Gestational Diabetes: Risk Factors and Screening

What Is Gestational Diabetes?

Gestational diabetes mellitus (GDM) is defined as glucose intolerance that is first identified during pregnancy. Unlike type 1 or type 2 diabetes, GDM is specifically a pregnancy complication — it arises because the placenta produces hormones (including human placental lactogen) that cause increasing insulin resistance as pregnancy progresses. When the pancreas cannot compensate by producing enough extra insulin, blood glucose levels rise above normal.

GDM typically develops in the second trimester (around weeks 24–28) when insulin resistance peaks, and it usually resolves within 6 weeks after delivery. However, it has significant implications during pregnancy and for long-term health.

Risk Factors for Gestational Diabetes

ACOG identifies several evidence-based risk factors that increase the likelihood of developing GDM. No single factor is definitively predictive — risk is cumulative across multiple factors:

High-Risk Factors

• Pre-pregnancy BMI ≥30: Obesity is one of the strongest risk factors, associated with 2–3× increased GDM risk
• Previous gestational diabetes: Women with prior GDM have a 33–50% recurrence risk in subsequent pregnancies
• Prediabetes or fasting glucose 100–125 mg/dL: Indicates pre-existing insulin dysregulation
• Family history of type 2 diabetes: First-degree relative with T2D increases risk substantially
• Age ≥35: Risk increases with maternal age

Moderate-Risk Factors

• Ethnicity: Higher prevalence in South Asian, East Asian, Hispanic, Black, and Indigenous populations
• PCOS: Polycystic ovary syndrome is associated with insulin resistance
• Previous macrosomic baby: Baby over 9 lbs (4,000g) in a prior pregnancy
• Previous unexplained stillbirth: May indicate undetected GDM in prior pregnancy
• Excessive gestational weight gain: Particularly in the first half of pregnancy

ACOG Screening Guidelines

ACOG recommends a two-tier approach to GDM screening:

Universal Screening at 24–28 Weeks

All pregnant women without known diabetes should be screened for GDM between 24 and 28 weeks of gestation. The standard screen is the 1-hour 50g glucose challenge test (GCT). This is a non-fasting test — you drink a 50g glucose solution and have blood drawn 1 hour later.

• A result below 130–140 mg/dL (threshold varies by provider) is considered negative — no GDM workup needed
• A result at or above the threshold triggers the diagnostic 3-hour OGTT

Early Screening for High-Risk Women

Women with high-risk factors (prior GDM, BMI ≥30, strong family history, or fasting hyperglycemia) are screened at the first prenatal visit — sometimes as early as 6–10 weeks. If the early screen is negative, they repeat universal screening at 24–28 weeks.

The Glucose Tests Explained

Step 1: 1-Hour 50g Glucose Challenge Test (GCT)

This screening test does not diagnose GDM — it identifies women who need further testing. You do not need to fast. You drink a standardized 50g glucose solution, wait one hour, then have a blood draw. A positive result (≥130–140 mg/dL) means you need the diagnostic test. Approximately 15–23% of women test positive on the GCT.

Step 2: 3-Hour 100g Oral Glucose Tolerance Test (OGTT)

This is the diagnostic test. You must fast for 8–14 hours before the test. Blood is drawn at fasting, then 1, 2, and 3 hours after drinking 100g of glucose. GDM is diagnosed if 2 or more values meet or exceed:

• Fasting: ≥95 mg/dL
• 1-hour: ≥180 mg/dL
• 2-hour: ≥155 mg/dL
• 3-hour: ≥140 mg/dL

Some providers use the one-step approach: a 2-hour 75g OGTT with fasting required. ACOG endorses either approach.

Managing Gestational Diabetes

A GDM diagnosis is not a crisis — it is a manageable condition that most women successfully control with monitoring and lifestyle adjustments.

Medical Nutrition Therapy (MNT)

The cornerstone of GDM management is a structured meal plan developed with a registered dietitian. Key principles:

• Distribute carbohydrates evenly across 3 meals and 2–3 snacks
• Limit simple/refined carbohydrates; choose complex carbs with fiber
• Keep breakfast carbohydrates lower (insulin resistance is highest in the morning)
• Pair carbohydrates with protein and fat to blunt glucose spikes

Blood Glucose Monitoring

Women with GDM typically check blood glucose 4× daily: fasting (before breakfast) and 1–2 hours after each meal. Target ranges are:

• Fasting: <95 mg/dL
• 1-hour postmeal: <140 mg/dL
• 2-hour postmeal: <120 mg/dL

Medication When Needed

If diet and glucose monitoring show consistently elevated values after 1–2 weeks, your provider may start medication. Insulin is the first-line pharmacological option because it does not cross the placenta. Metformin is also used and has a good safety profile, though some studies suggest a small percentage may need supplemental insulin.

Long-Term Implications of GDM

GDM does not just affect the current pregnancy. Women with GDM have:

• 50% lifetime risk of type 2 diabetes — ACOG recommends a 75g OGTT at 6–12 weeks postpartum to screen for persistent diabetes or prediabetes
• Increased risk of metabolic syndrome and cardiovascular disease
• 33–50% recurrence risk of GDM in subsequent pregnancies

For babies born to mothers with GDM, there is increased risk of macrosomia, neonatal hypoglycemia, and a higher lifetime risk of obesity and type 2 diabetes.

Tools to Assess Your GDM Risk