Pregnancy health
Gestational Diabetes Test Interpreter
Got glucose-test numbers you don't understand? Enter your 1-hour challenge (GCT) value, or the four values from the 3-hour tolerance test (OGTT), and see them in plain English against the published cutoffs — the same Carpenter-Coustan thresholds your OB uses. Everything is read on your own device.
A positive 1-hour screen is not a diagnosis — it just means the 3-hour test is next. Only your OB, reading the full report, diagnoses gestational diabetes.
Gestational diabetes test interpreter
Compared against the published cutoffs (1-hour lab cutoff you selected; 3-hour Carpenter-Coustan thresholds from ACOG Practice Bulletin 190). This is general information, not a diagnosis — a positive 1-hour screen isn't a verdict, and only your OB, reading the full report, diagnoses gestational diabetes.
How it works
The two-step glucose screen — and what each number means
Most US practices screen for gestational diabetes in two steps. Step one is the 1-hour glucose challenge (GCT): you drink a 50 g glucose drink with no fasting, and blood is drawn an hour later. This is a screen — its only job is to sort out who needs the longer test. A value at or above your lab's cutoff is a positive screen, not a diagnosis, and a large share of people who screen positive go on to a perfectly normal 3-hour test.
Step two, if step one is positive, is the 3-hour oral glucose tolerance test (OGTT): after an overnight fast you drink 100 g of glucose and have blood drawn four times — fasting, then at 1, 2, and 3 hours. Each draw is compared to its Carpenter-Coustan threshold (fasting 95, 1 h 180, 2 h 155, 3 h 140 mg/dL). If two or more of the four are met or exceeded, the result meets the criteria clinicians use to diagnose gestational diabetes — but the diagnosis, and what it means for your care, is your OB's to make.
Two honest footnotes. The 1-hour cutoff isn't universal — labs use 130, 135, or 140 mg/dL, all acceptable, which is why this tool asks you to pick yours. And the two-step path above isn't the only one: some practices use a one-step 75 g test (a single 2-hour test with its own thresholds, where one abnormal value is enough), and some labs read the 3-hour test with the slightly higher NDDG thresholds instead of Carpenter-Coustan. Your report says which was used — this tool reads the common two-step, Carpenter-Coustan path.
The reference
The cutoffs, laid out
The published values this tool compares your numbers against. The 1-hour cutoff you choose; the 3-hour thresholds are fixed.
| Step | The drink | Blood draws | What the result means |
|---|---|---|---|
| Step 1 — 1-hour glucose challenge (GCT) | 50 g glucose drink, no fasting needed | One blood draw an hour later | A value at or above the lab's cutoff (commonly 130, 135, or 140 mg/dL) is a positive screen → go on to Step 2. It is not a diagnosis. |
| Step 2 — 3-hour glucose tolerance test (OGTT) | 100 g glucose drink, after an overnight fast | Four blood draws: fasting, then 1, 2, and 3 hours | Two or more values at or above the Carpenter-Coustan thresholds = gestational diabetes, confirmed and interpreted by your OB. |
3-hour test: the Carpenter-Coustan thresholds
From ACOG Practice Bulletin 190. A value at or above a threshold is "met." Two or more met = the diagnostic criteria.
| Draw | When | Threshold (met if ≥) |
|---|---|---|
| Fasting | Before the drink | 95 mg/dL |
| 1 hour | 1 hour after the drink | 180 mg/dL |
| 2 hours | 2 hours after the drink | 155 mg/dL |
| 3 hours | 3 hours after the drink | 140 mg/dL |
1-hour screen cutoffs in common use: 130, 135, or 140 mg/dL — all acceptable; your report prints the one your lab applied. Some labs read the 3-hour test with the slightly higher NDDG thresholds instead.
Read this before you worry
The number is a threshold — your OB is the interpreter
These cutoffs are lines on a chart; they don't know your risk factors, your prior pregnancies, or the rest of your report. A positive 1-hour screen mostly means "take the longer test." A single high value on the 3-hour test usually doesn't meet the diagnostic criteria on its own. And even two values that do — the pattern clinicians use to diagnose gestational diabetes — is the start of a care conversation, not a verdict on your pregnancy.
Gestational diabetes, when it is diagnosed, is one of the most manageable pregnancy conditions: blood-sugar monitoring, an eating plan worked out with your care team, activity your provider okays, and medication such as insulin only if numbers stay high. Managed well, the large majority of pregnancies end healthily for both of you. So take your actual report to your OB and ask what your specific numbers mean for your care — that conversation, not this tool, is the real answer.
Next steps
Related tools
Gestational diabetes risk check
Before the test: a private check of the risk factors that decide whether earlier screening is worth asking about.
Prenatal visit schedule
Every visit and test through pregnancy — including exactly where the glucose screen lands.
Pregnancy checklist
What to sort trimester by trimester, so no screen or milestone slips by.
All pregnancy tools
The full suite — from timing conception to dating and tracking a pregnancy.
Sources
Where this comes from
The 3-hour thresholds are the Carpenter-Coustan values published in ACOG Practice Bulletin 190 — fasting 95, 1 h 180, 2 h 155, 3 h 140 mg/dL — with the two-of-four rule exactly as published. The two-step approach and the acceptable 1-hour cutoffs (130/135/140) follow ACOG and the ADA Standards of Care. We don't reweight or rescore anything: the tool reports how your numbers compare to the published cutoffs, and it never diagnoses — that's your OB's job.
Medically Reviewed & Fact-Checked · Updated
Reviewed by EasySTD Editorial Team
Compiled and checked by EasySTD's editorial team against CDC and public-health sources. This is educational information, not a substitute for advice from a licensed clinician. Our editorial guidelines →
6 Sources
Data & references
- ACOG Practice Bulletin No. 190: Gestational Diabetes Mellitus (Carpenter-Coustan thresholds)https://www.acog.org/clinical/clinical-guidance/practice-bulletin/articles/2018/02/gestational-diabetes-mellitus
- ACOG Clinical Practice Update (2024): Screening for Gestational and Pregestational Diabetes in Pregnancy and Postpartum (updates Practice Bulletin No. 190; the Carpenter-Coustan thresholds and two-of-four rule are unchanged)https://journals.lww.com/greenjournal/fulltext/2024/07000/acog_clinical_practice_update__screening_for.34.aspx
- ACOG: Gestational Diabetes (patient FAQ)https://www.acog.org/womens-health/faqs/gestational-diabetes
- ADA Standards of Care in Diabetes — screening and diagnosis criteriahttps://professional.diabetes.org/standards-of-care
- USPSTF: Screening for Gestational Diabetes (recommendation statement)https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/gestational-diabetes-screening
- CDC: Gestational Diabeteshttps://www.cdc.gov/diabetes/about/gestational-diabetes.html
EasySTD is an information and comparison directory, not a healthcare provider. This tool is general information based on published ACOG and ADA cutoffs: it is not medical advice, a diagnosis, or a lab result, and it does not create a doctor-patient relationship. Only your OB, reading the full report, can diagnose gestational diabetes and decide what your numbers mean for your care.
Good to Know
Glucose test results: frequently asked questions
The difference between the 1-hour and 3-hour tests, what a positive screen does and doesn't mean, the Carpenter-Coustan thresholds, one high value, why cutoffs vary, and how private this is.
What's the difference between the 1-hour and 3-hour glucose tests?
They're the two steps of the most common US screen. The 1-hour test (the glucose challenge test, or GCT) is a screen: you drink a 50 g glucose drink — no fasting needed — and have blood drawn an hour later. If that number is at or above your lab's cutoff, you move on. The 3-hour test (the oral glucose tolerance test, or OGTT) is the diagnostic step: after an overnight fast you drink 100 g of glucose and have blood drawn four times — fasting, then at 1, 2, and 3 hours. The 1-hour test only decides whether you need the 3-hour test; the 3-hour test is what actually establishes a diagnosis, and your OB interprets it.
I failed the 1-hour test — do I have gestational diabetes?
No. A value at or above the cutoff on the 1-hour challenge is a positive screen, not a diagnosis — it simply means the next step is the 3-hour test. Screens are deliberately set to catch more people than actually have the condition, so a large share of people who screen positive on the 1-hour test go on to have a completely normal 3-hour test. Try not to read anything into the single number beyond "time for the longer test." (One nuance: some practices treat a very high 1-hour result as diagnostic on its own and may begin management without the 3-hour test — that's a call your OB makes.)
What are the Carpenter-Coustan thresholds?
They're the four cutoffs used to read the 3-hour 100 g tolerance test, from ACOG Practice Bulletin 190: fasting 95 mg/dL, 1 hour 180 mg/dL, 2 hours 155 mg/dL, and 3 hours 140 mg/dL. A value at or above a threshold counts as "met." If two or more of the four are met, the result meets the criteria clinicians use to diagnose gestational diabetes. Some labs use the slightly higher National Diabetes Data Group (NDDG) thresholds instead — your report says which set was applied, and this tool uses the more widely used Carpenter-Coustan values.
Only one of my four 3-hour values was high — what does that mean?
By the standard two-of-four rule, a single elevated value does not meet the criteria for a gestational-diabetes diagnosis. But one abnormal value isn't nothing: some clinicians treat it as "impaired" glucose tolerance and will either repeat the test, watch your numbers, or in some cases still recommend treatment, especially alongside risk factors. This is exactly the kind of borderline result where the interpretation belongs to your OB, who can weigh the single value against your whole picture. This tool can tell you how many thresholds were met; it can't make that clinical call.
Why does the 1-hour cutoff change from one lab to another?
Because there's no single national standard for the 1-hour screen. Practices choose a cutoff — most commonly 130, 135, or 140 mg/dL — and each is a trade-off: a lower cutoff (130) catches more cases but sends more people to the 3-hour test, while a higher cutoff (140) sends fewer people on but can miss a few. All three are considered acceptable, which is why this tool asks you to pick the cutoff printed on your report rather than assuming one. What matters for you is the specific cutoff your lab used — that's the line your result is judged against.
Is what I enter here private?
Yes. This tool runs entirely in your browser: the numbers you type are compared on your own device and are never sent to a server, saved, or shared — close the tab and they're gone. We treat pregnancy data as sensitive by design, so there's no account and nothing to delete. It's general educational information based on published ACOG and ADA cutoffs; it isn't medical advice, a diagnosis, or a lab result, and it can't replace the OB who ordered your test and reads the report.
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