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.

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## 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)

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## 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

| Time Point | Threshold |
|---|---|
| Fasting | ≥ 95 mg/dL |
| 1 hour | ≥ 180 mg/dL |
| 2 hours | ≥ 155 mg/dL |
| 3 hours | ≥ 140 mg/dL |

**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

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## 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:**

| Time Point | Target |
|---|---|
| Fasting | < 95 mg/dL |
| 1-hour postprandial | < 140 mg/dL |
| 2-hour postprandial | < 120 mg/dL |

- 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

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## 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)

| Type | Timing | Typical Starting Dose |
|---|---|---|
| **NPH insulin** | Bedtime (for fasting hyperglycemia) | 0.1–0.2 units/kg/day |
| **NPH insulin** | Before breakfast (for lunch postprandial) | 0.1 units/kg |
| **Rapid-acting (lispro or aspart)** | Before meals (for postprandial hyperglycemia) | 2–4 units per meal, titrate by 1–2 units q 3 days |
| **Total daily dose** | Divided basal/bolus | 0.7–1.0 units/kg/day at term (increases with advancing GA) |

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

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## 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

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## Step 5: Delivery Timing and Intrapartum Management

### Delivery Timing per ACOG

| GDM Classification | Recommended Delivery GA |
|---|---|
| Diet-controlled, well-managed | 39 + 0 to 40 + 6 weeks (do not induce before 39 weeks solely for GDM if well-controlled) |
| Medication-controlled, well-managed | 39 + 0 weeks |
| Poorly controlled (persistently above target) | 37 + 0 to 38 + 6 weeks (individualize based on glucose control and comorbidities) |

### 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

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## 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?

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## 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)

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## 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).