What are the guidelines for CBG (Continuous Blood Glucose) monitoring in pregnant patients?

CBG Monitoring in Pregnant Patients

All pregnant patients with preexisting type 1 or type 2 diabetes should perform fasting and postprandial self-monitoring of blood glucose (SMBG) as the primary method of glycemic control, with continuous glucose monitoring (CGM) strongly recommended as an adjunctive tool for those with type 1 diabetes. 1, 2

Primary Monitoring Approach: Self-Monitoring of Blood Glucose

Fasting and postprandial SMBG is mandatory for all pregnant patients with diabetes and serves as the foundation for achieving glycemic targets, regardless of whether CGM is used. 1

Specific Blood Glucose Targets

• Fasting glucose: 70-95 mg/dL (3.9-5.3 mmol/L) 1, 3
• 1-hour postprandial: 110-140 mg/dL (6.1-7.8 mmol/L) 1, 3
• 2-hour postprandial: 100-120 mg/dL (5.6-6.7 mmol/L) 1, 3

Testing Frequency and Timing

• Preprandial testing is essential for patients using insulin pumps or basal-bolus therapy to adjust rapid-acting insulin doses before meals. 1, 2
• Postprandial monitoring is associated with better glycemic control and lower risk of preeclampsia compared to preprandial monitoring alone. 1, 3
• Testing should occur at multiple time points throughout the day, preferably during periods when blood glucose is relatively stable (before meals and bedtime). 1

Continuous Glucose Monitoring: Evidence-Based Recommendations

Type 1 Diabetes (Strongest Indication)

CGM is strongly recommended for all pregnant patients with type 1 diabetes based on the CONCEPTT trial, which demonstrated improved maternal and neonatal outcomes. 1, 2

Benefits demonstrated in type 1 diabetes:

• Mild improvement in A1C without increased hypoglycemia 1, 2
• Reduction in large-for-gestational-age births 1, 2
• Decreased neonatal hypoglycemia 1, 2
• Shorter neonatal hospital length of stay 1, 2

Type 2 Diabetes and Gestational Diabetes

There is insufficient evidence to support routine CGM use in type 2 diabetes or gestational diabetes. 1 The CONCEPTT trial specifically enrolled only type 1 diabetes patients, and CGM time in range does not provide actionable data for addressing fasting versus postprandial hyperglycemia in these populations. 1

However, Chinese clinical guidelines suggest CGM may be considered for gestational diabetes patients and women with diabetes during pregnancy as part of diabetes education and treatment guidance. 1

CGM Metrics and Targets (Type 1 Diabetes Only)

When CGM is used in pregnant patients with type 1 diabetes, the following targets apply:

• Target glucose range: 63-140 mg/dL (3.5-7.8 mmol/L) 1, 2
• Time in range (TIR) goal: >70% 1, 2
• Time below range (<63 mg/dL): <4% 1, 2
• Time below range (<54 mg/dL): <1% 1, 2
• Time above range (>140 mg/dL): <25% 1

Lower mean glucose, lower standard deviation, and higher percentage of time in target range are associated with lower risk of large-for-gestational-age births and other adverse neonatal outcomes. 1, 2

Critical Implementation Considerations

CGM Does Not Replace SMBG

CGM metrics cannot substitute for self-monitoring of blood glucose to achieve optimal pre- and postprandial glycemic targets. 1, 2 Pregnant patients must continue fasting and postprandial SMBG even when using CGM, as these specific measurements are required for insulin dose adjustments. 2

CGM Calibration Requirements

• Blood glucose concentrations for calibration should be obtained using the same glucose meter and same batch of test strips. 1
• Calibration values should be entered immediately after the result is displayed on the meter. 1
• CGM systems require at least 12 hours of wear before data becomes reliable. 2
• Calibration should show <15% difference between CGM and meter glucose values. 2

Recording and Documentation

During CGM monitoring, diet, exercise, medication, and other glucose-related events should be recorded in detail to facilitate interpretation of glucose patterns. 1

A1C Monitoring as Secondary Measure

A1C should be used as a secondary measure of glycemic control in pregnancy, not the primary metric, because it may not fully capture postprandial hyperglycemia which drives macrosomia. 1, 3, 2

A1C Targets and Monitoring Frequency

• Optimal A1C target: <6% (42 mmol/mol) if achievable without significant hypoglycemia 1, 3
• Acceptable target if hypoglycemia risk: <7% (53 mmol/mol) 1, 3
• Monitoring frequency: Monthly during pregnancy due to altered red blood cell kinetics 1, 3, 2

A1C levels fall physiologically during normal pregnancy due to increased red blood cell turnover, making the normal pregnancy A1C range naturally lower than in non-pregnant individuals. 1, 3

Common Pitfalls to Avoid

Over-reliance on A1C

A1C falls physiologically in normal pregnancy and doesn't capture postprandial hyperglycemia that drives macrosomia—use A1C only as a secondary measure after SMBG. 2

Insufficient Insulin Dose Escalation

Insulin resistance increases 5% weekly from week 16-36, typically doubling total daily insulin requirements—CGM patterns showing rising mean glucose demand proactive dose increases. 3, 2

CGM Use in Inappropriate Settings

CGM is not suitable for intensive care units due to skin edema, vasoconstrictors, hypotension, hypoxemia, and high-dose acetaminophen affecting accuracy. 1, 2

Special Populations

Gestational Diabetes and Women with Diabetes During Pregnancy

CGM is listed as applicable for gestational diabetes patients and women with diabetes during pregnancy, particularly for diabetes education to facilitate understanding of glucose changes resulting from diet, exercise, stress, sleep, and treatment. 1 However, the strongest evidence supports its use primarily in type 1 diabetes. 1

Patients with Dramatic Glycemic Variability

CGM is recommended for diabetes patients experiencing dramatic glycemic variability, including those with gastroparesis or special types of diabetes. 1