, lab staff usually draw fasting plasma glucose before you drink the glucose solution. The fasting sample gives your prenatal team a starting value before the glucose load. 1

When are blood samples collected?

For the commonly used 100-gram OGTT, samples are collected while fasting and at one, two, and three hours after the glucose drink. The timing matters: a draw taken too early or late may not match the laboratory’s validated interpretation method. 1

How long will I be at the lab?

Plan for more than three hours at the lab because check-in, the fasting draw, the drink, and timed collections all take time. Many labs ask patients to remain nearby or on site so each blood draw happens on schedule. Ask your lab whether you may leave the waiting area, eat, drink anything besides water, exercise, or use nicotine during the test. 1

How should I prepare for a 3-hour pregnancy glucose test?

Preparation affects test accuracy. One published laboratory protocol advises an unrestricted diet with at least 150 grams of carbohydrate for three days before the test, then fasting for 8 to 14 hours before the appointment; however, your own lab may use different instructions. 1

Do not make medication changes on your own to prepare for testing. Some medicines can affect glucose metabolism, and the ordering clinician or pharmacist should tell you whether and when to take your regular medicines. 1

What should I ask the ordering clinic before the appointment?

• Do I need to fast, and what is the exact fasting window?
• May I drink water, brush my teeth, or take my usual medicines?
• Should I avoid exercise, nicotine, or caffeine before or during the test?
• Should I bring a snack for after the final blood draw?
• What should I do if I become sick, arrive late, or vomit after drinking the solution?

Why can preparation instructions differ by laboratory?

Laboratories follow specific collection and handling procedures. They may also use different diagnostic systems. Following the written instructions from the clinic that ordered your test is more useful than copying a preparation plan from a friend, a social post, or a different lab. 1

What can I expect to feel during or after the test?

Some people tolerate the drink without much trouble. Others feel nauseated, warm, shaky, hungry, tired, lightheaded, or headachy while fasting or after consuming a concentrated glucose drink. These symptoms can be uncomfortable, but they do not by themselves tell you whether the result will be normal or abnormal. 1

When should I tell the lab staff that I feel unwell?

Tell staff right away if you feel faint, have severe nausea, vomit, have trouble staying awake, or think you cannot safely continue. Vomiting may mean the test cannot be interpreted and may need to be rescheduled, but the lab and your prenatal team should decide the next step. 1

How are 3-hour glucose test results interpreted?

Results are interpreted by comparing each timed sample with the cutoff system used by your laboratory. The two commonly discussed systems are Carpenter-Coustan thresholds and National Diabetes Data Group thresholds; they use different glucose values, so there is no single result chart that fits every lab. 1 4 5

Why do the glucose amount, timing, and cutoff system matter?

The 100-gram glucose load and the timed blood draws are part of the test definition. In the Carpenter-Coustan system, thresholds are 95 mg/dL fasting, 180 mg/dL at one hour, 155 mg/dL at two hours, and 140 mg/dL at three hours. The National Diabetes Data Group system uses higher thresholds: 105, 190, 165, and 145 mg/dL at those same time points. 1 4 5

What does an abnormal result mean?

Under the laboratory protocol described by the University of Florida, two or more values at or above the relevant cutoffs support a diagnosis of gestational diabetes. Some clinicians may give additional attention to a single elevated value based on the full pregnancy picture, which is one reason not to self-interpret a portal result. 1 2

An abnormal test is not a measure of effort or personal failure. It is information that helps a prenatal team decide whether nutrition counseling, glucose monitoring, medication, or closer follow-up may be appropriate. Those decisions depend on pregnancy-specific risks and should be made with the clinician caring for you. 2 8

What happens after a 3-hour glucose test?

The next step depends on the result and your pregnancy history. If the result does not meet gestational diabetes criteria, your prenatal team may simply continue routine care. If it does suggest gestational diabetes, care commonly focuses on glucose monitoring, food and activity guidance tailored to pregnancy, and medication only when the prenatal team decides it is needed. 2 8

Why might postpartum follow-up matter after gestational diabetes?

Gestational diabetes often improves after delivery, but it can identify people who need continued glucose follow-up. A 2022 randomized study examined postpartum glucose intolerance after early gestational diabetes, supporting the importance of discussing postpartum testing with the care team rather than assuming glucose changes always end at delivery. 7

After pregnancy, your clinician may use fasting glucose, an OGTT, or another test based on your history and guideline-based care plan. Our articles on prediabetes, high glucose in a blood test, and insulin resistance treatment explain these topics outside of pregnancy, but prenatal and postpartum clinicians should guide your individual testing schedule.

Where does Chia fit after a 3-hour glucose test?

For gestational diabetes or any glucose concern during pregnancy, the right next step is an obstetric, maternal-fetal medicine, endocrinology, or prenatal care team that can provide pregnancy-specific evaluation. Chia does not offer treatment for gestational diabetes or medication for use during pregnancy.

In particular, do not use weight-loss medications, including compounded GLP-1 medicines, to manage an abnormal pregnancy glucose test. Chia offers clinician-reviewed care only where clinically appropriate; prescriptions are never guaranteed, and our compounded medications are not FDA-approved.

When should I get help now?

Contact your prenatal team or the testing laboratory promptly if you vomit during the test, faint, cannot keep fluids down, or are unsure whether you completed the timed steps correctly. Same-day guidance is especially important if you feel severely unwell or have symptoms your prenatal team has told you require urgent assessment. 1

If you have severe trouble breathing, chest pain, loss of consciousness, heavy bleeding, or another emergency, seek emergency care. For non-emergency questions, bring your result report and the name of the laboratory method to your prenatal visit.

What are common questions about the 3-hour glucose test?