Managing Gestational Diabetes

Managing Gestational Diabetes

Guides GDM screening using Carpenter-Coustan criteria, structured glucose monitoring, medical nutrition therapy, pharmacologic management, and delivery timing per ACOG Practice Bulletin No. 190.

Why This Skill Exists

Gestational diabetes mellitus (GDM) complicates 6–9% of pregnancies in the United States and is associated with macrosomia, shoulder dystocia, neonatal hypoglycemia, operative delivery, and long-term maternal risk of type 2 diabetes. ACOG Practice Bulletin No. 190 (Gestational Diabetes Mellitus) recommends universal screening at 24–28 weeks using the two-step approach (1-hour GCT followed by 3-hour GTT if abnormal), with earlier screening for patients with risk factors.

The Carpenter-Coustan criteria define diagnostic thresholds for the 3-hour 100 g GTT and are the standard in US practice. Proper glucose monitoring, dietary counseling, timely initiation of pharmacotherapy, and evidence-based delivery timing directly reduce perinatal morbidity. This skill structures every phase of GDM management from screening through postpartum follow-up.

Checkpoint A: Pre-Draft Intake (Mandatory)

1. GDM risk factors — BMI ≥ 25 (≥ 23 in Asian Americans), prior GDM, prior macrosomic infant (≥ 4000 g), first-degree relative with DM, PCOS, A1c ≥ 5.7%? (Default: from chart review)
2. Screening results — 1-hour GCT value? If abnormal (≥ 135 or ≥ 140 per institutional threshold), 3-hour GTT values? (Default: from lab results)
3. Current gestational age — weeks + days? (Default: from prenatal record)
4. Glucose monitoring data — fasting and 1-hour or 2-hour postprandial values? (Default: from patient glucose log)
5. Dietary compliance — MNT initiated? Caloric target? Carbohydrate distribution? (Default: from dietitian notes)
6. Current medications — insulin (type, dose, timing), metformin, glyburide? (Default: from medication list)
7. Fetal growth — most recent EFW and percentile? (Default: from ultrasound report)
8. Pre-gestational diabetes ruled out — A1c at first prenatal visit? Fasting glucose? (Default: differentiate GDM from pre-existing DM)

Documents to Request

• 1-hour GCT result
• 3-hour GTT results (fasting, 1-hr, 2-hr, 3-hr)
• A1c value (first trimester — to rule out pre-gestational DM)
• Patient glucose log (minimum 2 weeks of data)
• Dietary consult/MNT plan
• Medication list with insulin regimen (if applicable)
• Fetal growth ultrasound reports
• Antenatal testing results (NST, BPP)

Step 1: Screening and Diagnosis

Two-Step Approach (ACOG Recommended)

Step 1: 50 g Glucose Challenge Test (GCT) at 24–28 Weeks

• Non-fasting; 1-hour blood glucose after 50 g oral glucose load
• Abnormal: ≥ 135 mg/dL (higher sensitivity) or ≥ 140 mg/dL (higher specificity) — institution-specific threshold
• If GCT ≥ 200 mg/dL, some institutions diagnose GDM without proceeding to GTT

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

• Fasting × 8 hours; blood drawn fasting, 1-hr, 2-hr, 3-hr

Carpenter-Coustan Diagnostic Thresholds

Diagnosis: ≥ 2 abnormal values = GDM

Alternative: National Diabetes Data Group (NDDG) thresholds are slightly higher (fasting ≥ 105, 1-hr ≥ 190, 2-hr ≥ 165, 3-hr ≥ 145). Specify which criteria are used.

Early Screening (First Trimester)

• Indicated for patients with risk factors (BMI ≥ 25, prior GDM, A1c 5.7–6.4%)
• If first-trimester A1c ≥ 6.5% or fasting glucose ≥ 126 → classify as pre-gestational diabetes (not GDM)
• If early GCT is normal, repeat at 24–28 weeks

Step 2: Medical Nutrition Therapy (MNT) and Glucose Monitoring

Dietary Prescription

• Caloric goal: 30 kcal/kg/day for normal BMI; 25 kcal/kg/day for overweight; ≥ 12 kcal/kg/day minimum for obese
• Carbohydrate distribution: 33–40% of total calories, distributed across 3 meals + 2–3 snacks
• Bedtime snack: complex carbohydrate + protein to prevent overnight ketosis and morning fasting hyperglycemia
• Referral to registered dietitian is standard of care

Self-Monitoring of Blood Glucose (SMBG)

• Frequency: 4 times daily — fasting + 1-hour or 2-hour postprandial after each meal
• Targets per ACOG:

• Document: percentage of values at target, pattern analysis (which meals are problematic), and glucose log review at each visit
• Allow 1–2 weeks of MNT before concluding it is insufficient

Step 3: Pharmacologic Therapy

Initiation Criteria

• Fasting glucose consistently ≥ 95 mg/dL and/or postprandial values consistently above target despite MNT × 1–2 weeks

• 30% of glucose values above target is a commonly used threshold for starting medication

Insulin (ACOG Preferred Agent)

Titration: increase by 10–20% every 3–7 days based on glucose patterns.

Oral Agents

• Glyburide: Not recommended as first-line per ACOG 2018 — higher rate of neonatal hypoglycemia and macrosomia vs. insulin
• Metformin: May be used if patient refuses insulin or for insulin-sensitizing effect; crosses placenta; long-term offspring effects unknown
• Document patient refusal of insulin if oral agents are used, with informed consent about limitations

Step 4: Fetal Surveillance and Growth Monitoring

Antenatal Testing

• GDM — diet-controlled, well-controlled: Antenatal testing (NST or modified BPP) starting at 40 weeks or earlier if complications arise
• GDM — medication-controlled: NST or modified BPP starting at 32 weeks, weekly or twice weekly per institutional protocol
• GDM — poorly controlled or with comorbidities: Increased surveillance (twice-weekly NST/BPP from 32 weeks)

Growth Ultrasound

• Perform at 36–37 weeks to assess EFW for delivery planning
• EFW > 4500 g (diabetic): Counsel on cesarean delivery (ACOG threshold for offering cesarean in GDM is 4500 g)
• EFW < 10th percentile: Evaluate for FGR — increased surveillance per SMFM guidelines
• AC growth velocity > 75th percentile may predict macrosomia before total EFW is abnormal

Step 5: Delivery Timing and Intrapartum Management

Delivery Timing per ACOG

Intrapartum Glucose Management

• Target blood glucose 70–110 mg/dL during labor
• Hold long-acting insulin on day of induction/labor
• D5LR or D5NS infusion if glucose < 70 mg/dL
• Insulin drip protocol if glucose > 110–120 mg/dL
• Monitor blood glucose every 1–2 hours during active labor

Neonatal Considerations

• Neonatal blood glucose monitoring starting 30 minutes after birth
• Watch for neonatal hypoglycemia, polycythemia, hyperbilirubinemia, respiratory distress
• Early breastfeeding to maintain neonatal glucose

Checkpoint B: Post-Draft Alignment (Mandatory)

1. Is the GDM diagnosis documented with specific GCT/GTT values and diagnostic criteria used (Carpenter-Coustan vs. NDDG)?
2. Is the glucose log reviewed with percentage of values at target?
3. Is the MNT plan documented with caloric goals and carbohydrate distribution?
4. Are pharmacotherapy changes documented with dosing rationale?
5. Is delivery timing specified and consistent with ACOG guidelines for the GDM classification?

Quality Audit

• GCT value and GTT values (all 4 time points) documented
• Diagnostic criteria specified (Carpenter-Coustan or NDDG)
• A1c documented to differentiate GDM from pre-gestational DM
• Registered dietitian referral documented
• SMBG frequency documented (4 times daily minimum)
• Glucose targets documented (fasting < 95, 1-hr < 140, 2-hr < 120)
• Glucose log reviewed with pattern analysis at each visit
• Medication initiation criteria met and documented
• Insulin regimen documented with type, dose, timing, and titration plan
• Oral agent use (if any) documented with informed consent about limitations
• Antenatal testing schedule documented and appropriate for GDM classification
• Growth ultrasound performed with EFW percentile and AC assessment
• Delivery timing planned per ACOG guidelines with GA and rationale
• Intrapartum glucose management plan documented
• Postpartum OGTT ordered (75 g, 2-hour at 4–12 weeks postpartum)

Guidelines

1. Use the two-step approach — ACOG recommends the 50 g GCT → 100 g GTT pathway in US practice. Document which diagnostic criteria (Carpenter-Coustan vs. NDDG) are used.
2. Insulin is the preferred pharmacologic agent — per ACOG, insulin does not cross the placenta and has the most evidence for safety. Glyburide is specifically NOT recommended as first-line.
3. Diet is always the foundation — even when medication is started, MNT continues and should be reinforced at every visit.
4. Do not over-treat — glucose targets are not "tight control." Overly aggressive insulin dosing causes maternal hypoglycemia, which is dangerous in pregnancy.
5. Order the postpartum OGTT — 50% of women with GDM develop type 2 diabetes within 10 years. The 75 g, 2-hour OGTT at 4–12 weeks postpartum identifies women who already have impaired glucose tolerance.
6. Differentiate GDM from pre-gestational DM early — a first-trimester A1c ≥ 6.5% or fasting glucose ≥ 126 is pre-existing diabetes, NOT GDM, and requires different management and delivery timing.
7. Document the EFW threshold for cesarean discussion — ACOG recommends discussing elective cesarean when EFW ≥ 4500 g in diabetic patients (vs. 5000 g in non-diabetic patients).