ObGyn Intelligence — Pregnancy

GDM Risk & Glucose Guide

Gestational diabetes risk assessment, screening timing, and glucose test interpretation using ACOG two-step and ADA/IADPSG one-step criteria.

Evidence Base

American College of Obstetricians and Gynecologists. Gestational Diabetes Mellitus. ACOG Practice Bulletin No. 190. Obstet Gynecol. 2018;131(2):e49–e64. — Primary guideline for GDM screening, diagnosis, and management.
Carpenter MW, Coustan DR. Criteria for screening tests for gestational diabetes. Am J Obstet Gynecol. 1982;144(7):768–773. — Defines the Carpenter-Coustan thresholds used in the two-step OGTT.
International Association of Diabetes and Pregnancy Study Groups 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. — Defines the IADPSG one-step 75g OGTT criteria.
National Diabetes Data Group. Classification and diagnosis of diabetes mellitus and other categories of glucose intolerance. Diabetes. 1979;28(12):1039–1057. — Defines the alternative NDDG thresholds for the two-step approach.

Risk Assessment

Two-Step Interpreter

One-Step Interpreter

Reference

Two-Step Screening Interpreter

Enter glucose values from the standard two-step approach: 50 g GCT (Step 1), then 100 g 3-hour OGTT (Step 2) if Step 1 is positive.

Step 1 — 50 g Glucose Challenge Test (GCT)

Non-fasting. 1-hour plasma glucose after 50 g oral glucose load.

1-Hour Glucose (mg/dL)

mg/dL — 1 hour after 50 g glucose

Cutoff Used at Your Center

Threshold varies by institution (ACOG accepts 130–140)

Step 2 — 100 g 3-Hour OGTT

Fasting state required (8–14 hours). GDM diagnosed if 2 or more values meet or exceed the threshold. Two criteria sets are accepted by ACOG.

Fasting (mg/dL)

1-Hour (mg/dL)

2-Hour (mg/dL)

3-Hour (mg/dL)

One-Step 75 g 2-Hour OGTT

IADPSG/ADA criteria (endorsed by WHO 2013). Fasting state required. GDM diagnosed if any one value meets or exceeds the threshold. Detects more GDM but has not been shown to improve clinical outcomes vs. two-step (ACOG PB 190).

Fasting (mg/dL)

1-Hour (mg/dL)

2-Hour (mg/dL)

Screening Thresholds Reference

50 g GCT — Step 1 (Two-Step Approach)

Cutoff	Sensitivity for GDM	Positive Rate	ACOG Recommendation
≥130 mg/dL	~99%	~20–25%	Acceptable
≥135 mg/dL	~96%	~15–18%	Acceptable
≥140 mg/dL	~80–90%	~14–18%	Most widely used

100 g 3-Hour OGTT — Diagnostic Thresholds

Timepoint	Carpenter-Coustan	NDDG
Fasting	≥95 mg/dL	≥105 mg/dL
1-Hour	≥180 mg/dL	≥190 mg/dL
2-Hour	≥155 mg/dL	≥165 mg/dL
3-Hour	≥140 mg/dL	≥145 mg/dL

Diagnosis: GDM is diagnosed when 2 or more values meet or exceed the threshold. Some experts diagnose GDM with 1 abnormal value (especially elevated fasting), but ACOG requires 2 or more.

75 g 2-Hour OGTT (One-Step / IADPSG)

Timepoint	IADPSG Threshold (mg/dL)
Fasting	≥92 mg/dL
1-Hour	≥180 mg/dL
2-Hour	≥153 mg/dL

IADPSG diagnosis: Any single value meeting or exceeding threshold is sufficient for GDM diagnosis — more sensitive than two-step.

Screening Timing

Population	Timing	Rationale
Average-risk	24–28 weeks	Standard universal screening (ACOG PB 190)
High-risk (see Tab 1)	First prenatal visit	Early detection; screen again at 24–28 wks if first screen negative
Overt diabetes suspected	First visit (HbA1c, fasting glucose)	Values meeting diabetes thresholds = pre-existing DM, not GDM