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Gestational Diabetes Mellitus (GDM) - Exam Notes

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1. DEFINITION

Mnemonic: "GDM = Glucose Disturbance in Maternity"

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2. EPIDEMIOLOGY

• Affects 5-9% of pregnant women (two-step criteria, USA)
• Prevalence is rising, paralleling obesity trends
• More common in: Hispanics, Native Americans, Asians, African Americans

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3. PATHOPHYSIOLOGY

• Human Placental Lactogen (HPL) - most important
• Progesterone
• Cortisol
• Prolactin
• Glucagon

Mnemonic: "HAPPY C-PUMP" - HPL, Progesterone, Cortisol, Prolactin, Glucagon

• Insulin sensitivity falls ~50-60% by late gestation
• GDM women have increased basal endogenous glucose production
• Insulin's ability to suppress hepatic glucose output is impaired (~80% vs 95% in normals)

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4. RISK FACTORS

Mnemonic: "ABCDE-F"

• A - Age >25 years
• B - Body weight (obesity, BMI >30)
• C - Certain ethnicities (Hispanic, Asian, Indigenous, African American)
• D - Diabetes family history (first-degree relative with T2DM)
• E - Ex-GDM (previous GDM)
• F - Fetal macrosomia in prior pregnancy / glycosuria

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5. SCREENING

When to screen:

• Universal screening: 24-28 weeks gestation (USPSTF B recommendation)
• Early screening for high-risk women: at first prenatal visit (obesity, prior GDM, glycosuria, strong family history)
• If early screen negative, repeat at 24-28 weeks

Mnemonic: "2-4-2-8 - Screen Late" (24-28 weeks)

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6. DIAGNOSIS

Two-Step Approach (USA Standard)

• 50 g glucose oral load, non-fasting
• Blood glucose at 1 hour
• Positive if ≥140 mg/dL → proceed to Step 2

• 100 g glucose, fasting state (overnight fast + 3 days adequate carbs before)
• Fasting, 1h, 2h, 3h values
• ≥2 abnormal values = GDM diagnosis

Diagnostic Thresholds for 100-g OGTT:

Mnemonic for Carpenter & Coustan values: "95 - 180 - 155 - 140" → think "9-1-1 (emergency) minus small numbers" or: "95, then drop by 25, then 15, then 15"

One-Step Approach (IADPSG/WHO - 75 g OGTT):

Mnemonic for IADPSG: "92-180-153" → "9 to 1 to 53" - Fasting 92, 1-hour 180, 2-hour 153

• Based on HAPO study (~25,000 participants, multinational)
• IADPSG criteria: diagnoses based on 1.75x odds ratio for adverse perinatal outcomes

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7. MANAGEMENT

Stepwise approach: "D-E-I-M"

• Diet
• Exercise
• Insulin (if targets not met)
• Monitoring

Diet:

• 30-35 kcal/kg lean body weight/day
• Carbohydrate-restricted diet
• 3 main meals + 3 daily snacks
• Avoid concentrated sweets

Exercise:

• Walking is first-line
• Decreases insulin resistance

Glucose Targets:

Mnemonic: "95 Fasting, 120 at Two, 140 at One" → "Fast 95, PP120, PP140"

When to start insulin:

• Fasting glucose >95-105 mg/dL persistently OR
• 2-hour postprandial >120 mg/dL on diet/exercise

Insulin regimen (Goldman-Cecil):

• Total dose: 0.7-1.0 units/kg/day
• 2/3 in morning (before breakfast): 1/3 regular + 2/3 NPH
• 1/3 in evening (before dinner): 1/2 regular + 1/2 NPH

Mnemonic: "Two-thirds Morning, One-third Evening" (2/3 : 1/3 split) Sub-mnemonic for morning: "One-Two Rule" - 1 part regular : 2 parts NPH

Oral agents:

• Metformin - used in some centers (crosses placenta, long-term data limited)
• Glyburide - alternative (less preferred due to higher neonatal hypoglycemia risk)
• Insulin remains the gold standard in most guidelines

HbA1c:

• Check every 4-6 weeks
• Not elevated unless fasting hyperglycemia is present

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8. MONITORING & ANTENATAL SURVEILLANCE

• SMBG (Self-Monitoring Blood Glucose): 4x/day - fasting + 2h after each meal
• Weekly review of glucose values (insulin resistance increases with gestation)
• Ultrasound for fetal growth every 4-6 weeks
• Antenatal testing in 3rd trimester for insulin-requiring patients
• Growth scans to detect macrosomia

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9. FETAL & MATERNAL COMPLICATIONS

Mnemonic for fetal complications: "MACROSOMIA + HASP"

• Macrosomia (primary complication of GDM)
• Hypoglycemia (neonatal)
• Anterior shoulder dystocia / birth trauma
• Stillbirth
• Polyhydramnios

Mnemonic for neonatal metabolic complications: "4 H's"

• Hypoglycemia
• Hypocalcemia
• Hyperbilirubinemia
• H-polycythemia (High hematocrit/polycythemia)

• Preeclampsia
• C-section (due to macrosomia)
• T2DM development postpartum (see below)

• Spontaneous abortion
• Congenital malformations (doubled - related to glycemic control during organogenesis)
• Fetal growth restriction (if maternal vascular disease)
• Diabetic ketoacidosis (at lower glucose levels than non-pregnant)

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10. POSTPARTUM MANAGEMENT

Testing after delivery:

• Glucose tolerance test at 4-12 weeks postpartum (Tietz recommends 4-12 weeks; Goldman-Cecil says 6-12 weeks)
• Then yearly screening for T2DM
• Use HbA1c or 75-g OGTT

Long-term T2DM risk:

• 30-60% lifetime risk of developing T2DM (Harrison's: 35-60% in next 10-20 years)
• Risk is 5x higher than women without GDM history

Risk reduction:

• Weight loss
• Regular exercise
• Metformin (in high-risk women)
• Breastfeeding is protective - reduces postpartum T2DM risk

Mnemonic: "30-60 Rule" - 30-60% chance of T2DM lifelong after GDM

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11. QUICK SUMMARY TABLE

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12. HIGH-YIELD EXAM POINTS

1. HPL is the main diabetogenic hormone of pregnancy
2. GDM diagnosis requires ≥2 abnormal values on 100-g OGTT (2-step)
3. IADPSG uses 75-g OGTT, only 1 abnormal value needed (more sensitive, diagnoses more women)
4. GDM is usually asymptomatic - always screen
5. Macrosomia is the primary GDM complication (not seen as much with pre-gestational DM which causes congenital malformations)
6. Congenital malformations are a risk of pre-gestational diabetes, not pure GDM
7. Insulin requirements increase as pregnancy advances
8. After GDM, test postpartum - do not just assume she is fine
9. Breastfeeding reduces T2DM risk postpartum
10. Women with prior GDM: 5x elevated lifetime T2DM risk

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13. MASTER MNEMONIC SUMMARY

• G - Glucose challenge (50g, 1h, non-fasting)
• D - Diagnose with 100g OGTT (2 abnormals)
• M - Macrosomia = primary fetal risk

• Diet first
• Exercise (walking)
• Insulin if targets missed

• 30-60% lifetime T2DM risk
• Screen at 4-12 weeks postpartum, then yearly

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