Glucose Tolerance Test Calculator for Pregnancy (Gestational Diabetes OGTT)
Check pregnancy glucose tolerance test results against the official cut-offs: one-step 75 g (IADPSG/WHO), three-hour 100 g (Carpenter–Coustan or NDDG), NICE 75 g and the 50 g one-hour screen, in mg/dL or mmol/L.
How it works
75 g one-step (IADPSG/WHO/ADA): fasting ≥ 92 mg/dL (5.1 mmol/L), 1 h ≥ 180 (10.0), 2 h ≥ 153 (8.5) — one value diagnoses · 100 g Carpenter–Coustan: 95 / 180 / 155 / 140 mg/dL — two values diagnose · 100 g NDDG: 105 / 190 / 165 / 145 mg/dL — two values · NICE 75 g: fasting ≥ 5.6 or 2 h ≥ 7.8 mmol/L · 50 g screen: 1 h ≥ 140 mg/dL (7.8 mmol/L); some centres use 130 or 135 · mmol/L = mg/dL ÷ 18
Gestational diabetes is usually tested at 24–28 weeks. In the one-step approach (used by WHO, ADA, IADPSG and many Latin American countries) you drink 75 g of glucose and a single value at or above the cut-off makes the diagnosis. In the two-step approach common in the US and Spain, a non-fasting 50 g one-hour screen is followed, if positive, by a 3-hour 100 g test where two or more high values are needed (Carpenter–Coustan or the stricter NDDG thresholds). The UK NICE criteria use fasting and 2-hour values only. Your clinic's chosen criteria decide the diagnosis.
Worked example
On a 75 g two-hour test, fasting 88 mg/dL is below 92, the 1-hour value of 185 is at or above 180, and the 2-hour value of 150 is below 153. One high value is enough under the IADPSG criteria, so the result is Gestational diabetes — criteria met (1 of 3 values at or above the threshold). Under the 100 g Carpenter–Coustan rules one high value would not be enough.
Sources: ADA Standards of Care: Diabetes in pregnancy (Section 15) · ACOG Practice Bulletin 190: Gestational diabetes mellitus · NICE NG3: Diabetes in pregnancy
Frequently asked questions
What are normal glucose tolerance test results in pregnancy?
For the 75 g two-hour test: fasting below 92 mg/dL (5.1 mmol/L), 1 hour below 180 (10.0) and 2 hours below 153 (8.5). For the 100 g three-hour test (Carpenter–Coustan): below 95, 180, 155 and 140 mg/dL.
What happens if I fail the 1-hour glucose test?
A positive 50 g screen (often 140 mg/dL or more) is not a diagnosis — most people who screen positive have a normal result on the full test. You will be booked for the 3-hour 100 g test, done fasting.
If only one value is high on the 3-hour test, do I have gestational diabetes?
Not by Carpenter–Coustan or NDDG criteria, which need two abnormal values. Many clinicians still recommend diet advice or a repeat test later, since one abnormal value is linked with larger babies.
How do I convert mg/dL to mmol/L?
Divide by 18 (more precisely 18.016). For example, 92 mg/dL ≈ 5.1 mmol/L and 153 mg/dL ≈ 8.5 mmol/L.
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Educational information only, not medical advice or a diagnosis. Discuss your results and any decision with your doctor, midwife or pharmacist.