What is the recommended approach to Continuous Blood Glucose (CBG) monitoring in pregnancy?

Continuous Glucose Monitoring in Pregnancy

Primary Recommendation

Continuous glucose monitoring should be used as an adjunct to—not a replacement for—self-monitoring of blood glucose in pregnant women with type 1 diabetes, where it demonstrably reduces macrosomia, neonatal hypoglycemia, and hospital length of stay. 1

Evidence-Based Approach by Diabetes Type

Type 1 Diabetes: Strong Recommendation for CGM

• CGM is strongly recommended for all pregnant women with type 1 diabetes based on the CONCEPTT trial, which showed mild A1C improvement without increased hypoglycemia, plus reductions in large-for-gestational-age births, neonatal hypoglycemia, and hospital length of stay. 1

• CGM must be used in addition to traditional self-monitoring of blood glucose (SMBG) targeting fasting <95 mg/dL and 1-hour postprandial <140 mg/dL—CGM metrics alone cannot replace these specific pre- and postprandial targets. 1

• The international consensus endorses specific CGM targets for type 1 diabetes in pregnancy: target range 63-140 mg/dL with >70% time in range (TIR), <4% time below 63 mg/dL, and <1% time below 54 mg/dL. 1

Type 2 Diabetes and Gestational Diabetes: Limited Evidence

• For type 2 diabetes and gestational diabetes, CGM may be used as an adjunct tool, but there is insufficient evidence to support TIR metrics or to replace SMBG. 1, 2

• These patients should rely primarily on SMBG at least 4 times daily (fasting and 1-hour postprandial after each meal for GDM on diet alone) or 6 times daily (before and after each meal for insulin-treated patients). 3

• CGM can identify patterns of hyperglycemia that predict need for pharmacological treatment—specifically, time above range and hyperglycemia before breakfast, after breakfast, before dinner, and overnight correlate with treatment needs. 4

Critical Implementation Details

CGM as Adjunct, Not Substitute

• The most common pitfall is using CGM metrics instead of SMBG for insulin dosing decisions. CGM should guide overall patterns, but insulin dose adjustments must be based on SMBG values targeting specific fasting and postprandial goals. 1

• CGM-reported mean glucose is superior to estimated A1C or glucose management indicator calculations, which should never be used in pregnancy due to altered red blood cell kinetics. 1

Monitoring Frequency Requirements

• Pregnant women with diabetes require fasting and postprandial SMBG regardless of CGM use—preprandial testing is essential for insulin pump or basal-bolus therapy to adjust rapid-acting insulin doses. 1

• Postprandial monitoring (not just CGM TIR) is specifically associated with better glycemic control and lower preeclampsia risk. 1

Specific Glycemic Targets

• Fasting: 70-95 mg/dL (preexisting diabetes) or <95 mg/dL (GDM) 1
• 1-hour postprandial: 110-140 mg/dL (preexisting diabetes) or <140 mg/dL (GDM) 1
• 2-hour postprandial: 100-120 mg/dL (preexisting diabetes) or <120 mg/dL (GDM) 1

Practical CGM Interpretation

Key Metrics to Monitor

• Lower mean glucose, lower standard deviation, and higher percentage of time in target range (63-140 mg/dL) are associated with lower risk of large-for-gestational-age births and adverse neonatal outcomes. 1

• Time in hyperglycemia after lunch specifically correlates with macrosomia and large-for-gestational-age infants—this pattern warrants intensified postprandial insulin coverage. 4

• Every additional percentage point of time above range increases the probability of needing pharmacological treatment by 24%. 4

Technical Considerations

• CGM systems require at least 12 hours of wear before data becomes reliable, and calibration should show <15% difference between CGM and meter glucose values. 1

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

• Avoid MRI, conventional X-ray, and CT scanning while wearing CGM devices. 1

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. 1

• 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. 1, 5

• Fear of hypoglycemia leading to inadequate control: Early pregnancy has enhanced insulin sensitivity with increased hypoglycemia risk, but this reverses by 16 weeks—adjust targets accordingly rather than accepting hyperglycemia. 1, 5

• Ignoring carbohydrate consistency: CGM data becomes uninterpretable without consistent carbohydrate intake matched to insulin doses—referral to a registered dietitian is essential. 5, 6