# Glucose Tolerance Test (3-hour)

Source: https://www.pregnancypowerhour.com/tests/glucose-tolerance-test
Last updated: 2026-07-01

> The 3-hour Glucose Tolerance Test (OGTT) is a diagnostic test performed after a positive 1-hour screen to confirm or rule out gestational diabetes during pregnancy.

The 3-hour Glucose Tolerance Test (OGTT) is the diagnostic follow-up after an initial screening suggests a potential for gestational diabetes, offering clarity on your blood sugar regulation during pregnancy. This test provides a more detailed picture than the initial 1-hour screen, helping you and your provider make informed decisions about your care.

## What it is

This test is specifically designed to confirm or rule out gestational diabetes, a condition where your body doesn't make enough insulin or use it well during pregnancy. It's part of what's known as the ACOG two-step approach in the United States, meaning it's performed after an initial 1-hour glucose challenge test (GCT) indicates a need for further investigation. Understanding this process can bring a sense of calm, knowing you're gathering comprehensive information. Like other prenatal tests, such as [Genetic Carrier Screening](/tests/genetic-carrier-screening), the 3-hour OGTT provides valuable insights, allowing you to proactively engage with your health.

## When and how it happens

Typically, this diagnostic test is recommended between weeks 24 and 32 of pregnancy, specifically after you've had an elevated result on your 1-hour glucose challenge screen. The process requires a bit of preparation: you'll need to fast overnight, usually for 8-14 hours, before your appointment.

Upon arrival, a fasting blood sample will be drawn. Following this, you'll be asked to drink a solution containing 100 grams of glucose – it's quite sweet, and many people report feeling nauseated during this part of the test. While alternatives to the glucose drink are rarely offered, it's always worth discussing any significant discomforts with your provider. After consuming the drink, you'll have three more blood draws: one at the 1-hour mark, another at 2 hours, and a final one at 3 hours. This means you'll be at the clinic for several hours. This extended monitoring helps your care team observe how your body processes sugar over time. For some, tools like [Continuous Glucose Monitoring (CGM) in Pregnancy](/tests/glucose-monitoring-cgm) offer a different perspective on daily blood sugar trends, but for diagnostic confirmation of GD, the OGTT is the standard.

## What the results mean

The results of your 3-hour OGTT are evaluated using specific criteria, most commonly the Carpenter-Coustan criteria. Gestational diabetes is diagnosed if two or more of your blood sugar values meet or exceed these thresholds: a fasting level of 95 mg/dL or higher, a 1-hour level of 180 mg/dL or higher, a 2-hour level of 155 mg/dL or higher, or a 3-hour level of 140 mg/dL or higher.

If only one of your values is abnormal, some providers might diagnose mild gestational diabetes, while others might suggest repeating the test in about four weeks to see if your body's glucose processing has changed. You get to decide, in collaboration with your provider, how to interpret and proceed with this information. It’s also helpful to know that international guidelines, such as those from the WHO and IADPSG, often use a different approach: a one-step 75-gram 2-hour OGTT with different diagnostic thresholds. This highlights that there are various evidence-based pathways to diagnosis. Understanding these nuances, much like exploring options for [Preeclampsia Screening (PE Risk Assessment)](/tests/preeclampsia-screening), empowers you to have a deeper conversation with your care team.

## Questions worth asking

Approaching any test with questions can help you feel more clear and confident. Here are a few to consider for your 3-hour OGTT:

*   What are my options if only one of my blood sugar values is abnormal?
*   What kind of support and resources are available if I am diagnosed with gestational diabetes?
*   Can we discuss the differences between the US diagnostic approach and international guidelines, and how that might inform my care?
*   What are the next steps in my care plan based on these results?
*   Are there any strategies to help manage potential nausea during the glucose drink portion of the test?

This information is designed to help you prepare for and understand the 3-hour OGTT, empowering you to have a collaborative conversation with your healthcare provider. Your provider is your best resource for your specific situation.