Pregnancy

The 3-Hour Glucose Test: How It Works

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The 3-hour glucose tolerance test confirms or rules out gestational diabetes after a positive screen. It uses an overnight fast, a 100-gram glucose drink, and four blood draws over three hours. Two or more values above the thresholds are needed to diagnose, so one high number alone does not qualify.

Last updated: July 2026

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What is the 3-hour glucose tolerance test?

The 3-hour glucose tolerance test is the confirmatory diagnostic step after a positive 1-hour screen. Unlike the screen, it requires an overnight fast and uses a stronger 100-gram glucose drink 1. According to the American College of Obstetricians and Gynecologists, technicians draw blood while you are fasting and then again at one, two, and three hours, four samples across the morning 1. The test exists because the 1-hour screen is intentionally oversensitive, so many who screen positive turn out not to have gestational diabetes 1. A diagnosis, which affects roughly 6 to 9 in 100 pregnancies, hinges on this longer test rather than the initial screen 1. Reaching this step is common and expected, so a referral to the 3-hour test is not a sign that something is wrong.

How should I prepare for the fasting draw?

Preparation makes the fasting stretch more bearable and the results more reliable. In the three days beforehand, eating your usual carbohydrates rather than restricting helps avoid a falsely abnormal result 1. You then fast for 8 to 14 hours overnight, typically finishing dinner and skipping breakfast, though small sips of water are usually fine 1. Scheduling the earliest morning slot shortens the hungry wait, and lining up a ride is wise if fasting tends to leave you lightheaded. Feeling shaky or queasy overlaps with ordinary pregnancy nausea and tends to ease after the final draw. Our gestational diabetes guide explains what a diagnosis would involve if two values come back high.

What happens during the four blood draws?

The test itself is mostly waiting, punctuated by four quick blood draws. After the fasting sample, you drink the 100-gram solution within about five minutes, then remain at the lab for the timed one-, two-, and three-hour draws 1. Staying seated, hydrated with water, and occupied helps the hours pass; strenuous walking can affect readings, so most people rest between samples 1. The drink is sweeter and larger than the screening version, and some feel warm or nauseated afterward. Bringing a phone charger, headphones, and a light snack for immediately after the last draw makes the morning easier. The Office on Women's Health lists this multi-draw test among the standard checks of prenatal care 4.

How are the results read?

Results are read against fixed thresholds, and it takes two abnormal values to diagnose gestational diabetes. According to ACOG, a single high number among the four draws does not meet the criteria; two or more must exceed the cutoffs 1. This two-value rule is why one borderline result does not seal a diagnosis. The thresholds reflect research linking higher maternal glucose to larger babies and delivery complications, a continuous relationship the HAPO study of more than 23,000 pregnancies established 2. If two or more values are elevated, treatment, which clearly improves outcomes in randomized trials 3, usually starts with diet and monitoring rather than medication. Your clinician interprets the full pattern of four numbers, not any single draw in isolation.

When a glucose tolerance result needs a clinician

A glucose tolerance result is best read by a clinician who can place it in your full prenatal context. A provider can confirm whether two values crossed the line, explain what diet-first management looks like, and arrange the growth monitoring a diagnosis brings at your prenatal appointments. The same 3-hour protocol applies whether this is a teenage first pregnancy or a later one in the perimenopausal forties, and a diagnosis flags higher type 2 diabetes risk well beyond the pregnancy itself 2. If your numbers are borderline, questions written down ahead of time help the visit stay focused. You can book prenatal follow-up to review them. Gale can help you prepare for that conversation.

Common questions

Plan for at least three and a half hours at the lab, since blood is drawn fasting and then at one, two, and three hours after the drink. With check-in and waiting, most people spend a full morning there.

Small sips of plain water are usually allowed during the fast and the test, but check with your lab. You cannot eat, chew gum, or have other beverages, since anything with calories would affect your blood-sugar readings.

Tell the staff right away. Vomiting can invalidate the test, and it often needs to be rescheduled for another day. Drinking it slowly within the allowed few minutes and keeping it chilled can help you keep it down.

No. A diagnosis requires two or more of the four values to exceed the thresholds. A single elevated number is considered borderline, and your clinician interprets the whole pattern rather than any single draw.

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When to contact your prenatal clinician

  • —Feeling faint, very shaky, or unwell during the fast is a reason to tell lab staff and seek clinician review
  • —Vomiting the glucose drink, which can invalidate the test, is a reason to contact your prenatal clinician about rescheduling
  • —Decreased or absent fetal movement in the third trimester is a reason to seek same-day obstetric evaluation
  • —Vaginal bleeding or severe abdominal pain during pregnancy is a reason to seek urgent care

Decreased or absent fetal movement, heavy vaginal bleeding, or severe abdominal pain in pregnancy needs urgent evaluation: contact your obstetric provider or labor and delivery unit right away, and call 911 for fainting, heavy bleeding, or a seizure.

This article is general health education, not medical advice. Preparing for and interpreting the 3-hour glucose tolerance test are decisions to make with an obstetric clinician who knows your pregnancy.

References

  1. 1.American College of Obstetricians and Gynecologists (2018). ACOG Practice Bulletin No. 190: Gestational Diabetes Mellitus. Obstetrics & Gynecology. doi:10.1097/AOG.0000000000002501 ✓The confirmatory 3-hour, 100-gram test with fasting plus one-, two-, and three-hour draws, the two-abnormal-values diagnostic rule, guidance to eat normal carbohydrates beforehand, and gestational diabetes prevalence
  2. 2.HAPO Study Cooperative Research Group / Metzger BE, et al. (2008). Hyperglycemia and adverse pregnancy outcomes. New England Journal of Medicine. doi:10.1056/NEJMoa0707943 ✓The continuous relationship between maternal glucose and adverse outcomes across more than 23,000 pregnancies that underlies the diagnostic thresholds, and the link to later type 2 diabetes risk
  3. 3.Crowther CA, Hiller JE, Moss JR, et al. / Australian Carbohydrate Intolerance Study in Pregnant Women (ACHOIS) (2005). Effect of treatment of gestational diabetes mellitus on pregnancy outcomes. New England Journal of Medicine. doi:10.1056/NEJMoa042973 ✓Randomized-trial evidence that treating gestational diabetes, usually starting with diet and monitoring, improves pregnancy outcomes
  4. 4.Office on Women's Health (U.S. HHS) (2025). Prenatal care. Office on Women's Health (womenshealth.gov), U.S. HHS. link ✓Glucose tolerance testing as a standard component of routine prenatal care

4 sources, numbered by first appearance. General health information, not medical advice. AI-assisted editorial content — every citation independently verified. Editorial policy

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