What are the blood sugar targets and management of insulin in gestational diabetes mellitus (GDM)?

Blood Sugar Targets and Insulin Management in Gestational Diabetes Mellitus

For women with gestational diabetes mellitus, maintain fasting glucose <95 mg/dL (5.3 mmol/L) and either 1-hour postprandial <140 mg/dL (7.8 mmol/L) or 2-hour postprandial <120 mg/dL (6.7 mmol/L), and initiate insulin as first-line pharmacological therapy when lifestyle modifications fail to achieve these targets. 1, 2

Glycemic Targets for GDM

The American Diabetes Association and American College of Obstetricians and Gynecologists recommend identical blood glucose targets for gestational diabetes: 1, 2

• Fasting glucose: <95 mg/dL (5.3 mmol/L) 1, 2
• 1-hour postprandial: <140 mg/dL (7.8 mmol/L) OR 1, 2
• 2-hour postprandial: <120 mg/dL (6.7 mmol/L) 1, 2

You should monitor either 1-hour OR 2-hour postprandial values—not necessarily both—as both approaches are equally acceptable. 2 Postprandial monitoring is associated with better glycemic control and lower risk of preeclampsia compared to preprandial monitoring alone. 1

A1C Targets

While blood glucose monitoring remains the primary assessment tool, A1C targets in pregnancy are: 1, 2

• Ideally <6% (42 mmol/mol) if achievable without significant hypoglycemia 1, 2
• May be relaxed to <7% (53 mmol/mol) if necessary to prevent hypoglycemia 1, 2

A1C is slightly lower in normal pregnancy due to increased red blood cell turnover, making it a secondary measure after blood glucose monitoring. 1, 2

Continuous Glucose Monitoring

For women using CGM, the targets are more stringent: 2

• Target sensor glucose range: 63-140 mg/dL (3.5-7.8 mmol/L) 2
• Time in range: >70% 2
• Time below range (<63 mg/dL): <4% 2
• Time below range (<54 mg/dL): <1% 2

Insulin Management in GDM

When to Initiate Insulin

Insulin is the preferred first-line medication for GDM when lifestyle modifications (medical nutrition therapy and physical activity) fail to achieve glycemic targets. 1, 2 This is a critical distinction: insulin does not cross the placenta to a measurable extent, while oral agents like metformin and glyburide do cross the placenta and lack long-term safety data. 1

Approximately 70-85% of women diagnosed with GDM using Carpenter-Coustan or NDDG criteria can control their condition with lifestyle modification alone, meaning 15-30% will require insulin. 1, 2 This proportion is expected to be even higher (more women controlled with lifestyle alone) when using the lower IADPSG diagnostic thresholds. 1

Practical Insulin Initiation Strategy

When blood glucose values consistently exceed targets despite lifestyle modifications, initiate insulin therapy. 3 The fasting glucose value on the diagnostic OGTT is the strongest predictor of insulin need—women with fasting values ≥105 mg/dL have an 81.1% likelihood of requiring insulin, compared to 54.0% for those with fasting values ≥95 mg/dL. 4

Start with basal insulin if fasting glucose is elevated (typically NPH or long-acting insulin analogs), and add prandial rapid-acting insulin if postprandial values remain elevated despite basal insulin optimization. 3 Insulin requirements should be evaluated every 2-3 weeks as pregnancy progresses, with insulin resistance increasing exponentially during the second and early third trimesters. 1, 5

Monitoring Requirements

Women on insulin therapy require: 1, 5

• Pre- and postprandial blood glucose monitoring 4-6 times daily 5
• Fasting urine ketone testing to identify those severely restricting carbohydrates to control blood glucose 1
• Comprehensive hypoglycemia education for patients and family members about prevention, recognition, and treatment 5

Critical Pitfalls to Avoid

Do not use metformin or glyburide as first-line agents. 1 While metformin is associated with lower risk of neonatal hypoglycemia and less maternal weight gain than insulin, it readily crosses the placenta and lacks long-term safety data in offspring. 1 The evidence is clear that insulin remains the gold standard for fetal safety. 1

Do not rely solely on self-monitoring of blood glucose (SMBG) to assess control. Research using continuous glucose monitoring systems reveals that SMBG-based therapy, while effective at achieving mean glucose targets, misses long asymptomatic periods of both hyperglycemia and hypoglycemia in women with GDM. 6 This underscores the importance of frequent monitoring and potentially incorporating CGM when available. 2

Lifestyle Management Foundation

Before and alongside insulin therapy, all women with GDM require: 1

• Medical nutrition therapy with a registered dietitian familiar with GDM management 1
• Minimum 175 g carbohydrate daily (35% of a 2,000-calorie diet), emphasizing nutrient-dense, complex carbohydrates 1
• Minimum 71 g protein and 28 g fiber daily 1
• Moderate-intensity physical activity if not contraindicated 7
• Consistent carbohydrate intake matched to insulin dosing to avoid hyperglycemia or hypoglycemia 1

The food plan should emphasize monounsaturated and polyunsaturated fats while limiting saturated fats and avoiding trans fats. 1 Severely restricting carbohydrates to control glucose can enhance lipolysis, promote elevated free fatty acids, and worsen maternal insulin resistance—check fasting urine ketones to identify this problem. 1