# Gestational Diabetes: Screening, Diagnosis, and Management

## Introduction

Gestational diabetes mellitus (GDM) is defined as glucose intolerance first recognized during pregnancy. It affects approximately 6 to 9% of pregnancies in the United States and is associated with significant maternal and neonatal complications if not adequately managed. Early detection and treatment reduce adverse outcomes.

## Risk Factors

Risk factors for GDM include obesity (BMI of 30 kg/m2 or greater), age 35 or older, family history of type 2 diabetes in a first-degree relative, history of GDM in a prior pregnancy (with a recurrence rate of 30 to 70%), prior delivery of a macrosomic infant (birth weight greater than 4000 g), polycystic ovary syndrome (PCOS), and race/ethnicity including Hispanic, African American, Native American, South Asian, and Pacific Islander populations. A history of prediabetes or A1c of 5.7% or greater also increases risk.

## Screening Strategies

### Two-Step Approach (ACOG Preferred)

The first step involves a 50 g oral glucose challenge test (GCT) at 24 to 28 weeks, performed in the non-fasting state. A positive screen is defined as a 1-hour glucose of 130 or 140 mg/dL or greater, with the threshold varying by institution. Patients with a positive screen proceed to step 2, a 100 g oral glucose tolerance test (OGTT) performed fasting, with glucose measured at fasting, 1 hour, 2 hours, and 3 hours. Diagnosis requires two or more values meeting or exceeding the Carpenter-Coustan thresholds: fasting 95 mg/dL or greater, 1 hour 180 mg/dL or greater, 2 hours 155 mg/dL or greater, and 3 hours 140 mg/dL or greater.

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

The one-step approach uses a 75 g OGTT at 24 to 28 weeks, performed fasting. Diagnosis requires one or more values meeting or exceeding the following thresholds: fasting 92 mg/dL or greater, 1 hour 180 mg/dL or greater, or 2 hours 153 mg/dL or greater.

| Approach | Glucose Load | Diagnostic Thresholds | Criteria for Diagnosis |
|----------|-------------|----------------------|----------------------|
| Two-Step (ACOG) - Step 1 | 50 g GCT (non-fasting) | 1-hr ≥130-140 mg/dL | Positive screen → proceed to Step 2 |
| Two-Step (ACOG) - Step 2 | 100 g OGTT (fasting) | Fasting ≥95, 1-hr ≥180, 2-hr ≥155, 3-hr ≥140 | ≥2 abnormal values |
| One-Step (IADPSG) | 75 g OGTT (fasting) | Fasting ≥92, 1-hr ≥180, 2-hr ≥153 | ≥1 abnormal value  |  This approach diagnoses more women with GDM, though the clinical benefit of treating milder cases remains debated. |

### Early Screening

High-risk patients should be screened at the first prenatal visit using standard diagnostic criteria for pre-existing diabetes. If initial screening is negative, rescreening occurs at 24 to 28 weeks. Pre-existing diabetes is diagnosed by a fasting glucose of 126 mg/dL or greater, an A1c of 6.5% or greater, or a random glucose of 200 mg/dL or greater with symptoms.

![Flowchart comparing the one-step and two-step GDM screening approaches with diagnostic thresholds](images/gdm-screening-flowchart.jpg)

## Complications of GDM

### Maternal

Maternal complications include a 2- to 4-fold increased risk of preeclampsia, higher rates of cesarean delivery due to macrosomia, polyhydramnios, and a significantly increased lifetime risk of developing type 2 diabetes, with approximately 50% progressing within 5 to 10 years.

### Fetal and Neonatal

Macrosomia (birth weight greater than 4000 g) increases the risk of shoulder dystocia and birth injury. Neonatal complications include hypoglycemia, hyperbilirubinemia, and respiratory distress syndrome. Stillbirth risk increases primarily with uncontrolled GDM. Long-term offspring risks include childhood obesity, metabolic syndrome, and type 2 diabetes.

## Management

### Medical Nutrition Therapy

Medical nutrition therapy is the first-line treatment for all patients with GDM. Referral to a registered dietitian for individualized meal planning is recommended. A carbohydrate-controlled diet with a minimum of 175 g of carbohydrates per day, distributed across 3 meals and 2 to 3 snacks, is the foundation. Complex carbohydrates, fiber, and lean protein are emphasized while simple sugars and concentrated sweets are avoided. Caloric goals are based on pre-pregnancy BMI.

### Blood Glucose Monitoring

Self-monitoring of blood glucose (SMBG) is performed four times daily: fasting and 1 or 2 hours after each meal. ACOG targets are fasting glucose of 95 mg/dL or less, 1-hour postprandial of 140 mg/dL or less, and 2-hour postprandial of 120 mg/dL or less. If targets are not consistently met with diet and exercise within 1 to 2 weeks, pharmacotherapy is initiated.

### Exercise

Thirty minutes of moderate-intensity exercise most days of the week, such as walking, swimming, or prenatal yoga, is recommended. Exercise improves insulin sensitivity and may reduce the need for pharmacotherapy. Supine exercises should be avoided after 20 weeks, and modifications should be made based on obstetric complications.

### Pharmacotherapy

Insulin is the preferred pharmacologic agent because it does not cross the placenta. Common regimens include basal insulin (NPH) and prandial rapid-acting analog insulin, adjusted based on glucose patterns. Metformin crosses the placenta and is used as an alternative when insulin is refused or impractical, though long-term offspring effects remain under study. Glyburide also crosses the placenta and is associated with higher rates of neonatal hypoglycemia, causing it to fall out of favor.

![Table showing blood glucose targets for GDM management with corresponding insulin adjustment strategies](images/gdm-glucose-targets.jpg)

## Antepartum Surveillance

Fetal surveillance begins at 32 to 36 weeks depending on disease severity and glycemic control. Non-stress testing once or twice weekly is standard for patients on pharmacotherapy. For diet-controlled GDM with good glycemic control, surveillance timing is institution-dependent, with some beginning at 36 to 40 weeks. Serial growth ultrasounds every 4 weeks monitor for macrosomia or growth restriction. Regarding delivery timing, patients with diet-controlled GDM may await spontaneous labor up to 40 to 41 weeks, while those on medication are typically delivered by 39 weeks.

## Intrapartum and Postpartum Care

### Intrapartum

Glucose is monitored every 1 to 2 hours during labor with a target of 70 to 110 mg/dL. An insulin drip may be needed for patients on high doses of insulin. All diabetes medications are discontinued after delivery.

### Postpartum

Most patients with GDM return to normoglycemia after delivery. A 75 g OGTT at 4 to 12 weeks postpartum screens for persistent diabetes or prediabetes. Lifetime screening for type 2 diabetes every 1 to 3 years is recommended thereafter. Counseling addresses weight management, exercise, and breastfeeding, which improves maternal glucose metabolism. Contraception is discussed, as GDM is not a contraindication to any method.

![Postpartum follow-up algorithm for women with gestational diabetes](images/gdm-postpartum-followup.jpg)

## Key Clinical Pearls

GDM is a strong predictor of future type 2 diabetes, making postpartum OGTT and lifelong screening essential steps that are frequently overlooked. Medical nutrition therapy is first-line, and most patients (70 to 85%) achieve glycemic targets with diet and exercise alone. Insulin remains the gold standard pharmacotherapy, with metformin serving as an acceptable alternative, though long-term offspring data are limited. Patients should be counseled that breastfeeding, weight management, and regular exercise significantly reduce their risk of progression to type 2 diabetes.

## References

1. ACOG Practice Bulletin No. 190: Gestational Diabetes Mellitus. *Obstet Gynecol*. 2018;131(2):e49-e64.
2. HAPO Study Cooperative Research Group. Hyperglycemia and adverse pregnancy outcomes. *N Engl J Med*. 2008;358(19):1991-2002.
3. Landon MB, et al. A multicenter randomized trial of treatment for mild gestational diabetes. *N Engl J Med*. 2009;361(14):1339-1348.
4. ADA Standards of Medical Care in Diabetes: Management of Diabetes in Pregnancy. *Diabetes Care*. 2024;47(Suppl 1):S282-S294.