# Gestational Diabetes: Screening, Management, and Delivery Planning

## Definition and Epidemiology

Gestational diabetes mellitus (GDM) is defined as glucose intolerance that is first recognized during pregnancy. It complicates approximately 6 to 9% of pregnancies in the United States, with prevalence varying by population and the diagnostic criteria used. The incidence is rising in parallel with the obesity epidemic. Risk factors include obesity, advanced maternal age, family history of diabetes mellitus, prior GDM, polycystic ovary syndrome, a history of a macrosomic infant, and membership in certain higher-risk ethnic groups (Hispanic, African American, South Asian, and Pacific Islander populations).

## Screening Strategies

### Two-Step Approach (ACOG Preferred)

The two-step approach is the screening method preferred by ACOG. The first step is a 50 g glucose challenge test (GCT), which does not require fasting. A venous glucose level is measured one hour after ingestion. The threshold for a positive screen varies by institution -- 130 mg/dL (higher sensitivity), 135 mg/dL, or 140 mg/dL. If the result is 200 mg/dL or above, some institutions diagnose GDM directly without proceeding to a confirmatory test. Patients who screen positive undergo the second step: a 100 g oral glucose tolerance test (OGTT) performed in the fasting state with glucose measurements at fasting, 1 hour, 2 hours, and 3 hours. Using the Carpenter-Coustan criteria (the most commonly applied), the diagnostic thresholds are fasting 95 mg/dL or above, 1-hour 180 mg/dL or above, 2-hour 155 mg/dL or above, and 3-hour 140 mg/dL or above. Two or more abnormal values are required for the diagnosis. The older National Diabetes Data Group (NDDG) criteria use slightly higher thresholds. Screening is performed at 24 to 28 weeks' gestation for average-risk patients.

| Two-Step Screening | Timepoint | Carpenter-Coustan Threshold |
|---|---|---|
| 50 g GCT (non-fasting) | 1 hour | ≥130, 135, or 140 mg/dL (varies by institution) |
| 100 g OGTT (fasting) | Fasting | ≥95 mg/dL |
| 100 g OGTT | 1 hour | ≥180 mg/dL |
| 100 g OGTT | 2 hours | ≥155 mg/dL |
| 100 g OGTT | 3 hours | ≥140 mg/dL |
| **Diagnosis** | — | **2 or more abnormal values required** |

### One-Step Approach (IADPSG/WHO)

The one-step approach uses a 75 g OGTT in the fasting state with glucose measurements at fasting, 1 hour, and 2 hours. Diagnostic thresholds are fasting 92 mg/dL or above, 1-hour 180 mg/dL or above, and 2-hour 153 mg/dL or above, with only one abnormal value required for diagnosis. This method identifies approximately twice as many women as the two-step approach. It is endorsed by the ADA, IADPSG, and WHO but is not currently recommended by ACOG as the primary strategy due to concerns about overdiagnosis, cost, and unclear net benefit.

| One-Step Screening (75 g OGTT) | Timepoint | IADPSG Threshold |
|---|---|---|
| 75 g OGTT (fasting) | Fasting | ≥92 mg/dL |
| 75 g OGTT | 1 hour | ≥180 mg/dL |
| 75 g OGTT | 2 hours | ≥153 mg/dL |
| **Diagnosis** | — | **1 abnormal value sufficient** |

### Early Screening

Women with risk factors for pregestational diabetes should be screened at their first prenatal visit. If early screening is normal, repeat screening at 24 to 28 weeks is still indicated. A GDM diagnosis before 20 weeks may actually represent undiagnosed pregestational diabetes.

<image>Side-by-side comparison diagram of the one-step (75g OGTT) versus two-step (50g GCT followed by 100g OGTT) screening approaches for gestational diabetes, showing diagnostic thresholds, timing, and number of abnormal values required for diagnosis</image>

## Glycemic Targets and Monitoring

### Blood Glucose Goals (ACOG/ADA)

The glycemic targets for GDM are fasting glucose below 95 mg/dL (some experts advocate below 90 mg/dL), 1-hour postprandial below 140 mg/dL, and 2-hour postprandial below 120 mg/dL. Patients check their blood glucose four times daily: fasting and after each of the three main meals.

### Monitoring

Daily glucose logs should be reviewed regularly. Continuous glucose monitoring (CGM) is increasingly being used in GDM management and may improve glycemic control, though it is not yet standard of care. Hemoglobin A1C reflects average glucose over 2 to 3 months and should be checked at diagnosis and periodically thereafter, with a target below 6% in GDM.

## Medical Nutrition Therapy and Lifestyle

### Dietary Counseling

Medical nutrition therapy is the foundation of GDM management and achieves adequate glycemic control in 70 to 85% of patients without pharmacotherapy. The diet should be carbohydrate-controlled, with a minimum of 175 g of carbohydrates per day distributed across 3 meals and 2 to 3 snacks. Complex carbohydrates are preferred over simple sugars. Caloric recommendations are individualized by BMI: 30 kcal/kg/day for normal weight, 25 kcal/kg/day for overweight patients, and 12 to 15 kcal/kg/day for obese patients (with a minimum of 1,500 to 1,800 kcal/day to prevent ketosis). Referral to a registered dietitian is recommended.

### Exercise

Regular exercise complements dietary management. Thirty minutes of moderate-intensity exercise on most days is recommended. Walking after meals is a particularly effective strategy for improving postprandial glucose values. Safe exercise options during pregnancy include swimming, stationary cycling, and prenatal yoga.

## Pharmacologic Therapy

### Insulin (First-Line per ACOG)

Insulin is the first-line pharmacologic agent when diet and exercise fail to achieve glycemic targets, typically after 1 to 2 weeks of persistently elevated values. Basal insulin with NPH at bedtime is used for fasting hyperglycemia, while rapid-acting insulin (lispro or aspart) before meals addresses postprandial hyperglycemia. Starting doses are 0.7 to 1.0 units/kg/day, divided approximately equally between basal and prandial components. Insulin requirements typically increase throughout pregnancy, especially in the third trimester. Insulin does not cross the placenta.

### Oral Agents

Metformin is a reasonable alternative when insulin is refused or impractical. It is started at 500 mg daily and titrated to 1,000 to 2,500 mg daily. However, metformin crosses the placenta, and emerging long-term data suggest possible increased adiposity in exposed offspring during childhood. Up to 40% of patients started on metformin ultimately require supplemental insulin. Glyburide was previously widely used but is now less favored after evidence showed inferior outcomes compared to insulin, including higher rates of macrosomia, neonatal hypoglycemia, and treatment failure. ACOG does not recommend glyburide as a first-line agent.

## Fetal Surveillance

### Growth Monitoring

Serial growth ultrasound should be performed every 4 weeks starting at 28 to 32 weeks to assess for macrosomia (estimated fetal weight above the 90th percentile) or growth restriction. The abdominal circumference is the most sensitive ultrasound parameter for detecting excessive fetal growth.

### Antenatal Testing

The intensity of fetal surveillance depends on the treatment regimen and glycemic control. For well-controlled GDM managed with diet alone, antenatal testing may be deferred, though some institutions begin nonstress testing or biophysical profile at 36 to 40 weeks. For GDM requiring medication or with poor glycemic control, nonstress testing or biophysical profile should begin at 32 weeks and be performed twice weekly. Polyhydramnios on ultrasound may indicate poor glycemic control.

<image>Ultrasound image demonstrating fetal macrosomia in a gestational diabetic pregnancy, showing a large abdominal circumference measurement with labeled soft tissue and subcutaneous fat thickness</image>

## Delivery Planning

### Timing of Delivery

Delivery timing in GDM is stratified by disease severity. For well-controlled, diet-managed GDM, delivery is recommended at 39 weeks 0 days to 40 weeks 6 days, with induction at 39 to 40 weeks being reasonable. For GDM requiring medication with good glycemic control, delivery is recommended at 39 weeks 0 days to 39 weeks 6 days. For poorly controlled GDM, delivery may be warranted as early as 37 weeks 0 days to 38 weeks 6 days, individualized to the clinical situation. No patient with GDM should go beyond 41 weeks.

| GDM Category | Recommended Delivery Timing |
|---|---|
| Diet-controlled, well-managed | 39+0 to 40+6 weeks |
| Medication-controlled, good glycemic control | 39+0 to 39+6 weeks |
| Poorly controlled | 37+0 to 38+6 weeks (individualized) |
| Maximum gestational age | Do not exceed 41 weeks |

### Mode of Delivery

Vaginal delivery is preferred. An elective cesarean delivery should be offered when the estimated fetal weight is 4,500 g or greater (some guidelines use 4,000 g for diabetic patients because of the higher risk of shoulder dystocia at a given fetal weight). The risk of shoulder dystocia should be discussed with patients carrying macrosomic fetuses.

### Intrapartum Glucose Management

During labor, the target glucose range is 70 to 110 mg/dL. Most GDM patients, especially those managed with diet alone, do not require insulin during labor. Blood glucose should be checked every 1 to 2 hours. An insulin drip is rarely needed for GDM (it is more commonly required in pregestational diabetes).

## Postpartum Management

### Immediate Postpartum

Insulin and oral hypoglycemic agents are discontinued after delivery. A fasting glucose should be checked before discharge. In most cases, GDM resolves immediately once the placenta is delivered and the source of insulin-antagonizing hormones is removed.

### Postpartum Glucose Screening

ACOG and ADA recommend a 75 g OGTT at 4 to 12 weeks postpartum to determine whether the patient has diabetes, prediabetes, or normal glucose tolerance. Unfortunately, only about 50% of eligible women complete postpartum screening. If results are normal, rescreening should occur every 1 to 3 years, as the lifetime risk of developing type 2 diabetes remains elevated.

### Long-Term Risk

Women with GDM face a 50% lifetime risk of developing type 2 diabetes, with the risk increasing with obesity and additional pregnancies complicated by GDM. The Diabetes Prevention Program (DPP) trial showed that lifestyle modification reduces the progression from prediabetes to diabetes by 58%. The recurrence rate of GDM in future pregnancies is 30 to 50%. Offspring of mothers with GDM are at increased risk for childhood obesity and metabolic syndrome.

## Clinical Pearls

Diet and exercise achieve glycemic control in the majority of GDM patients. Pharmacotherapy is reserved for the minority who cannot meet glycemic targets with lifestyle modification alone.

ACOG recommends insulin over metformin or glyburide as first-line pharmacotherapy for GDM when medication is needed.

A single abnormal value on the 3-hour OGTT does not meet diagnostic criteria for GDM, but these patients are at increased risk for adverse outcomes and may benefit from dietary counseling and glucose monitoring.

Postpartum glucose screening is critical but frequently missed. Building it into discharge planning and postpartum visit scheduling improves completion rates.

GDM is a cardiovascular risk factor that extends well beyond the pregnancy itself. Patients should be counseled about the need for long-term metabolic monitoring.

Fasting hyperglycemia is the most difficult glucose value to control in GDM and often requires the addition of bedtime NPH insulin.

The HAPO study demonstrated a continuous relationship between maternal glucose levels and adverse pregnancy outcomes with no clear threshold, supporting the importance of glycemic optimization even when values are near diagnostic cutoffs.

## References

- ACOG Practice Bulletin No. 190: Gestational Diabetes Mellitus (2018, reaffirmed 2023)
- ADA Standards of Care in Diabetes -- 2024: Management of Diabetes in Pregnancy
- Landon MB et al. A multicenter, randomized trial of treatment for mild gestational diabetes. N Engl J Med. 2009;361:1339-1348
- HAPO Study Cooperative Research Group. Hyperglycemia and adverse pregnancy outcomes. N Engl J Med. 2008;358:1991-2002
- Balsells M et al. Glibenclamide, metformin, and insulin for GDM: systematic review and meta-analysis. BMJ. 2015;350:h102
- Crowther CA et al. Effect of treatment of gestational diabetes mellitus on pregnancy outcomes (ACHOIS). N Engl J Med. 2005;352:2477-2486