Quick Facts
- Sample: Venous blood (4 draws over 3 hours)
- Fasting: 8–14 hours required
- Turn-around: 1–2 business days. May take longer based on weather, holiday, or lab delays.
Benefits
- Definitive diagnosis — Accurately diagnoses gestational diabetes with high specificity
- Prevents complications — Enables early intervention to protect maternal and fetal health
- No waiting — Skip physician referrals and get tested directly
- Clear results — Established diagnostic criteria with secure online access
- Cost-effective — Helps avoid $5,800 in potential complications per case
- Comprehensive — Four-point glucose assessment provides complete tolerance profile
Who Is This Test For?
- Pregnant women with abnormal 1-hour glucose screening results (≥140 mg/dL)
- Women with risk factors like family diabetes history or previous gestational diabetes
- Expectant mothers over age 25 from higher-risk ethnic backgrounds
- Pregnant women with PCOS, high blood pressure, or previous large babies
- Those seeking comprehensive glucose tolerance assessment during pregnancy
How It Works – Just 3 Steps
- Fast overnight for 8-14 hours (water allowed), then arrive at lab
- Drink glucose solution after baseline blood draw, remain for 3 hours
- Get results online within 1-2 days in secure online portal
FAQ
When should I take this test? Typically between 24-28 weeks of pregnancy after an abnormal 1-hour glucose screening test.
What preparation is needed? Fast for 8-14 hours before your appointment. Eat normally in the days prior and bring entertainment for the 3-hour wait.
How is gestational diabetes diagnosed? Two or more abnormal glucose values indicate gestational diabetes using established Carpenter and Coustan criteria.
What happens if I'm diagnosed? You'll need blood glucose monitoring, dietary changes, and regular follow-up with your obstetric care team.
Can I eat or drink during the test? No food or beverages except small sips of water are allowed during the 3-hour testing period.
More Details
What is the purpose of this test?
The 3-hour glucose tolerance test definitively diagnoses gestational diabetes mellitus (GDM) in pregnant women by measuring how effectively your body processes glucose over time (American College of Obstetricians and Gynecologists, 2023). This comprehensive diagnostic test is typically performed after an abnormal 1-hour glucose challenge screening test and provides critical information to prevent maternal and fetal complications.
Gestational diabetes affects approximately 6.9% of pregnancies in the United States and can lead to serious health risks including preeclampsia, cesarean delivery, and macrosomia (large babies) (PLOS Medicine, 2024). Early diagnosis enables timely intervention through dietary modifications, blood glucose monitoring, and medical management when necessary.
Who would benefit from this test?
This test is essential for pregnant women who have had an abnormal 1-hour glucose screening test result, typically ≥140 mg/dL (University of Washington Medicine, 2020). Women with risk factors for gestational diabetes should also consider this comprehensive assessment.
High-risk factors include being over age 25, having a family history of diabetes, belonging to certain ethnic groups (Latino, Black, Native American, Asian, or Pacific Islander), previous gestational diabetes, PCOS, obesity, or history of delivering babies weighing over 9 pounds. Women with these risk factors may require earlier screening and closer monitoring throughout pregnancy.
When should I order a Gestational Glucose Tolerance Diagnostic Test (3-Hour)?
This test is typically ordered between 24-28 weeks of pregnancy following an abnormal 1-hour glucose challenge test (ACOG, 2023). High-risk women may be screened earlier in pregnancy, though routine screening before 24 weeks lacks sufficient evidence according to current guidelines.
The optimal timing allows for early enough detection to implement management strategies while providing enough pregnancy progression to accurately assess glucose tolerance changes. If you have multiple risk factors or previous gestational diabetes, your healthcare provider may recommend testing in both early and standard timeframes.
How do I interpret the results?
Fasting
- Normal: <95 mg/dL
- Abnormal: ≥95 mg/dL
1-hour
- Normal: <180 mg/dL
- Abnormal: ≥180 mg/dL
2-hour
- Normal: <155 mg/dL
- Abnormal: ≥155 mg/dL
3-hour
- Normal: <140 mg/dL
- Abnormal: ≥140 mg/dL
Gestational diabetes is diagnosed when two or more values exceed the normal thresholds using Carpenter and Coustan criteria.
Disclaimer: Reference ranges may vary by laboratory. Listed ranges are general guidelines and may differ from those used by the performing lab. Always consult your healthcare provider for interpretation.
A diagnosis requires two or more abnormal values based on established criteria (University of Washington Medicine, 2020). One abnormal value may indicate impaired glucose tolerance requiring additional monitoring but does not constitute a gestational diabetes diagnosis. ACOG does not support diagnosis based on only one abnormal value.
Pre-test preparation
Fast for 8-14 hours before your appointment, consuming only small sips of water. Eat normally in the days leading up to the test without restricting carbohydrates, as this can affect results accuracy. Inform the laboratory about any medications you're taking that might influence glucose levels.
Schedule your test for morning hours and plan to remain at the testing facility for the entire 3-hour duration. Bring reading materials, electronic devices, or other quiet activities to keep yourself occupied during waiting periods. Avoid strenuous activity before and during the test.
How often should I get tested?
- Normal pregnancy (low risk) – Once at 24–28 weeks
- High-risk pregnancy – Early pregnancy + 24–28 weeks
- Previous gestational diabetes – Early pregnancy + standard timing
- Post-delivery follow-up – 4–12 weeks after delivery
Most women require only one 3-hour glucose tolerance test during pregnancy, typically following an abnormal 1-hour screening. However, high-risk women may need multiple assessments. After delivery, women diagnosed with gestational diabetes should have postpartum glucose testing to screen for persistent diabetes (ACOG, 2023).
Why early detection matters
Early detection of gestational diabetes is crucial for preventing serious complications for both mother and baby. Untreated GDM increases risks of preeclampsia, cesarean delivery, shoulder dystocia during delivery, and neonatal hypoglycemia (Diabetes Care Journal, 2019).
Each case of gestational diabetes is associated with approximately $5,800 in higher medical expenditures, with a total economic burden reaching nearly $1.6 billion annually in the United States. Early diagnosis enables timely interventions including dietary modifications, blood glucose monitoring, and medical management that can significantly reduce these complications and associated costs.
Related tests you may consider
Hemoglobin A1c (HbA1c) - provides information about average blood glucose levels over 2-3 months, useful for early screening in high-risk women or postpartum follow-up to assess for type 2 diabetes conversion.
Fasting Plasma Glucose Test - offers a simpler alternative measuring blood glucose after overnight fasting, useful for monitoring diagnosed GDM patients or alternative screening protocols.
Glucose Tolerance Test (GTT), 2-Hour (Oral WHO Protocol) – Measures how your body processes sugar over two hours after drinking a glucose solution, helping screen for diabetes and other blood sugar disorders.
