What are the predicting factors for shoulder dystocia?

Predicting Factors for Shoulder Dystocia

The strongest predictors of shoulder dystocia include fetal macrosomia (especially birth weight >4,500g), maternal diabetes, prior shoulder dystocia, and maternal obesity, though more than 50% of cases occur without identifiable risk factors.

Major Risk Factors

Maternal Factors

• Diabetes mellitus

  • Pregestational or gestational diabetes increases risk 2.2-fold 1
  • Risk is significantly higher in diabetic women (2.0% vs 0.9% overall) 1
  • In diabetic pregnancies, shoulder dystocia risk increases dramatically when birth weight exceeds 4,250g (12.1% vs 1.9%) 1

• Obesity

  • BMI ≥30 kg/m² increases risk (1.9% vs 0.9% overall) 1
  • Diabetic women with shoulder dystocia have significantly higher pre-pregnancy weight (83.4±23.8 kg vs. 62.5±10.9 kg) and BMI (30.2±6.8 kg/m² vs. 22.9±4.3 kg/m²) 2

• Previous macrosomic infant delivery

  • History of delivering a large infant is a significant risk factor (p<0.01) 3

• Other maternal factors

  • Multiparity
  • Maternal age <17 years
  • Post-term pregnancy (>40 weeks)
  • Maternal height and ethnicity 4

Fetal Factors

• Macrosomia

  • The strongest independent risk factor for shoulder dystocia 5
  • Risk increases significantly with birth weights >4,000g (p<0.01) 3
  • Risk increases dramatically at 4,250-4,499g compared to 4,000-4,249g (6.1% vs 1.6%) 1
  • When birth weight exceeds 4,500g:
    • Risk is 9.2-24% in non-diabetic women
    • Risk is 19.9-50% in diabetic women 4

• Estimated fetal weight (EFW) ≥4,250g

  • Independent risk factor (OR 3.8,95% CI 1.5-9.4) 1

• Fetal biometric measurements

  • Abdominal circumference minus head circumference (AC-HC) ≥2.5 cm is an independent risk factor (OR 3.1,95% CI 1.3-7.5) 1
  • Male sex 4

Labor and Delivery Factors

• Operative vaginal delivery

  • Significantly increases risk (RR 9.58,95% CI 3.70-24.81) 5
  • Midpelvic operative delivery is a risk factor, especially with macrosomia 6

• Labor abnormalities

  • Arrest and protraction disorders of labor 6

Important Considerations

Unpredictability

• Despite known risk factors, shoulder dystocia remains largely unpredictable
• More than 50% of cases occur without identifiable risk factors 6
• Shoulder dystocia can occur in infants of normal birth weight, not just those with macrosomia 7

Glucose Control

• Severity of maternal fasting hyperglycemia correlates with shoulder dystocia risk
• Each 1 mmol increase in fasting OGTT leads to a relative risk of 2.09 (95% CI 1.03-4.25) 5

Prevention Considerations

• Labor induction is not recommended for suspected fetal macrosomia as it doubles cesarean delivery risk without reducing shoulder dystocia risk 7
• Prophylactic cesarean delivery may be considered for:
  • Estimated fetal weights >5,000g in non-diabetic women
  • Estimated fetal weights >4,500g in diabetic women 4, 7
• Vaginal delivery is not contraindicated for estimated fetal weights up to 5,000g in non-diabetic women 4, 7

Complications of Shoulder Dystocia

• Brachial plexus injury

  • Higher risk in infants of diabetic mothers (61.5% vs 17.1%) 2
  • Most resolve within 6 months 3

• Clavicular fracture

  • Occurs approximately 10 times more frequently in macrosomic infants 7
  • Most heal without intervention 7

• Maternal complications

  • Higher incidence of perineal tears in diabetic women (23.1% vs 0%) 2
  • Potential for symphysis pubis dehiscence 2

By understanding these risk factors, clinicians can better identify pregnancies at higher risk for shoulder dystocia, though it remains important to be prepared for this emergency in all deliveries given its unpredictable nature.