In a pregnant woman with diabetes taking metformin 500 mg twice daily who has an evening blood glucose of 200 mg/dL, what is the appropriate next management?

Management of Inadequate Evening Glycemic Control in Pregnancy

Add insulin immediately to achieve target glucose levels, as metformin alone is insufficient for your current glycemic control. 1, 2

Why Insulin is Required Now

Insulin is the preferred and recommended first-line medication for treating hyperglycemia in pregnancy because it does not cross the placenta to a measurable extent and has the most extensive safety record. 1 Your evening glucose of 200 mg/dL is significantly above the target of <140 mg/dL at 1 hour postprandial or <120 mg/dL at 2 hours postprandial. 1, 2

Metformin should not be used as a first-line agent because it crosses the placenta, achieving fetal cord concentrations equal to or higher than maternal levels, and lacks long-term offspring safety data. 1 More importantly, approximately 25-28% of women fail to achieve glycemic targets with metformin monotherapy, necessitating supplemental insulin. 3, 4

Specific Insulin Regimen

Initial Dosing Strategy

Start with rapid-acting insulin (lispro or aspart) before dinner to address your elevated evening glucose. 2, 5 Calculate your total daily insulin dose as 0.5 units/kg of current body weight, with approximately 40% allocated to basal insulin and 60% distributed as prandial insulin across three meals. 2, 3

For your evening hyperglycemia specifically, begin with 4-6 units of rapid-acting insulin before dinner, then titrate upward by 2-4 units every 2-3 days until your 1-hour postprandial glucose consistently falls below 140 mg/dL or your 2-hour postprandial glucose falls below 120 mg/dL. 2

Basal Insulin Consideration

If your fasting glucose is also elevated (≥95 mg/dL), add NPH insulin or insulin detemir as basal coverage. 2, 5 Start with 10-20% of your total daily insulin dose given at bedtime, then increase by 2-4 units every 2-3 days until fasting glucose is 70-95 mg/dL. 2

Critical Monitoring Requirements

Perform self-monitoring of blood glucose 4-6 times daily: fasting upon waking, and either 1 hour or 2 hours after each main meal (breakfast, lunch, dinner). 2, 3 Choose either 1-hour or 2-hour postprandial measurements consistently. 3

Your glycemic targets are:

• Fasting: 70-95 mg/dL 1, 2
• 1-hour postprandial: 110-140 mg/dL 1, 2
• 2-hour postprandial: 100-120 mg/dL 1, 2

Check A1C monthly with a target <6% if achievable without significant hypoglycemia, or <7% if hypoglycemia risk is high. 2 However, A1C is a secondary metric only—it may miss postprandial hyperglycemia that drives fetal macrosomia, so daily glucose monitoring cannot be replaced by A1C. 1, 2

Insulin Dose Adjustments During Pregnancy

Insulin resistance rises markedly beginning around week 16-17, with requirements increasing approximately 5% per week through week 36. 2, 3 Your total daily insulin dose may double or triple by late pregnancy, requiring weekly or bi-weekly dose escalations. 2

A sudden unexplained drop in insulin requirements may indicate placental insufficiency and warrants immediate obstetric evaluation. 2, 5

What to Do With Metformin

Continue metformin 500 mg twice daily while adding insulin, as combination therapy may provide additional benefit. 6 However, discontinue metformin immediately if you develop hypertension, preeclampsia, or any sign of placental insufficiency, as it may worsen fetal growth restriction or cause metabolic acidosis. 3

Safety Education

Receive comprehensive hypoglycemia education before starting insulin, including recognition (shakiness, sweating, confusion) and treatment (15-20g fast-acting carbohydrate such as glucose tablets or 4-6 oz juice). 2, 5 Pregnancy attenuates counter-regulatory hormone responses, reducing awareness of hypoglycemia, especially in the first trimester. 2, 5

Monitor for diabetic ketoacidosis at lower glucose thresholds (<200-250 mg/dL) during pregnancy, as the ketogenic metabolic environment increases risk. 2, 5

Specialized Care Referral

Referral to a specialized diabetes-and-pregnancy center with a multidisciplinary team (maternal-fetal medicine, endocrinology, diabetes education, nutrition) is strongly recommended to manage the complex insulin adjustments required throughout pregnancy. 2, 3

Common Pitfalls to Avoid

Do not delay insulin initiation hoping metformin will eventually work—your glucose of 200 mg/dL indicates metformin failure and requires immediate escalation. 1, 3

Do not reduce monitoring frequency even when control improves, as insulin requirements change rapidly, especially after week 16. 2, 3

Do not rely solely on fasting glucose—postprandial values are the primary drivers of fetal macrosomia and must be controlled. 1, 2