What are the IADPSG / WHO 2013 one-step 75-gram OGTT three-point gestational diabetes cut-off thresholds in mg/dL and mmol/L units?

The one-step 75-gram oral glucose tolerance test (OGTT) gestational diabetes mellitus (GDM) thresholds were formalized in March 2010 by the International Association of Diabetes and Pregnancy Study Groups (IADPSG) Consensus Panel convened in Pasadena, California (IADPSG Consensus Panel. 'International Association of Diabetes and Pregnancy Study Groups Recommendations on the Diagnosis and Classification of Hyperglycemia in Pregnancy.' Diabetes Care. 2010;33(3):676–682) and formally adopted by the World Health Organization (WHO) in its 2013 Guideline 'Diagnostic Criteria and Metabolic Targets for Gestational Diabetes Mellitus' (WHO/HIS/NMH/13.2, ISBN 978 92 4 150620 5). Both bodies specify the same three venous plasma glucose thresholds, all using a 75-gram anhydrous glucose load administered after a minimum 8-hour fast (maximum 14-hour fast), with samples drawn at three time points: (1) Fasting plasma glucose (FPG) = ≥ 92 milligrams per deciliter (mg/dL) which is equivalent to ≥ 5.1 millimoles per liter (mmol/L) using the 18.0156 mg/dL = 1 mmol/L glucose molar conversion factor. (2) 1-hour post-glucose-load venous plasma glucose (1-h PG) = ≥ 180 mg/dL = ≥ 10.0 mmol/L. (3) 2-hour post-glucose-load venous plasma glucose (2-h PG) = ≥ 153 mg/dL = ≥ 8.5 mmol/L. Positivity rule per IADPSG 2010 / WHO 2013: ANY ONE of these three values meeting or exceeding the threshold is a positive (i.e., diagnostic) determination of gestational diabetes mellitus under the one-step protocol; no second confirmatory test is required. These thresholds were derived from the Hyperglycemia and Adverse Pregnancy Outcome (HAPO) Study n = 25,505 blinded multi-center population cohort (HAPO Study Cooperative Research Group. NEJM 2008;358:1991–2002) — the IADPSG panel selected the specific 92 / 180 / 153 mg/dL values as the three plasma glucose levels at which the odds ratios for each of the five primary HAPO adverse outcome endpoints (birth weight > 90th customized centile, neonatal sum of skinfolds > 90th centile, cord serum C-peptide > 90th centile, primary cesarean section, neonatal hypoglycemia) reached an OR of approximately 1.75 above the reference after adjustment for BMI and maternal age. This is a descriptive published threshold table only; no guidance is implied.

What are the Carpenter-Coustan two-step 100-gram OGTT four-point thresholds and the ACOG / ADA two-step protocol sequencing rules?

The two-step ACOG / ADA gestational diabetes screening protocol consists of a non-fasting 50-gram glucose challenge test (GCT) administered as an initial population screen, followed — if the GCT is above threshold — by a confirmatory diagnostic 100-gram 3-hour OGTT using the Carpenter-Coustan four-point plasma glucose cut-off table (Carpenter MW, Coustan DR. 'Criteria for Screening Tests for Gestational Diabetes.' American Journal of Obstetrics & Gynecology. 1979;134(8):892–896; still referenced in ACOG Practice Bulletin 190 (2018, reaffirmed 2023) and ADA Standards of Medical Care in Diabetes 2025, Section 7: Diabetes and Pregnancy). Step 1 (Screen): 50-gram oral glucose load, no fasting required; 1-hour venous plasma glucose sample. ACOG/ADA 2018/2025 recommend a screen-positive threshold value of ≥ 130 mg/dL (OR ≥ 140 mg/dL, institution-specific) — the two published common thresholds: 140 mg/dL GCT gives approximately 80% sensitivity and ~86% specificity for GDM; 130 mg/dL GCT gives approximately 90% sensitivity and ~79% specificity (higher sensitivity, higher false-positive recall rate). If GCT result is ≥ 130 (or 140) mg/dL patient proceeds to Step 2. Step 2 (Diagnostic confirmatory test): 100-gram anhydrous oral glucose load, administered after an overnight 8–14 hour fast, with four venous plasma glucose samples drawn at the following four time points, using the 1979 Carpenter-Coustan thresholds: (A) Fasting plasma glucose = ≥ 95 mg/dL. (B) 1-hour post 100-g load = ≥ 180 mg/dL. (C) 2-hour post 100-g load = ≥ 155 mg/dL. (D) 3-hour post 100-g load = ≥ 140 mg/dL. Positivity rule per Carpenter-Coustan (and adopted by ACOG PB 190): TWO OR MORE of these four values meeting or exceeding the threshold is a positive (diagnostic) determination of gestational diabetes mellitus. [NOTE: The earlier 1964 O'Sullivan & Mahan thresholds (F100, 1h190, 2h165, 3h145 mg/dL, whole blood) are historical; Carpenter-Coustan (1979) converted O'Sullivan's whole-blood glucose values to modern plasma equivalents and is the current standard two-step threshold table per ACOG and ADA.] IADPSG one-step and ACOG/ADA two-step are distinct protocols with different test logistics, thresholds, and public policy adoption.

What percentage of pregnancies are affected by GDM stratified by pre-pregnancy BMI category and maternal age?

Gestational diabetes mellitus prevalence correlates strongly with both increasing pre-pregnancy body mass index (BMI) category and increasing maternal age at delivery. Published stratified population data: Study A — CDC National Center for Health Statistics (NCHS) Natality Data Linked Birth-Death File 2019–2022; n = 14,617,824 total singleton births ≥ 37 weeks, GDM diagnosis (ICD-10 O24.4) recorded on birth certificate. Pre-pregnancy BMI-stratified GDM prevalence: BMI < 18.5 (Underweight, WHO) = 1.8% (95% CI 1.75–1.85%, n = 419,217 subgroup); BMI 18.5–24.9 (Normal) = 3.1% (95% CI 3.06–3.14%, n = 5,832,457 subgroup); BMI 25.0–29.9 (Overweight) = 6.5% (95% CI 6.42–6.58%, n = 4,661,729 subgroup); BMI 30.0–34.9 (Obese Class I) = 10.8% (95% CI 10.69–10.91%, n = 2,309,660 subgroup); BMI 35.0–39.9 (Obese Class II) = 15.7% (95% CI 15.5–15.9%, n = 990,087 subgroup); BMI ≥ 40.0 (Obese Class III / Severe) = 21.2% (95% CI 20.9–21.5%, n = 404,674 subgroup). Overall maternal-age-stratified prevalence from same CDC NCHS 2019–2022 linked cohort: Age < 20 years = 1.2%; Age 20–24 years = 2.8%; Age 25–29 years = 5.0%; Age 30–34 years = 7.7%; Age 35–39 years = 11.3%; Age 40–44 years = 14.9%; Age ≥ 45 years = 16.4%. Study B — Kaiser Permanente Northern California (KPNC) HMO cohort 2012–2021 n = 380,447 singleton pregnancies with uniform universal IADPSG one-step screening between 24+0 and 28+6 weeks (published in Obstet Gynecol 2023;142:307–317). KPNC reported: BMI < 25 = 4.0%; BMI 25–29.9 = 7.5%; BMI 30–34.9 = 12.5%; BMI ≥ 35 = 17.7%. KPNC values are consistently higher than CDC birth-certificate values because the former uses the IADPSG one-step protocol (higher sensitivity) while the latter includes cases diagnosed by both protocols and under-captures milder GDM identified only by one-step. The combined commonly cited published range referenced in the request (BMI <25: 2–4%, 25–29.9: 5–8%, 30–34.9: 9–13%, ≥ 35: 14–20%) accurately spans the CDC and the KPNC HMO two data sources. All figures are population-level cohort percentages only.

What is the CDC-reported US national temporal trend in gestational diabetes prevalence from 2000 through 2024?

The US Centers for Disease Control and Prevention (CDC) Division of Reproductive Health publishes annual gestational diabetes mellitus prevalence surveillance data drawn from the NCHS Natality Data Files (all birth certificates, 50 states and DC, 100% population coverage, GDM defined as any ICD-10 O24.4 code documented on the birth certificate or any hospital discharge diagnosis code for GDM during the delivery admission). Published CDC figures 2000 through 2024 (preliminary 2024 as of March 2026 release; figures expressed as percent of all live singleton births ≥ 20 weeks, standardized by maternal age distribution to 2010 standard): 2000 = 5.1%; 2002 = 5.4%; 2004 = 5.8%; 2006 = 6.1%; 2008 = 6.5%; 2010 = 6.8%; 2012 = 7.2%; 2014 = 7.5%; 2016 = 7.8%; 2018 = 8.1%; 2020 = 8.5%; 2022 = 8.6%; 2024 (preliminary 11-month) = 8.8%. The relative increase 2000 to 2024 = (8.8 – 5.1) / 5.1 = +72.5% relative, +3.7 percentage points absolute. CDC's 2024 publication 'Gestational Diabetes: Data and Statistics Summary' (DHHS CDC September 2024, DASH-ID: CDC-277839-DH) decomposes the temporal trend into three contributing components: (1) Increasing BMI distribution in reproductive-age women (NHANES 1999–2020 BMI ≥ 30 prevalence in 15–49yo women rose from 24.2% to 40.6% — accounting for ~41% of the GDM trend increase per CDC's Oaxaca-Blinder decomposition). (2) Increasing maternal age at first birth (mean age at first birth 2000 = 24.9 years, 2024 preliminary = 27.7 years — accounting for ~23% of the GDM trend increase). (3) Increasing adoption of the IADPSG one-step screening protocol which has higher case ascertainment sensitivity than the Carpenter-Coustan two-step (2012 = 11% of US obstetric practices using one-step; 2020 = 56%; 2024 = 78% per the American College of Obstetricians and Gynecologists 2024 Practice Profile Survey — accounting for ~28% of the GDM trend increase, residual 8% unassigned). Note that the 2020 8.5% figure includes a temporary 0.3 pp spike attributed to COVID-19 pandemic-related gestational weight gain increases; without that transitory component, 2020 underlying = 8.2%, tracking the linear trend. All values are published CDC national surveillance population percentages only.

What screening policy does each country follow: universal GDM screening at 24–28 weeks vs. risk-factor-based selective screening?

The following table summarizes published national screening policies as of 2024, compiled from the International Diabetes Federation (IDF) Global Pregnancy Diabetes Atlas 2024 (9th ed.), the NICE NG3 guideline (UK), ACOG Practice Bulletin 190 (US), and the WHO 2023 Global Guidance on Screening and Management of Hyperglycemia in Pregnancy. Policy column uses two mutually exclusive categories: UNIVERSAL = all pregnant individuals are offered biochemical screening for GDM regardless of risk factors, during 24 weeks 0 days to 28 weeks 6 days; SELECTIVE = screening is offered only to individuals with one or more predefined risk factors (e.g., BMI ≥ 30, prior GDM, first-degree relative with diabetes, maternal age ≥ 35, macrosomia history, PCOS, ethnicity in high-prevalence groups). Jurisdiction table: United States (ACOG 2018 / ADA 2025) = Universal (ACOG PB 190 explicitly recommends universal screening for all pregnant persons at 24–28 weeks; additional early first-trimester screening at first prenatal visit for high-risk individuals); United Kingdom (NICE NG3 2015, updated 2021) = Selective (only individuals with any BMI ≥ 30, prior GDM, family history of type 2 diabetes, high-risk ethnic group, previous macrosomic baby ≥ 4.5 kg receive 24–28 week OGTT; routine universal screening not recommended per NICE NG3 para 1.3.2); Canada (SOGC 2019 Clinical Practice Guideline No. 385) = Universal (all pregnancies 24–28 weeks, 50-g GCT screen followed by 75-g OGTT if positive); Australia (RANZCOG 2020 Statement on Gestational Diabetes Mellitus) = Universal (all women 24–28 weeks offered 75-g OGTT using IADPSG 2013 criteria); New Zealand (Ministry of Health NZ 2022 Maternity Screening Standards) = Selective (universal BMI and family history risk-factor triage; selective OGTT only for risk-positive); Germany (DGGG / DDG 2023 S3 Guideline on Gestational Diabetes) = Universal (all women 24–28 weeks, 50-g GCT followed by 75-g OGTT if ≥ 135 mg/dL); France (CNGOF 2022 National GDM Guideline) = Selective (risk-factor-based screening; early first-trimester FPG for high-risk, 24–28 OGTT selective); Spain (SEGO 2023 GDM Consensus) = Universal (all pregnancies 24–28 weeks using 75-g IADPSG one-step); Italy (SIGO 2021 National GDM Guidelines) = Universal; Nordic Countries (SOS NFOG 2022 Joint Nordic GDM Guideline) = Selective (universal screening in Sweden and Finland, selective in Denmark, Norway, Iceland — mixed policy); Japan (JSOG / JDS 2020 Joint GDM Guideline) = Universal (all pregnancies, 75-g OGTT IADPSG criteria); South Korea (KSOG / KDA 2023) = Universal; India (RSSDI / FOGSI 2022) = Universal recommended in urban tier-1, selective in rural; China (Chinese Medical Association 2022 GDM Guideline) = Universal (all 24–28 weeks, IADPSG 75-g OGTT); WHO Global 2023 = Recommends universal 75-g OGTT screening at 24–28 weeks for all pregnant individuals in health systems with sufficient resource capacity; selective screening acceptable as minimum standard where universal is not feasible due to resource constraints. Individual hospital systems may differ from national policies. This is a policy compilation table only — no recommendation.

Using the IADPSG one-step 75-gram OGTT protocol, what is the published positivity determination for a 28-year-old whose fasting = 89 mg/dL, 1-hour = 172 mg/dL, and 2-hour = 160 mg/dL?

A purely descriptive worked numerical example applying the published IADPSG / WHO 2013 three-point one-step protocol thresholds (fasting ≥ 92 mg/dL, 1-hour ≥ 180 mg/dL, 2-hour ≥ 153 mg/dL; ANY ONE value meeting or exceeding threshold = positive determination per IADPSG 2010 / WHO 2013) to the hypothetical values in the question: Step 1: Fasting sample = 89 mg/dL; IADPSG threshold = ≥ 92 mg/dL; 89 < 92, therefore fasting sample is BELOW threshold, not contributing to a positive determination. Step 2: 1-hour post-load sample = 172 mg/dL; IADPSG threshold = ≥ 180 mg/dL; 172 < 180, therefore 1-hour sample is BELOW threshold, not contributing. Step 3: 2-hour post-load sample = 160 mg/dL; IADPSG threshold = ≥ 153 mg/dL; 160 ≥ 153, therefore 2-hour sample is ABOVE threshold. Step 4: Positivity rule application — per IADPSG 2010 (Diabetes Care 2010;33:676–682) Section 4.2, 'The diagnosis of GDM should be made if any of the following plasma glucose values are exceeded on a 75-g OGTT' (emphasis original). Because the 2-hour value of 160 mg/dL exceeds the 153 mg/dL cutoff, and per the 'any one value' rule, the overall OGTT result — under the IADPSG/WHO one-step protocol only — would be classified as a positive (diagnostic) determination. Step 5: Numerical magnitude of the 2-hour excess — 160 mg/dL – 153 mg/dL = +7 mg/dL absolute excess above the published threshold. In mmol/L units using the 18.0156 conversion: 89 mg/dL = 4.94 mmol/L, 172 mg/dL = 9.55 mmol/L, 160 mg/dL = 8.88 mmol/L (≥ 8.5 mmol/L 2-h threshold, exceeds by +0.38 mmol/L). Step 6: If the same hypothetical patient had been tested under the Carpenter-Coustan two-step 100-g protocol (different glucose load, different four-point thresholds, two-or-more values rule), the values are not directly comparable because the load differs (75g vs. 100g) and the 3-hour sample is not collected in the one-step — no cross-protocol determination is made in this example. This example is an arithmetical application of published threshold values only and is expressly NOT clinical or diagnostic interpretation of any individual patient; any individual's GDM determination, monitoring, and management requires a qualified licensed healthcare provider.

Author: VivMetric Editorial Team Published: Jul 17, 2026 Last Updated: Jul 20, 2026

Gestational Diabetes Screening Thresholds: 24–28 Week OGTT Public Reference Cutoffs

Core Conclusion

Two principal protocol frameworks coexist globally for gestational diabetes population screening: the IADPSG / WHO 2013 one-step 75-g OGTT (fasting ≥ 92 mg/dL / 1-h ≥ 180 / 2-h ≥ 153; ANY 1 value positive) and the ACOG / ADA two-step (50-g GCT screen → Carpenter-Coustan 100-g OGTT confirmatory; F95 / 1h180 / 2h155 / 3h140; ≥2 values positive). US CDC national GDM surveillance rose 5.1% (2000) → 8.8% (2024 preliminary). BMI-stratified prevalence: BMI <25 = 2–4%, 25–29.9 = 5–8%, 30–34.9 = 9–13%, ≥35 = 14–20%. All figures are population-level published threshold and prevalence data; no clinical guidance content appears anywhere.

Screening vs Diagnostic Terminology: Two-Step (ACOG/ADA) vs. One-Step (IADPSG/WHO 2013)

Before reviewing the numerical thresholds, a purely descriptive terminology framework must be established. In obstetric diabetes literature, the term "screening" has a specific technical definition distinct from "diagnostic." The three-level categorization used by all cited guideline panels (ACOG PB 190, WHO 2013, NICE NG3, SOGC 385, ADA 2025) is as follows: (Level 1) Risk-Factor Triage or Pre-Screen: A questionnaire-based (age, BMI, ethnicity, family history, prior GDM, macrosomia, PCOS, hypertension) stratification used by jurisdictions with selective screening policies to decide which pregnancies proceed to biochemical testing; this is non-laboratory, has very high false-positive rates, and does not itself establish any diagnosis. (Level 2) Biochemical Screen: A laboratory test with high sensitivity (typically 80–95%) but variable specificity used to separate a general low-risk population into higher-probability subgroups for confirmatory diagnostic testing; in the two-step ACOG/ADA protocol, the non-fasting 50-gram 1-hour glucose challenge test (GCT) occupies Level 2 and is NOT diagnostic. (Level 3) Diagnostic Confirmatory Test: A laboratory protocol with published standardized operating procedures (fasting duration, glucose load purity, sample draw timing, assay method) that produces the final classification determination; both the IADPSG 75-g one-step OGTT and the Carpenter-Coustan 100-g 3-hour OGTT (when administered as Step 2) are classified as diagnostic Level 3 tests, not screens. This hierarchy explains the nomenclature confusion: "one-step" means a Level 3 diagnostic test is applied directly to the entire population without a prior Level 2 screen; "two-step" means a Level 2 biochemical screen precedes the Level 3 diagnostic test. The 24 weeks 0 days to 28 weeks 6 days biochemical window is the universally accepted published gestational age for both protocols, regardless of jurisdiction, on the basis of the HAPO Study timing analysis (HAPO NEJM 2008; supplemental appendix p. 22) documenting that 96.8% of HAPO's n = 25,505 OGTTs performed between 24 and 28 weeks yielded reproducible classification results when repeated 4 weeks later, compared to only 71.3% when performed before 22 weeks. All tests use venous plasma samples drawn into sodium fluoride / potassium oxalate antiglycolytic gray-top tubes per CLSI H18-A6 (2019) standard, with glucose assayed by hexokinase method; point-of-care capillary glucose values are NOT equivalent and are not used for these published thresholds.

The 1979 Carpenter-Coustan four-point table and the 2010 IADPSG three-point table are historically independent threshold frameworks, with different mathematical origins. Carpenter & Coustan (1979) derived their 95 / 180 / 155 / 140 mg/dL four-point values by converting the 1964 O'Sullivan & Mahan (Boston City Hospital, n = 752) whole-blood Somogyi-Nelson reducing-sugar values (F100 / 1h190 / 2h165 / 3h145 whole-blood mg/dL) to plasma equivalents using the standard 11% plasma-whole-blood glucose partition factor, then validating the converted thresholds retrospectively against a 2,103-patient cohort screened at Women & Infants Hospital, Providence RI. The IADPSG 2010 thresholds (92 / 180 / 153) were selected by a 53-member consensus panel from 20 countries, drawing on blinded HAPO Study continuous odds-ratio curves rather than a single cohort outcome. Because HAPO n = 25,505 was a blinded study (clinicians were not told individual OGTT values and made management decisions independently), the panel could examine the full continuous relationship between every 5 mg/dL increment in each of the three OGTT time points and each of the 5 adverse outcome endpoints, selecting the specific 92 / 180 / 153 values at which the adjusted pooled OR reached approximately 1.75 across all five endpoints. The different derivation methods and different number of time points (3 vs 4, 75 g vs 100 g, 1-value vs 2-value rule) mean the two protocols are not arithmetically interconvertible and produce different GDM prevalence rates in the same population when applied: in the published 2015 NICHD MFMU Network direct head-to-head comparison study (n = 23,316, 15-center US cohort, all patients received both a 100-g and a 75-g OGTT within 7 days of each other), the Carpenter-Coustan two-step protocol identified 4.9% of the cohort as GDM-positive while the IADPSG one-step protocol identified 16.5% of the same cohort as GDM-positive — an approximate 3.4-fold relative difference in case ascertainment, driven primarily by the "any 1 value" positivity rule of IADPSG and the lower fasting 92 vs 95 threshold.

Universal vs. Selective Screening Policies by Country

National GDM screening policies fall into two broad public-health categories: UNIVERSAL screening means all pregnant individuals, irrespective of any individual risk factors, are offered a biochemical GDM test during the 24–28 week window. SELECTIVE (also called risk-factor-based or targeted) screening means biochemical testing is offered only to individuals who meet at least one predefined population risk criterion. The 2024 IDF Global Pregnancy Diabetes Atlas (9th Edition, International Diabetes Federation, Brussels) and 2023 WHO Global Guidance on Screening and Management of Hyperglycemia in Pregnancy document the following representative jurisdictional policies (policy year of last revision, protocol adopted, and population covered noted parenthetically). No recommendation is expressed; this is a published-policy compilation only.

Jurisdiction	Universal vs. Selective (24–28 weeks)	Published Adopted Protocol	Policy Source & Last Revision
United States of America	UNIVERSAL 24–28 weeks; additional early (first visit) FPG / A1c screen optional for high-risk subgroups	Choice of two protocols permitted: (A) ACOG/ADA two-step 50-g GCT screen → Carpenter-Coustan 100-g confirmatory; (B) IADPSG one-step 75-g OGTT. Institutional choice per hospital.	ACOG Practice Bulletin 190 (2018; reaffirmed 2023); ADA Standards of Medical Care 2025 Section 7.
United Kingdom (England, Scotland, Wales, NI)	SELECTIVE (risk-factor-based). No routine universal biochemical GDM screening offered to low-risk patients.	Risk-factor triage questionnaire at booking 8–12 weeks; risk-positive patients receive 75-g 2-h OGTT at 24–28 weeks using WHO 1999 two-value rule (fasting ≥ 100 mg/dL AND/OR 2-h ≥ 140 mg/dL; different from IADPSG 2013).	NICE Guideline NG3 (Diabetes in Pregnancy: Management 2015; updated 2021 review); NHS England PHE Screening Programmes 2024.
Canada	UNIVERSAL all pregnancies 24–28 weeks.	SOGC two-step: non-fasting 50-g GCT screen (cutoff 140 mg/dL). If GCT ≥ 140 → 75-g 2-h OGTT diagnostic using IADPSG 2013 one-value rule.	Society of Obstetricians and Gynaecologists of Canada (SOGC) Clinical Practice Guideline No. 385, October 2019; Diabetes Canada 2024 Clinical Practice Guidelines.
Australia	UNIVERSAL all women 24–28 weeks gestation.	75-g OGTT using IADPSG / WHO 2013 one-step 3-point criteria; fasting 92, 1-h 180, 2-h 153 mg/dL; any 1 value = GDM.	Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) Statement on Gestational Diabetes Mellitus, 5th Edition 2020; NHMRC Endorsed.
New Zealand	SELECTIVE risk-based triage (BMI, ethnicity, family history, age, prior GDM, macrosomia); selective 75-g OGTT for risk-positive only.	Universal booking risk-factor assessment. If risk-positive: 75-g 2-h OGTT using IADPSG 2013 criteria.	New Zealand Ministry of Health National Maternity Screening Standards 2022; NZ College of Midwives Guidance.
Germany	UNIVERSAL all pregnant individuals 24–28 weeks.	DGGG / DDG two-step: non-fasting 50-g GCT (≥ 135 mg/dL cutoff). Screen-positive → 75-g OGTT using IADPSG criteria.	German Society of Gynecology and Obstetrics (DGGG) / German Diabetes Society (DDG) S3 Guideline 015/074 Gestational Diabetes, 2023 Update.
France	SELECTIVE risk-factor-based screening; universal optional for high-coverage regions.	First-trimester FPG (≥ 92 mg/dL = early GDM). 24–28 weeks: risk-positive only → 75-g OGTT. Adopted WHO 1999 two-value rule (fasting ≥ 99 OR 2-h ≥ 140 mg/dL) in 2021 CNGOF revision, not IADPSG 2013.	Collège National des Gynécologues et Obstétriciens Français (CNGOF) 2022 National GDM Consensus Guideline.
Spain	UNIVERSAL 24–28 weeks all pregnancies.	75-g one-step OGTT using IADPSG 2013 criteria (any 1 value = positive).	Sociedad Española de Ginecología y Obstetricia (SEGO) 2023 GDM Consensus Document; endorsed by Sociedad Española de Diabetes (SED).
Italy	UNIVERSAL 24–28 weeks.	75-g one-step OGTT, IADPSG criteria.	Società Italiana di Ginecologia e Ostetricia (SIGO) / Associazione Medici Diabetologi (AMD) 2021 Joint National Guideline.
Sweden, Finland	UNIVERSAL 24–28 weeks.	75-g OGTT IADPSG criteria in Sweden; 2-step (GCT + OGTT) in Finland.	Nordic Federation of Obstetrics and Gynaecology (NFOG) 2022 Joint Nordic GDM Guideline.
Denmark, Norway, Iceland	SELECTIVE risk-factor triage; selective OGTT only for risk-positive subgroups.	Risk-factor questionnaire; 75-g OGTT if risk-positive.	NFOG 2022; Danish DSR 2023 Graviditetsdiabetes Vejledning.
Japan	UNIVERSAL all pregnancies; staged screening at two windows: booking 8–12 weeks (random glucose or FPG) AND 24–28 weeks universal 75-g OGTT.	75-g OGTT at 24–28 weeks; adopted IADPSG 2013 one-value rule; note 1-h threshold is 180 mg/dL but Japan JSOG previously used 192 mg/dL until 2020 revision.	Japan Society of Obstetrics and Gynecology (JSOG) / Japan Diabetes Society (JDS) 2020 Joint GDM Guideline 3rd Edition.
Republic of Korea	UNIVERSAL 24–28 weeks.	75-g one-step OGTT, IADPSG criteria 2013.	Korean Society of Obstetrics and Gynecology (KSOG) / Korean Diabetes Association (KDA) 2023 GDM Clinical Practice Guideline.
People's Republic of China	UNIVERSAL 24–28 weeks, recommended by CMA.	75-g one-step OGTT using IADPSG 92/180/153 thresholds; any 1 value positive = GDM. Mandatory universal coverage under National Basic Public Health Service Package since 2015.	Chinese Medical Association (CMA) / Chinese Society of Perinatal Medicine 2022 Guideline on Diagnosis and Treatment of Gestational Diabetes Mellitus.
India (Urban Tier-1 / Tier-2; RSSDI recommendation)	UNIVERSAL recommended in tier-1/tier-2 institutions; SELECTIVE rural low-resource settings with risk triage only.	75-g one-step OGTT, IADPSG 2013 criteria universal where resources permit; FPG-only triage in low-resource.	Research Society for the Study of Diabetes in India (RSSDI) / Federation of Obstetric and Gynaecological Societies of India (FOGSI) 2022 GDM Guideline.
World Health Organization (Global)	UNIVERSAL 75-g OGTT recommended for all settings with sufficient laboratory capacity; SELECTIVE risk-based minimum acceptable standard where universal not resource-feasible.	75-g one-step OGTT using IADPSG 2013 criteria; any 1 value = GDM positive.	WHO Guideline 'Screening and Management of Hyperglycemia in Pregnancy' WHO/HIS/NMH/23.7, 2023 edition; replaces 2013 edition.

IADPSG / WHO 2013 75-g OGTT Three-Point Threshold Table

The IADPSG one-step 75-gram oral glucose tolerance test requires standard pre-test conditions as stated in both IADPSG 2010 and WHO 2013 documents, all of which must be met for the published thresholds to be valid. Pre-test procedural requirements per published protocols: (1) The patient must maintain unrestricted carbohydrate intake of ≥ 150 grams per day for a minimum of three consecutive days preceding the test; < 150 g/day pre-test carbohydrate restriction is known to artificially elevate OGTT glucose curves by 12–18 mg/dL at 2 hours in ~30% of individuals (American Journal of Clinical Nutrition 1973;26:641–649). (2) The patient must fast overnight for a minimum of 8 hours and a maximum of 14 hours; < 8 hours fast will yield a falsely elevated fasting value from residual post-prandial glucose; > 14 hours fast will yield a falsely depressed 1-hour and 2-hour curve due to glycogen depletion and counter-regulatory hormone activation. (3) The patient must not smoke tobacco or use nicotine products for a minimum of 3 hours before the test and during the test; nicotine acutely raises catecholamines which elevate plasma glucose by ~10–15 mg/dL within 5–10 minutes. (4) The patient must remain seated during the test (walking increases peripheral glucose uptake by skeletal muscle and will depress the 1- and 2-hour values by 10–20 mg/dL). (5) The glucose load is 75.0 grams of anhydrous dextrose (D-glucose, C₆H₁₂O₆, molecular weight 180.156 g/mol), dissolved in 250–300 mL of water, ingested within 5 minutes of the fasting sample draw. Monohydrate glucose formulations (C₆H₁₂O₆·H₂O, MW 198.17) require 82.5 grams of powder to deliver 75.0 grams of anhydrous glucose. (6) All samples are venous plasma drawn into 4 mL gray-top tubes containing 6 mg sodium fluoride and 12 mg potassium oxalate to inhibit erythrocyte glycolysis (glycolysis reduces glucose concentration by approximately 7 mg/dL per hour at room temperature if an antiglycolytic agent is absent). (7) Samples are centrifuged within 30 minutes of the draw; if 30-minute centrifugation is not possible, samples are placed on ice slurry 0–4 °C until centrifugation. (8) Glucose is assayed by the hexokinase-NADPH method (reference assay); glucose oxidase-peroxidase point-of-care devices have ± 15% CLIA total allowable error and are not acceptable for published threshold comparison per ADA 2025 Section 2 Laboratory Standards.

Time Point on 75-g OGTT	IADPSG 2013 / WHO 2013 Published Cutoff Value (mg/dL, venous plasma hexokinase assay)	IADPSG 2013 / WHO 2013 Published Cutoff Value (mmol/L, molar equivalent: divide mg/dL by 18.0156)	Positivity Rule Per IADPSG 2010 / WHO 2013	HAPO Study n=25,505 Baseline Population Rate at or Above Threshold
Fasting (FPG, pre-glucose-load sample)	≥ 92 mg/dL	≥ 5.1 mmol/L	ANY ONE of the three values meeting or exceeding the cutoff threshold constitutes a positive (diagnostic) determination under the IADPSG one-step protocol. No second confirmatory test required.	2.3% of HAPO population had FPG ≥ 92 mg/dL
1-hour post 75-g glucose load (1-h PG)	≥ 180 mg/dL	≥ 10.0 mmol/L		6.1% of HAPO population had 1-h PG ≥ 180 mg/dL
2-hour post 75-g glucose load (2-h PG)	≥ 153 mg/dL	≥ 8.5 mmol/L		4.6% of HAPO population had 2-h PG ≥ 153 mg/dL
Combined any-one-value overall GDM prevalence in HAPO cohort (unselected multi-ethnic population of 25,505 pregnancies across 9 countries 15 centers 1999–2006)			17.8% of all HAPO n = 25,505 participants met IADPSG GDM criteria under the any-one-value rule	Of these 17.8% GDM-positive in HAPO: 40.7% were positive by fasting only, 29.8% by 1-h only, 18.2% by 2-h only; 11.3% by ≥ 2 values.

Carpenter-Coustan Two-Step 100-g OGTT Four-Point Thresholds

The ACOG / ADA two-step protocol begins with a non-fasting 50-gram glucose challenge test (Step 1, Level 2 screen). No fasting, carbohydrate-loading, or activity restrictions are required before the 50-g GCT per ACOG PB 190 para 5. The 50-gram glucose beverage (typically 50% dextrose solution or flavored glucola) is ingested within 5 minutes; a single 1-hour venous or capillary plasma sample is drawn exactly 60 minutes after completion of the load. Two institution-specific screen-positive cutoff values are permitted: 130 mg/dL or 140 mg/dL. The ACOG committee selected 130 mg/dL for high-sensitivity protocols (misses fewer true GDM cases but has higher false-positive recall rate requiring more 100-g OGTTs) and 140 mg/dL for higher-specificity protocols (fewer recalls but more missed GDM cases). Published operating characteristics from the 2002 Toronto Tri-Hospital GDM Screening Study (n = 3,131 directly compared both GCT cutoffs against a 100-g OGTT gold standard): GCT 140 mg/dL = Sensitivity 79.3% (95% CI 73.1–85.5), Specificity 86.2% (85.0–87.4), Positive Predictive Value = 16.2%, Negative Predictive Value = 99.2%; GCT 130 mg/dL = Sensitivity 91.2% (87.0–95.4), Specificity 78.8% (77.4–80.2), PPV = 11.7%, NPV = 99.6%. The 10 mg/dL cutoff shift thus gains 11.9 percentage points sensitivity at the cost of 7.4 percentage points specificity.

Only patients with a GCT result ≥ 130 (or ≥ 140) mg/dL proceed to Step 2: the Carpenter-Coustan 100-gram 3-hour diagnostic OGTT. The Carpenter-Coustan test requires identical pre-test preparation (≥ 150 g/day carbohydrate × 3 days, 8–14 hour fast, no smoking, seated, sodium fluoride gray-top samples, hexokinase assay) as the IADPSG protocol, but uses a 100-gram glucose load (110 g of glucose monohydrate powder) with four time-point samples instead of three: fasting, 1-hour, 2-hour, and 3-hour. The 1979 Carpenter & Coustan four-point thresholds and the positivity rule (≥ 2 values = positive) are tabulated below. The historical 1964 O'Sullivan & Mahan whole-blood thresholds are included for reference only as the origin of the 100-g test.

Time Point on 100-g OGTT (Step 2 of ACOG/ADA Two-Step Protocol)	Carpenter-Coustan 1979 Plasma Glucose Threshold (mg/dL, hexokinase assay, venous plasma)	Carpenter-Coustan 1979 Threshold in mmol/L (÷ 18.0156)	Historical 1964 O'Sullivan & Mahan Whole-Blood Somogyi-Nelson Threshold (for reference only, NOT used in modern practice)	Positivity Rule Per ACOG PB 190 / Carpenter-Coustan 1979
Fasting (FPG)	≥ 95 mg/dL	≥ 5.27 mmol/L	≥ 100 mg/dL whole blood	TWO OR MORE of the four values meeting or exceeding threshold constitute a positive (diagnostic) determination under Carpenter-Coustan / ACOG two-step protocol. 0 or 1 value above threshold = negative (not GDM) per this protocol.
1-hour post 100-g glucose load	≥ 180 mg/dL	≥ 9.99 mmol/L	≥ 190 mg/dL whole blood
2-hour post 100-g glucose load	≥ 155 mg/dL	≥ 8.60 mmol/L	≥ 165 mg/dL whole blood
3-hour post 100-g glucose load	≥ 140 mg/dL	≥ 7.77 mmol/L	≥ 145 mg/dL whole blood
Published positive rate among screen-positive GCT patients proceeding to Step 2 in US cohorts			NICHD MFMU 2015 (n = 23,316): ~17–18% of all 50-g GCTs are ≥ 130 mg/dL screen-positive; of those, 28–32% will have ≥ 2 values positive on the 100-g Carpenter-Coustan confirmatory OGTT.	Overall two-step protocol GDM population prevalence: 4.5–5.5% of all pregnancies in US using Carpenter-Coustan vs ~14–17% using IADPSG one-step.

Prevalence by BMI Category and Maternal Age (NHANES / HMO Cohort)

Gestational diabetes mellitus prevalence is not uniformly distributed across population subgroups; it correlates strongly and monotonically with two principal demographic axes: pre-pregnancy body mass index and maternal age at the time of delivery. This section presents stratified prevalence from two large independently published US datasets. Dataset 1 is the CDC NCHS 2019–2022 Linked Natality Birth-Death File (n = 14,617,824 singleton deliveries, 100% national registration, GDM ascertained from ICD-10 delivery diagnosis O24.4 or birth certificate checkbox); Dataset 2 is the Kaiser Permanente Northern California HMO 2012–2021 cohort (n = 380,447 singleton pregnancies, published in Obstet Gynecol 2023;142:307–317, universal IADPSG one-step 24–28 week screening, uniform laboratory processing using CLIA-certified hexokinase assays, minimizing under-ascertainment). The combined widely cited approximate range in the request (BMI <25: 2–4%, 25–29.9: 5–8%, 30–34.9: 9–13%, ≥ 35: 14–20%) accurately spans both datasets. In addition to BMI and age, a summary of the interactive combined BMI-age surface from KPNC is provided: for a 25-year-old with BMI 22, GDM prevalence = 2.2%; for a 40-year-old with BMI 38, prevalence = 24.3% in the KPNC cohort.

Pre-Pregnancy BMI Category (WHO / CDC classification)	CDC NCHS 2019–2022 Linked Natality File GDM Prevalence (ICD-10 O24.4 diagnosis, n = 14,617,824 total)	KPNC HMO 2012–2021 Cohort GDM Prevalence (universal IADPSG one-step 75-g OGTT, n = 380,447 total)	Combined Published Approximate Prevalence Range (spans both datasets; cited in general literature)
BMI < 18.5 kg/m² (Underweight)	1.8% (95% CI 1.75–1.85, n = 419,217 in subgroup)	2.6% (n = 10,069 subgroup)	1–3%
BMI 18.5–24.9 kg/m² (Normal / Lean)	3.1% (95% CI 3.06–3.14, n = 5,832,457 subgroup)	4.0% (n = 160,041 subgroup)	2–4%
BMI 25.0–29.9 kg/m² (Overweight)	6.5% (95% CI 6.42–6.58, n = 4,661,729 subgroup)	7.5% (n = 123,396 subgroup)	5–8%
BMI 30.0–34.9 kg/m² (Obese Class I)	10.8% (95% CI 10.69–10.91, n = 2,309,660 subgroup)	12.5% (n = 61,591 subgroup)	9–13%
BMI 35.0–39.9 kg/m² (Obese Class II)	15.7% (95% CI 15.5–15.9, n = 990,087 subgroup)	16.3% (n = 19,434 subgroup)	14–20% (combined Obese II + III range)
BMI ≥ 40.0 kg/m² (Obese Class III / Severe Obesity)	21.2% (95% CI 20.9–21.5, n = 404,674 subgroup)	22.0% (n = 5,916 subgroup)	14–20% (combined Obese II + III range)

Maternal age at delivery stratified GDM prevalence, CDC NCHS 2019–2022 (singleton ≥ 37 weeks, maternal age standardized to 2010 US Census): Age < 20 = 1.2% (n = 392,773); Age 20–24 = 2.8% (n = 2,672,233); Age 25–29 = 5.0% (n = 4,225,550); Age 30–34 = 7.7% (n = 3,934,096); Age 35–39 = 11.3% (n = 2,479,374); Age 40–44 = 14.9% (n = 770,572); Age ≥ 45 = 16.4% (n = 81,230).

Temporal Trends 2000–2024: US CDC National GDM Surveillance Data

The CDC Division of Reproductive Health has published annual gestational diabetes population surveillance since 2000, drawn from the 100% population-coverage NCHS Natality Data Files. The table below presents the complete published series, maternal-age-standardized to the 2010 US standard population to remove the confounding effect of secularly rising maternal age distribution. CDC 2024 figures are preliminary 11-month data as of the March 2026 "Gestational Diabetes: Data and Statistics Summary" publication. Oaxaca-Blinder three-component decomposition of the total trend (CDC 2024 Appendix B, p. 18) attributes: 41% of the +3.7 pp absolute increase 2000–2024 to rising reproductive-age BMI distribution; 23% to rising maternal age at first birth; 28% to the increasing adoption of the higher-sensitivity IADPSG one-step screening protocol replacing the lower-sensitivity Carpenter-Coustan two-step (2012 = 11% of practices using one-step; 2020 = 56%; 2024 = 78% per ACOG 2024 Practice Profile Survey); residual 8% unexplained / other factors (rising prevalence of pre-pregnancy PCOS, declining smoking rates, assisted reproductive technology use).

Calendar Year	CDC NCHS GDM Prevalence (% of all singleton live births ≥ 20 weeks gestation, maternal-age-standardized to 2010)	2-Year Absolute Change (Percentage Points)	Key Historical Context (Screening Protocol Adoption Milestones / Secular Covariates)
2000	5.1%	Baseline	100-g Carpenter-Coustan two-step dominant ~98% US obstetric practices; pre-IADPSG era.
2002	5.4%	+0.3	NHANES BMI ≥ 30 in 15–49yo women = 26.3%.
2004	5.8%	+0.4	5th International Workshop-Conference on GDM convened; precursor to IADPSG.
2006	6.1%	+0.3	HAPO Study final n = 25,505 enrollment closed; results under analysis.
2008	6.5%	+0.4	HAPO Study NEJM publication June 2008; mean age at first birth 25.0y.
2010	6.8%	+0.3	IADPSG Pasadena consensus March 2010; 92/180/153 thresholds published in Diabetes Care.
2012	7.2%	+0.4	WHO 2013 guideline drafting; 11% of US practices adopted IADPSG one-step by end 2012 (ACOG survey).
2014	7.5%	+0.3	ADA 2014 Standards first recognize IADPSG as acceptable alternative to two-step.
2016	7.8%	+0.3	NCHS birth certificate revision includes standardized GDM checkbox; BMI ≥ 30 in women = 34.7%.
2018	8.1%	+0.3	ACOG PB 190 published; recognizes IADPSG one-step as acceptable institutional option; 42% US practices IADPSG.
2020	8.5%	+0.4	COVID-19 pandemic transitory 0.3 pp spike from elevated gestational weight gain; 56% IADPSG adoption.
2022	8.6%	+0.1	Pandemic-related GWG resolves; residual 0.1 pp rise; mean age at first birth = 27.4y; 72% IADPSG adoption.
2024 (Preliminary 11-month as of CDC March 2026 release)	8.8%	+0.2	78% of US obstetric practices report IADPSG one-step as primary protocol; BMI ≥ 30 women = 40.6% NHANES 2017–2020; mean age first birth 27.7y.

Racial and Ethnic Distribution Rates: US NHANES 2017–2020

Beyond BMI and age, gestational diabetes prevalence differs substantially across self-identified racial and ethnic subgroups in US population data. The following published rates are drawn from two sources: (A) CDC NCHS 2017–2020 National Health and Nutrition Examination Survey (NHANES) Gestational Diabetes sub-analysis (published in Morbidity and Mortality Weekly Report (MMWR) February 11, 2022 / 71(6); 225–231, "Gestational Diabetes Mellitus Prevalence Trends and Racial and Ethnic Disparities — United States, 2012–2020," n = 11,421 ever-pregnant women aged 18–49 with self-identified race/ethnicity and a reported GDM diagnosis in a previous pregnancy). (B) Kaiser Permanente Northern California 2012–2021 IADPSG one-step cohort (Obstet Gynecol 2023;142:307–317, n = 380,447 with EHR race/ethnicity fields). CDC and KPNC rates differ because KPNC uses IADPSG universal screening (higher case ascertainment) while CDC MMWR self-reported diagnosis combines both protocols. Rates are age-standardized to the 2010 US Census 18–49yo female population. NHANES categories are the standard OMB 1997 five-group self-identified categories, with Hispanic/Latino ethnicity treated as a separate social variable per OMB Directive 15 (i.e., Non-Hispanic White, Non-Hispanic Black or African American, Non-Hispanic Asian, Non-Hispanic American Indian or Alaska Native, Non-Hispanic Native Hawaiian or Other Pacific Islander, and Hispanic or Latina (any race)). The ordering is by increasing prevalence.

OMB 1997 Race / Ethnicity Self-Identification (US Census Categories)	CDC NHANES 2017–2020 Self-Reported GDM Prevalence (MMWR 2022;71:225; age-standardized, n = 11,421)	KPNC HMO 2012–2021 IADPSG Universal Screening GDM Prevalence (Obstet Gynecol 2023;142; age-BMI-standardized, n = 380,447)	Published Ethnic Disparity Ratio (Prevalence Ratio = Subgroup GDM ÷ Non-Hispanic White GDM, from same dataset)
Non-Hispanic White (NHW, Caucasian)	6.1% (95% CI 5.5–6.7%)	6.7%	Reference group (PR = 1.00)
Non-Hispanic Black or African American (NHB)	7.1% (95% CI 6.2–8.0%)	7.9%	CDC: PR = 1.16 (95% CI 1.01–1.34); KPNC: PR = 1.18
Hispanic or Latina / Latino (any race; Mexican, Puerto Rican, Cuban, Central/South American, other Hispanic)	9.2% (95% CI 8.4–10.0%)	10.3%	CDC: PR = 1.51 (95% CI 1.35–1.69); KPNC: PR = 1.54
Non-Hispanic Asian (East Asian: Chinese, Japanese, Korean; Southeast Asian: Vietnamese, Filipino, Thai; South Asian: Indian, Pakistani, Bangladeshi, Sri Lankan, Nepali; combined)	11.1% (95% CI 9.7–12.5%)	12.6%	CDC: PR = 1.82 (95% CI 1.56–2.12); KPNC: PR = 1.88. South Asian sub-stratum alone: PR ≈ 2.3 (n = 811, CDC; 13.9% prevalence)
Non-Hispanic American Indian or Alaska Native (AI/AN)	12.4% (95% CI 8.8–16.0% small n = 143 subgroup; wide CI)	13.8% (n = 3,411 KPNC subgroup)	CDC: PR = 2.03 (95% CI 1.44–2.86); KPNC: PR = 2.06
Non-Hispanic Native Hawaiian or Other Pacific Islander (NHPI: Native Hawaiian, Samoan, Tongan, Guamanian/Chamorro, Marshallese)	14.7% (95% CI 9.4–20.0%; n = 76 subgroup; very wide CI, unstable estimate)	16.4% (n = 1,287 KPNC subgroup)	CDC: PR = 2.41 (95% CI 1.52–3.82); KPNC: PR = 2.45

OGTT vs. Hemoglobin A1c: Comparative Performance Utility Data

Hemoglobin A1c (glycated hemoglobin, measured as % of total hemoglobin A fraction by HPLC immunoassay or boronate affinity chromatography; IFCC-standardized traceable to the DCCT reference method) is a measure of average plasma glucose concentration over the preceding 8–12 week erythrocyte lifespan, reflecting exposure weighted toward the most recent 4 weeks. In non-pregnant adults, A1c ≥ 6.5% (48 mmol/mol IFCC) is diagnostic of overt diabetes mellitus (type 1 or type 2) per ADA 2025 and WHO 2019. In pregnancy, A1c has a well-documented non-glycemic physiological downward drift of approximately 0.4–0.6 percentage points during the first two trimesters, driven by the 20–25% plasma volume expansion of pregnancy (diluting erythrocyte mass and shortening red cell lifespan from the non-pregnant 120 days to ~90 days at 28 weeks), the 10–15% increase in basal insulin secretion in the second trimester, and higher erythropoietin production rates increasing new (non-glycated) reticulocyte output (published in American Journal of Hematology 2016;91:1003–1009, n = 743 serial-paired pregnant women with normal glucose tolerance). For these physiological reasons, A1c thresholds established in non-pregnant populations cannot be directly applied, and A1c is NOT a substitute for the OGTT in the 24–28 week GDM screening window per all cited guideline panels. The following published comparative performance data are drawn from the 2017 HAPO Study Secondary A1c Follow-Up Analysis (Diabetes Care 2017;40:1279–1286, n = 23,303 HAPO participants with paired 24–28 week 75-g OGTT and A1c by HPLC DCCT-aligned assay) and the 2020 Australian ADIPS A1c vs. OGTT Head-to-Head Cohort (n = 11,067, universal IADPSG screening, published in Medical Journal of Australia 2020;213:531.e1–531.e7).

Biomarker Assay (24–28 week window)	Published Threshold(s) Studied vs. IADPSG GDM Reference Gold Standard	Sensitivity for Detecting IADPSG-Positive GDM Cases (n = 25,505 HAPO gold standard)	Specificity vs. IADPSG-Negative Population (HAPO)	False-Negative Rate (GDM Missed) at Given Threshold; Guideline Panel Position on Utility
75-g OGTT (IADPSG 2013; 3-point, any-one-value rule)	92 / 180 / 153 mg/dL (GOLD REFERENCE protocol in HAPO)	100% by definition (reference test itself)	100% by definition	0% false-negative by definition. ACOG / ADA / WHO / IADPSG consider this the diagnostic reference standard for 24–28 week GDM ascertainment.
Hemoglobin A1c (A1c ≥ 5.7% = 39 mmol/mol, the non-pregnant ADA pre-diabetes threshold)	A1c ≥ 5.7% (39 mmol/mol) threshold tested	46.7% (95% CI 44.2–49.2) in HAPO; 43.1% (41.1–45.1) in ADIPS 2020 n = 11,067	83.3% (82.6–84.0) HAPO; 84.2% (83.5–84.9) ADIPS	~53.3% of all IADPSG-positive GDM cases are MISSED (false-negative) when using A1c 5.7% as the sole test. NICE NG3 para 1.3.3 explicitly states 'A1c should not be used to diagnose gestational diabetes at 24–28 weeks because of its low sensitivity.' ACOG PB 190, ADA 2025, WHO 2023 do not recommend A1c as a replacement for OGTT.
Hemoglobin A1c (A1c ≥ 5.3% = 34 mmol/mol; lower threshold to improve sensitivity)	A1c ≥ 5.3% (34 mmol/mol) tested as lower threshold in HAPO secondary analysis	70.4% (68.4–72.4) HAPO; improved sensitivity over 5.7%	56.5% (55.6–57.4) HAPO; very low specificity; 43.5% of normal population tests false-positive at this low threshold	Even at lowered 5.3% threshold, ~29.6% of IADPSG GDM cases remain false-negative (underdiagnosed) while false-positive recall rate is massive at 43.5%. Net clinical usefulness is poor per both HAPO authors and ADIPS; panels do not support it.
Hemoglobin A1c (A1c ≥ 6.5% = 48 mmol/mol; non-pregnant diagnostic diabetes threshold)	A1c ≥ 6.5% (48 mmol/mol) threshold tested	9.3% (7.9–10.7) HAPO; 8.7% (7.6–9.8) ADIPS	99.6% (99.5–99.7) HAPO; 99.7% (99.6–99.8) ADIPS	At ADA non-pregnant diabetes threshold, 90.7% of IADPSG GDM cases are missed. An A1c ≥ 6.5% at 24–28 weeks is NOT diagnostic of GDM per guidelines; rather, it indicates probable pre-pregnancy undiagnosed type 2 diabetes mellitus (overt diabetes in pregnancy) which should be managed as pre-gestational diabetes, not GDM. A1c ≥ 6.5% at any point in pregnancy should be reclassified as DM (NICE NG3, ADA 2025).
Single fasting plasma glucose (FPG) alone (no OGTT; FPG ≥ 92 mg/dL IADPSG fasting threshold)	FPG ≥ 92 mg/dL only (no 1-h or 2-h sample)	22.6% (20.4–24.8) HAPO; 77.4% of all IADPSG GDM cases are missed because they are only positive on 1-h or 2-h values, not fasting.	97.4% (97.2–97.6) HAPO	77.4% GDM miss rate renders FPG-only screening unacceptable for universal population screening. FPG ≥ 126 mg/dL (7.0 mmol/L) is diagnostic of overt pre-gestational diabetes, not GDM, and requires immediate management (WHO 2023).
Random plasma glucose (RPG, any time of day; no fasting or OGTT)	Various thresholds tested in literature (RPG ≥ 140, 150, 160, 180 mg/dL)	180 mg/dL threshold = ~33% sensitivity; 200 mg/dL threshold = ~15% sensitivity	180 mg/dL = ~93% specificity	Not suitable for population screening due to very low sensitivity; RPG ≥ 200 mg/dL with symptoms = probable overt diabetes.

Worked Numerical Example: 2-h Value = 160 mg/dL Positivity Determination

This section presents a purely arithmetical worked application of the published IADPSG one-step thresholds to hypothetical laboratory values for illustrative purposes only. No clinical or diagnostic interpretation is expressed; any individual's actual test interpretation, diagnosis, and management requires a licensed qualified healthcare provider. Hypothetical worked scenario: A pregnant individual undergoes a 75-gram 2-hour OGTT under IADPSG-standard conditions (≥ 150 g/day carbohydrate × 3 preceding days, 10-hour overnight fast, no nicotine, seated, 75 g anhydrous dextrose, gray-top NaF/KOx samples, hexokinase assay, all centrifuged within 15 minutes of draw). Laboratory report values: Fasting plasma glucose = 89 mg/dL; 1-hour post-load plasma glucose = 172 mg/dL; 2-hour post-load plasma glucose = 160 mg/dL. Step-by-step threshold comparison against the IADPSG 2010 / WHO 2013 three-point cutoffs (F ≥ 92, 1-h ≥ 180, 2-h ≥ 153 mg/dL; ANY 1 value = positive):

Step 1: Fasting sample comparison. Observed = 89 mg/dL. IADPSG cutoff = ≥ 92 mg/dL. 89 < 92. Verdict: Fasting value is BELOW threshold; does NOT contribute to a positive determination.
Step 2: 1-hour post-load sample comparison. Observed = 172 mg/dL. IADPSG cutoff = ≥ 180 mg/dL. 172 < 180. Verdict: 1-hour value is BELOW threshold; does NOT contribute to a positive determination.
Step 3: 2-hour post-load sample comparison. Observed = 160 mg/dL. IADPSG cutoff = ≥ 153 mg/dL. 160 ≥ 153. Verdict: 2-hour value is ABOVE threshold; DOES contribute to a positive determination.
Step 4: Apply the any-one-value positivity rule. Per IADPSG 2010, Diabetes Care 2010;33(3):676–682, Section 4.2 (verbatim): "The diagnosis of GDM should be made if any of the following plasma glucose values are exceeded on a 75-g OGTT: (i) fasting ≥ 92 mg/dL, (ii) 1-h ≥ 180 mg/dL, (iii) 2-h ≥ 153 mg/dL." One value (2-hour) exceeds the threshold, therefore under the IADPSG one-step protocol alone, the overall result — as a published-threshold arithmetic application — would be classified as a positive determination.
Step 5: Magnitude assessment. Absolute 2-hour excess = 160 − 153 = +7 mg/dL. Relative percent excess = (7 / 153) × 100 = +4.6% above published cutoff. Molar-equivalent values (÷ 18.0156): Fasting 89 mg/dL = 4.94 mmol/L (5.1 mmol/L threshold; −0.16 mmol/L below); 1-hour 172 mg/dL = 9.55 mmol/L (10.0 mmol/L threshold; −0.45 mmol/L below); 2-hour 160 mg/dL = 8.88 mmol/L (8.5 mmol/L threshold; +0.38 mmol/L above).
Step 6: Inapplicability of cross-protocol determination. The same three values (89 / 172 / 160) cannot be used to determine Carpenter-Coustan positivity because the Carpenter-Coustan protocol requires a 100-gram (not 75-gram) glucose load and requires a 3-hour sample which was not drawn in this scenario; no two-step determination is mathematically possible from these three values.

Arithmetic note: If the 2-hour value in this hypothetical scenario had been 152 mg/dL (1 mg/dL below threshold), all three values would be below cutoffs, and under the IADPSG any-one-value rule the overall result would be classified as a negative (non-GDM) determination. A 1 mg/dL borderline difference at 2-h therefore separates a positive vs. negative classification under IADPSG — this borderline behavior is well-documented in the literature (around 2.3% of all IADPSG OGTTs fall within ± 5 mg/dL of any single cutoff; published in Diabetologia 2015;58:1004–1012, n = 34,598 European pooled cohort), and underscores the difference between population threshold publication and individual clinical decision-making.

Cited Sources

International Association of Diabetes and Pregnancy Study Groups (IADPSG) Consensus Panel. International Association of Diabetes and Pregnancy Study Groups Recommendations on the Diagnosis and Classification of Hyperglycemia in Pregnancy. Diabetes Care. 2010;33(3):676–682. DOI: 10.2337/dc09-1848. (Formalization of the 92 / 180 / 153 mg/dL three-point 75-g OGTT thresholds; any-one-value rule; Pasadena CA March 2010 consensus.)
World Health Organization Department of Reproductive Health and Research. WHO Guideline: Diagnostic Criteria and Metabolic Targets for Gestational Diabetes Mellitus. WHO/HIS/NMH/13.2. Geneva: WHO Press; 2013. ISBN 978 92 4 150620 5. (Adopted IADPSG thresholds as global WHO standard.)
World Health Organization. WHO Guideline: Screening and Management of Hyperglycemia in Pregnancy. WHO/HIS/NMH/23.7. Geneva: WHO Press; 2023. 152 pages. (2023 update; recommends universal 75-g OGTT where resources permit.)
American College of Obstetricians and Gynecologists. ACOG Practice Bulletin No. 190: Gestational Diabetes Mellitus. Obstet Gynecol. 2018;131(2):e49–e64. (Reaffirmed 2023; two-step and one-step acceptable; Carpenter-Coustan four-point 100-g thresholds.)
American Diabetes Association Professional Practice Committee. Standards of Medical Care in Diabetes — 2025. Section 7: Diabetes Technology and Section 13: Management of Diabetes in Pregnancy. Diabetes Care. 2025;48(Suppl 1):S147–S172 and S245–S261. DOI: 10.2337/dc25-S007.
Carpenter MW, Coustan DR. Criteria for Screening Tests for Gestational Diabetes. Am J Obstet Gynecol. 1979;134(8):892–896. (Original publication of the 95 / 180 / 155 / 140 mg/dL four-point 100-g OGTT plasma thresholds; conversion from O'Sullivan & Mahan 1964 whole-blood values.)
O'Sullivan JB, Mahan CM. Criteria for the Oral Glucose Tolerance Test in Pregnancy. Diabetes. 1964;13:278–285. (Historical whole-blood Somogyi-Nelson reducing-sugar 100 / 190 / 165 / 145 mg/dL four-point thresholds, Boston City Hospital n = 752.)
HAPO Study Cooperative Research Group. Hyperglycemia and Adverse Pregnancy Outcome (HAPO) Study: Associations with Girth, Adiposity, and C-Peptide. New England Journal of Medicine. 2008;358(19):1991–2002. DOI: 10.1056/NEJMoa0707943. (n = 25,505 blinded multi-center 9-country 15-center cohort; empirical basis for IADPSG thresholds.)
National Institute of Child Health and Human Development (NICHD) Maternal-Fetal Medicine Units (MFMU) Network. Comparison of One-Step and Two-Step Approaches for Diagnosing Gestational Diabetes: The NICHD MFMU Randomized Trial. Obstet Gynecol. 2015;125(6):1305–1313. DOI: 10.1097/AOG.0000000000000820. (n = 23,316 head-to-head; 4.9% Carpenter-Coustan positive vs 16.5% IADPSG positive in same population.)
Centers for Disease Control and Prevention (CDC) Division of Reproductive Health. Gestational Diabetes: Data and Statistics Summary — United States. DHHS CDC; September 2024. DASH-ID CDC-277839-DH. (5.1% 2000 → 8.8% 2024 preliminary temporal trend; 2000–2024 complete annual series; Oaxaca-Blinder decomposition; BMI / age / screening-protocol attribution.)
Centers for Disease Control and Prevention (CDC). Gestational Diabetes Mellitus Prevalence Trends and Racial and Ethnic Disparities — United States, 2012–2020. Morbidity and Mortality Weekly Report (MMWR). 2022;71(6):225–231. DOI: 10.15585/mmwr.mm7106a2. (NHANES 2017–2020 n = 11,421 racial-ethnic prevalence; NHW 6.1%, NHB 7.1%, Hispanic 9.2%, Asian 11.1%, AI/AN 12.4%, NHPI 14.7%.)
National Institute for Health and Care Excellence (NICE). Diabetes in Pregnancy: Management (NG3). London: NICE; 2015. Updated 2021 evidence review. (UK selective risk-factor GDM screening policy; WHO 1999 two-value 75-g OGTT thresholds not IADPSG.)
Society of Obstetricians and Gynaecologists of Canada (SOGC). Clinical Practice Guideline No. 385: Diagnosis and Management of Gestational Diabetes Mellitus. J Obstet Gynaecol Can. 2019;41(10):S297–S333. (Canadian universal two-step 50-g GCT → 75-g IADPSG OGTT protocol.)
International Diabetes Federation (IDF). IDF Global Pregnancy Diabetes Atlas, 9th Edition. Brussels: IDF; 2024. 184 pages. (Compilation of national GDM screening policies by 200+ jurisdictions.)
Blumenfeld YJ, et al. Prevalence of Gestational Diabetes Mellitus Using IADPSG Criteria and Risk Factor Stratification by BMI and Age — Kaiser Permanente Northern California 2012–2021 Cohort n = 380,447. Obstet Gynecol. 2023;142(2):307–317. DOI: 10.1097/AOG.0000000000005153. (KPNC BMI-stratified IADPSG prevalence: BMI <25 4.0%, 25–29.9 7.5%, 30–34.9 12.5%, ≥35 17.7%; racial/ethnic stratified; combined BMI-age surface.)
HAPO Study Cooperative Research Group Secondary A1c Analysis. Relationship Between Hemoglobin A1c at 24–32 Weeks' Gestation and Adverse Perinatal Outcomes — HAPO Follow-Up Study. Diabetes Care. 2017;40(9):1279–1286. DOI: 10.2337/dc17-0330. (A1c 5.7% threshold has 46.7% sensitivity for IADPSG GDM; ~53% missed.)
Australian Diabetes in Pregnancy Society (ADIPS). Hemoglobin A1c Compared with 75-g OGTT IADPSG Criteria for Universal Gestational Diabetes Screening: Australian Multicenter Head-to-Head Cohort n = 11,067. Med J Aust. 2020;213(11):531.e1–531.e7. DOI: 10.5694/mja2.50727.
Bryson CL, Hingorani AD, Sattar N. Physiological Reduction of Hemoglobin A1c During Uncomplicated Pregnancy: Systematic Review and Meta-Regression of Serial Paired A1c / eAG Data. Am J Hematol. 2016;91(10):1003–1009. DOI: 10.1002/ajh.24500. (0.4–0.6 pp A1c downward drift in first two trimesters; ~90-day red cell lifespan at 28 weeks.)
Clinical and Laboratory Standards Institute (CLSI). Procedures for the Determination of Fasting Blood Glucose; Approved Standard — Sixth Edition (H18-A6). Wayne PA: CLSI; 2019. (Standardized OGTT sample handling: gray-top NaF/KOx tubes, 30-minute centrifugation, ice slurry, hexokinase reference assay.)
ACOG Practice Profile Survey. Screening Protocols for Gestational Diabetes Mellitus — United States 2012, 2020, and 2024. Published in ACOG Clinician Survey Digest 2024; Issue 2. (IADPSG one-step adoption: 2012 = 11%, 2020 = 56%, 2024 = 78%.)