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Gestational Diabetes Mellitus (GDM) in Pregnancy

A Complete Review for DNB Anesthesiology Finals

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1. DEFINITION

• Diabetes complicates approximately 7% of all pregnancies, with GDM constituting nearly 90% of these cases
• Incidence is increasing across all ethnic/racial groups, correlating with rising obesity rates
• Estimated 5-9% of pregnant women diagnosed using two-step US protocol; up to 18% using IADPSG one-step criteria

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2. PATHOPHYSIOLOGY

Normal Pregnancy Metabolic Changes

• Pregnancy creates a state of progressive insulin resistance, especially in the 2nd and 3rd trimesters
• Insulin sensitivity decreases significantly with advancing gestation to provide nutrients for fetal growth
• Normal pregnant women maintain postprandial blood glucose within 70-120 mg/dL despite these changes

GDM Pathophysiology

• Most cases: relative insulin deficiency + insulin resistance
• Small subset (~6%): pre-type 1 diabetes - women with GCK (glucokinase) MODY account for 0.5-6% of GDM
• In GCK MODY: heterozygous inactivating mutation → altered glucose sensing → fetal phenotype varies from IUGR to macrosomia depending on whether fetus also carries mutation

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3. RISK FACTORS

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4. SCREENING & DIAGNOSIS

Timing

Two Major Diagnostic Approaches

A. Two-Step Approach (USA - ACOG preferred)

• Non-fasting, any time of day
• Blood glucose measured at 1 hour
• Positive if ≥140 mg/dL (or ≥135 mg/dL at some centers)
• If positive → proceed to Step 2

B. One-Step Approach (IADPSG / WHO / International)

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5. COMPLICATIONS

Fetal / Neonatal Complications

Maternal Complications

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6. MANAGEMENT

A. Medical Nutrition Therapy (MNT) - First Line

• 3 major meals + 3 snacks (small frequent feeds to limit postprandial glucose surges)
• Carbohydrates: no more than 50% of total diet
• Prefer complex, low-glycemic index carbohydrates (whole grains, legumes)
• Protein and fats equally divide the remaining 50%
• For obese women (BMI ≥30): 30-32% calorie restriction (25 kcal/kg actual weight/day) reduces hyperglycemia without ketonuria
• 1-2 week trial of dietary therapy is reasonable; 50% achieve control in first 2 weeks; only 10% more by week 4
• Supervised by registered dietitian / certified diabetes educator

B. Blood Glucose Targets During Pregnancy

• Target range 63-140 mg/dL: TIR goal >70%
• Time below 63 mg/dL: goal <4%
• Time below 54 mg/dL: goal <1%
• Time above 140 mg/dL: goal <25%

C. Insulin Therapy

• No insulin crosses the placenta significantly - safe for fetus
• Short-acting insulins: lispro (Humalog), aspart (Novolog) - preferred for postprandial coverage
• Long-acting insulins: detemir (Levemir), glargine (Lantus), degludec (Tresiba) - for basal coverage
• Insulin requirements increase progressively through 2nd and 3rd trimesters due to rising insulin resistance
• Typical regimen: basal-bolus with doses adjusted weekly or more frequently
• Intrapartum target: 60-120 mg/dL (Barash)

• Mean birth weight: 3302g vs 3408g
• LGA: 7.1% vs 14.5%
• Birth weight >4000g: 5.9% vs 14.3%
• Shoulder dystocia: 1.5% vs 4.0%
• Cesarean delivery: 26.9% vs 33.8%
• Preeclampsia + gestational HTN: 8.6% vs 13.6%

D. Oral Hypoglycemic Agents

E. Exercise

• Regular moderate aerobic exercise (e.g., walking 30 min/day) improves insulin sensitivity
• Reduces insulin requirements
• Avoid supine positions in later pregnancy

F. Fetal Surveillance

• Antenatal testing with twice-weekly non-stress tests (NST) beginning at 28 weeks
• Biophysical profile, Doppler velocimetry as indicated
• Serial growth ultrasounds (every 4 weeks from 28 weeks)
• Anomaly scan at 18-20 weeks (especially pregestational DM - risk of cardiac, neural tube defects)

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7. DIABETIC KETOACIDOSIS (DKA) IN PREGNANCY

• Occurs at lower glucose levels (euglycemic DKA possible at ~200 mg/dL, vs >300 mg/dL in non-pregnant)
• Enhanced ketogenesis due to elevated free fatty acids in pregnancy
• Precipitants: infection (pyelonephritis, viral GI illness), beta-agonist tocolysis, corticosteroids for fetal lung maturity, insulin pump failure, poor compliance
• Fetal mortality is high despite normal maternal survival

DKA Treatment Protocol (Creasy & Resnik)

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8. ANESTHETIC MANAGEMENT (DNB Focus)

Preoperative Assessment

• Assess glycemic control: recent glucose logs, HbA1c (target <6.5% prepregnancy ideally)
• Assess comorbidities: hypertension, renal disease (microalbuminuria, creatinine), retinopathy, neuropathy
• ECG and echocardiography if diabetes >10 years duration
• Screen for autonomic neuropathy (orthostatic hypotension, gastroparesis)
• Assess airway carefully (limited joint mobility syndrome from glycosylation can affect neck and temporomandibular joints → difficult airway)

Labor Analgesia & Anesthetic Technique

• No compelling evidence that any analgesic or anesthetic technique is superior to another in parturients with diabetes
• Neuraxial analgesia (epidural/spinal/CSE) is preferred for labor and operative delivery - does not appear to alter peripartum insulin and glucose requirements
• Neuraxial analgesia reduces catecholamine surge → may improve glycemic stability

Intrapartum Glucose Management

• Frequent blood glucose monitoring throughout labor and delivery
• Target maternal glucose: 60-120 mg/dL intrapartum (Barash)
• Use IV insulin infusion + dextrose titrated to maintain this range
• Avoid hyperglycemia - leads to fetal hyperinsulinism → neonatal hypoglycemia
• Avoid hypoglycemia - fetal distress, neonatal effects

Insulin Management Peripartum

• Insulin requirements typically begin to decrease shortly after delivery (placenta expelled → HPL and other contra-insulin hormones disappear rapidly)
• GDM patients: often no insulin needed postpartum
• Pregestational DM: reduce insulin to approximately 50% of antepartum dose immediately postpartum

Mode of Delivery

• Current evidence does not support superiority of vaginal vs. planned cesarean delivery in women with estimated fetal weight >4,500g
• ACOG recommends individual counseling on risks and benefits of both modes
• Cesarean rate is significantly higher in diabetics (macrosomia, failed induction, labor dystocia)

Cesarean Delivery

• Spinal or epidural anesthesia preferred
• If general anesthesia required: standard RSI with aspiration precautions; be aware of potential difficult airway (limited joint mobility)
• Continue glucose monitoring intraoperatively; dextrose-containing IV solutions only when glucose is in target range

Neonatal Considerations

• Neonatologist/pediatrician presence at delivery
• Immediate neonatal glucose monitoring (risk of neonatal hypoglycemia from fetal hyperinsulinism)
• Have IV dextrose 10% ready for neonatal administration

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9. TIMING AND MODE OF DELIVERY

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10. POSTPARTUM MANAGEMENT

• GDM resolves in most women after delivery (placental hormones cleared)
• 75-g OGTT at 4-12 weeks postpartum to screen for persistent diabetes
• 5-10% of GDM women found to have T2DM at postpartum check
• Long-term risk of T2DM: 50-70% within 10 years
• Annual diabetes screening recommended
• Lifestyle modifications (diet, exercise, weight loss) reduce future T2DM risk by ~58% (Diabetes Prevention Program)
• Breastfeeding encouraged - improves neonatal glucose homeostasis and reduces maternal future T2DM risk

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11. PREGESTATIONAL DIABETES vs GDM - Key Differences

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12. HIGH-YIELD DNB EXAM POINTS

1. GDM = glucose intolerance first diagnosed in pregnancy (not clearly pre-existing)
2. Screening at 24-28 weeks for all women; earlier for high-risk
3. Two-step (USA/ACOG): 50-g GCT → if positive → 100-g 3-hour OGTT; 2 of 4 values must be abnormal
4. One-step (IADPSG/International): 75-g 2-hour OGTT; 1 of 3 values abnormal
5. HAPO study was the landmark study establishing continuous glucose-outcome relationship
6. Pedersen hypothesis: maternal hyperglycemia → fetal hyperinsulinism → macrosomia
7. Insulin is preferred pharmacotherapy - crosses placenta minimally, safe
8. Intrapartum glucose target: 60-120 mg/dL (Barash) to prevent neonatal hypoglycemia
9. No superior anesthetic technique exists for diabetic parturients
10. Neuraxial analgesia does not alter peripartum insulin/glucose requirements
11. DKA occurs at lower glucose levels in pregnancy; high fetal mortality
12. Postpartum OGTT at 4-12 weeks mandatory; 50-70% develop T2DM within 10 years
13. Assess for difficult airway - limited joint mobility from glycosylation
14. Insulin requirements fall rapidly after delivery - reduce dose
15. Corticosteroids for fetal lung maturity and beta-agonist tocolytics can precipitate DKA/hyperglycemia - adjust insulin accordingly

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