Pregnancy · Diabetes
GDM OGTT Interpreter
Interpret your oral glucose tolerance test against the two main diagnostic standards: IADPSG / WHO / ADA / NICE 75g one-step (any 1 of 3 = GDM) or ACOG two-step Carpenter-Coustan 100g 3-hour (≥ 2 of 4 = GDM). Supports both mg/dL and mmol/L.
Last reviewed 25 May 2026
Gestational diabetes — OGTT interpreter
75 g (IADPSG) or 100 g (Carpenter-Coustan) OGTT
Diagnostic standard
Units
mg/dL
mg/dL
mg/dL
Enter at least one plasma glucose value to interpret.
Educational tool only — not medical advice. IADPSG (one-step) is the preferred standard in WHO 2013, ADA, NICE; ACOG accepts both. GDM diagnosis triggers: dietary counselling, glucose monitoring, exercise prescription, and (for ~30 %) metformin or insulin. Treatment significantly reduces macrosomia, shoulder dystocia, and neonatal hypoglycaemia risk (HAPO trial / ACHOIS / MFMU 2009).
What does this mean?
GDM is screened for around 24–28 weeks and earlier (booking) in higher-risk women. The HAPO study (NEJM 2008) showed adverse pregnancy outcomes rise continuously with maternal glucose, so any single elevated value on a 75 g OGTT (fasting ≥ 92, 1 h ≥ 180, 2 h ≥ 153 mg/dL) meets the IADPSG definition. About 14 % of pregnancies worldwide (IDF Atlas) — more in Asian and Hispanic populations. First- line treatment is medical nutrition therapy + 30 min walking after meals; ~70 % achieve targets this way. If not, add metformin (safe in pregnancy per MiG trial 2008 and NICE NG3) or move to insulin. Diagnosed GDM raises lifetime type-2 diabetes risk; a 6–12 week postnatal OGTT is recommended (ADA, NICE), then lifestyle surveillance.
Introduction
Gestational diabetes mellitus (GDM) is glucose intolerance first recognised in pregnancy. It affects 7-14 % of pregnancies globally (rising with maternal age and BMI). Diagnosis is by oral glucose tolerance test (OGTT) at 24-28 weeks; women with risk factors are screened earlier.
This calculator interprets OGTT values against the two main diagnostic standards:
- IADPSG / WHO / ADA / NICE 75 g one-step OGTT — any 1 of 3 values above threshold = GDM.
- ACOG two-step (Carpenter-Coustan) 100 g 3-hour OGTT — ≥ 2 of 4 values above threshold = GDM.
Background — the HAPO study
The Hyperglycemia and Adverse Pregnancy Outcomes (HAPO) study, NEJM 2008, was the foundational evidence for modern GDM diagnostic thresholds. 25,505 pregnant women across 15 centres in 9 countries underwent a blinded 75 g OGTT at 24-32 weeks. The investigators plotted glucose levels against birth-weight ≥ 90th percentile, primary caesarean, cord C-peptide elevation, and neonatal hypoglycaemia. The relationship was continuous — there is no sudden “cliff” below which glucose is safe. The IADPSG 2010 consensus chose thresholds at the level associated with a 1.75-fold increase in adverse outcomes.
How to interpret your result
IADPSG 75 g (one-step)
| Time | Threshold (mg/dL) | Threshold (mmol/L) |
|---|---|---|
| Fasting | ≥ 92 | ≥ 5.1 |
| 1-hour | ≥ 180 | ≥ 10.0 |
| 2-hour | ≥ 153 | ≥ 8.5 |
Any single value at or above its threshold = GDM diagnosed.
Carpenter-Coustan 100 g (two-step second test)
| Time | Threshold (mg/dL) | Threshold (mmol/L) |
|---|---|---|
| Fasting | ≥ 95 | ≥ 5.3 |
| 1-hour | ≥ 180 | ≥ 10.0 |
| 2-hour | ≥ 155 | ≥ 8.6 |
| 3-hour | ≥ 140 | ≥ 7.8 |
Two or more values at or above threshold = GDM diagnosed.
After diagnosis — the four pillars of treatment
- Diet — registered dietitian, carbohydrate counting, eating to glucose tolerance. About 70 % of women control with diet alone.
- Glucose monitoring — 4× daily (fasting + 1h or 2h after each main meal). Targets: fasting < 95 mg/dL (5.3 mmol/L), 1-h postprandial < 140 (7.8) or 2-h < 120 (6.7).
- Exercise — 30 min walking after meals significantly reduces postprandial peaks.
- Medication — ~30 % need metformin or insulin. Both are safe in pregnancy; insulin is the gold standard, metformin is increasingly first-line in many countries.
After birth — postpartum surveillance
- 75 g OGTT at 6-12 weeks postpartum to confirm resolution.
- Annual fasting glucose or HbA1c thereafter — 50 % of women with GDM develop type 2 diabetes within 10-20 years (Bellamy 2009 Lancet).
- Diabetes Prevention Program lifestyle modification (7 % weight loss + 150 min/week activity) reduces T2D progression by 53 % in this population.
Limitations
- The OGTT itself is uncomfortable and has imperfect reproducibility (~10 % variability on repeat testing). Don’t over-interpret a single borderline value.
- Self-reported home glucose monitoring is operator-dependent. Use a calibrated meter and follow technique precisely.
- HbA1c is not a substitute for OGTT in pregnancy — it under-detects GDM because plasma volume expansion lowers HbA1c by ~0.5 %.
- The IADPSG threshold has been debated — ACOG still accepts the two-step approach because the HAPO outcome differences at IADPSG thresholds are modest.
Sources
- HAPO Study Cooperative Research Group. Hyperglycemia and Adverse Pregnancy Outcomes. N Engl J Med 2008;358:1991-2002.
- IADPSG Consensus Panel. Recommendations on the diagnosis and classification of hyperglycemia in pregnancy. Diabetes Care 2010;33:676-82.
- ACOG. Practice Bulletin 190: Gestational Diabetes Mellitus. 2018 (reaffirmed 2024).
- American Diabetes Association. Standards of Medical Care in Diabetes — 2024.
- WHO. Diagnostic criteria and classification of hyperglycaemia first detected in pregnancy. 2013.
- NICE. Diabetes in pregnancy (NG3). 2015, updated 2020.
- Bellamy L, et al. Type 2 diabetes mellitus after gestational diabetes: a systematic review and meta-analysis. Lancet 2009;373:1773-9.
- Crowther CA, et al. Effect of treatment of gestational diabetes mellitus on pregnancy outcomes (ACHOIS). N Engl J Med 2005.
Frequently asked questions
75 g (one-step) vs 100 g (two-step) — which test should I have?
Both are valid. The 75 g one-step OGTT (IADPSG / WHO / ADA / NICE) is performed once at 24-28 weeks: fast overnight, drink 75 g glucose, draw blood fasting, at 1 hour, and at 2 hours. ANY ONE value above threshold = GDM. The two-step (ACOG / Carpenter-Coustan) starts with a 50 g non-fasting glucose challenge test; if ≥ 130-140 mg/dL, proceed to a 100 g 3-hour OGTT where ≥ 2 of 4 values above threshold = GDM. The one-step is simpler, identifies more women with GDM, and is increasingly preferred globally; the two-step is the historical US standard. ACOG accepts both.
What are the actual thresholds?
IADPSG 75g (one-step): fasting ≥ 92 mg/dL (5.1 mmol/L), 1-hour ≥ 180 mg/dL (10.0 mmol/L), 2-hour ≥ 153 mg/dL (8.5 mmol/L). Any one = GDM. Carpenter-Coustan 100g (two-step): fasting ≥ 95 mg/dL (5.3 mmol/L), 1-hour ≥ 180 (10.0), 2-hour ≥ 155 (8.6), 3-hour ≥ 140 (7.8). Two or more = GDM. The 1-hour value happens to be identical in both standards.
Where do these thresholds come from?
The HAPO study (Hyperglycemia and Adverse Pregnancy Outcomes), NEJM 2008 — 25,505 pregnant women across 15 centres, blinded glucose testing. The thresholds were set at the glucose level where the risk of large-for-gestational-age, primary caesarean, cord C-peptide, and neonatal hypoglycaemia rose by 1.75-fold above the cohort mean. IADPSG endorsed these thresholds in 2010; WHO adopted them in 2013; ADA followed.
I was diagnosed with GDM — what happens now?
Standard care has four pillars: (1) Dietary counselling — a registered dietitian, carbohydrate counting, eating to glucose tolerance. (2) Glucose monitoring — 4× daily (fasting + 1h after each main meal) until you and your team know the pattern. Targets: fasting < 95 mg/dL (5.3 mmol/L), 1-hour postprandial < 140 (7.8) or 2-hour < 120 (6.7). (3) Exercise — 30 minutes most days reduces glucose. (4) Medication — about 30 % of women need metformin or insulin in addition to diet/exercise. Treatment significantly reduces macrosomia, shoulder dystocia, and neonatal hypoglycaemia (ACHOIS trial, MFMU 2009).
Will I get diabetes later in life?
Risk is higher but not certain. About 50 % of women with GDM develop type 2 diabetes within 10-20 years (Bellamy 2009 Lancet meta-analysis). Postpartum screening at 6-12 weeks (75 g OGTT) is recommended, followed by annual fasting glucose or HbA1c surveillance. The good news: lifestyle modification (Diabetes Prevention Program — 7 % weight loss + 150 min/week activity) reduces T2D progression by 53 % in this population (Ratner 2008 J Clin Endocrinol Metab).
What about my baby — does GDM hurt them?
Treated GDM is much safer than untreated. Untreated GDM raises macrosomia (large baby) risk, shoulder dystocia, birth injury, neonatal hypoglycaemia, jaundice, and stillbirth (rare). Treated GDM has outcomes close to non-GDM pregnancies. Baby has slightly increased lifetime risk of obesity and type 2 diabetes (Boney 2005 Pediatrics) — the same lifestyle measures that help you reduce postpartum diabetes also benefit the child.
Does early GDM screening apply to me?
Early (first-trimester) screening with HbA1c or fasting glucose is recommended for women with risk factors: BMI ≥ 30, prior GDM, prior macrosomic baby, family history of T2D, PCOS, current corticosteroid use, ethnicity associated with higher T2D risk (South Asian, Black, Hispanic, Native American). NICE and ACOG agree on early screening for these groups; if normal, standard 24-28 week screening still applies.
What if my fasting is normal but the 1-hour is high?
Under IADPSG, that single elevated value diagnoses GDM. Under Carpenter-Coustan, two values are needed. Discuss with your team — many will treat as 'impaired glucose tolerance in pregnancy' even if it doesn't formally meet Carpenter-Coustan, because the HAPO data show outcomes worsen at lower glucose levels than the historical thresholds set.