What are the classifications and risk factors of placenta previa?

Placenta Previa Classification and Risk Factors

Placenta previa is classified into three main types: low-lying placenta, marginal/partial placenta previa, and complete placenta previa, with complete placenta previa carrying the highest risk of adverse maternal and neonatal outcomes. 1, 2

Classification of Placenta Previa

• Complete placenta previa: The placenta completely covers the internal cervical os, with the placental margin >2 cm from the os, associated with highest risk of complications 2, 3
• Marginal/partial placenta previa (sometimes grouped as "marpartial"):
  • Marginal placenta previa: Placental margin adjacent to the internal os 3
  • Partial placenta previa: Placenta covers the os but the margin is situated within 2 cm of the os 3
• Low-lying placenta: Placental edge is within 2 cm of the internal cervical os but does not cover it 2, 4

Transvaginal ultrasound is the gold standard for accurate diagnosis and classification of placenta previa with high sensitivity (90.7%) and specificity (96.9%) 1

Risk Factors for Placenta Previa

Major Risk Factors

• Prior cesarean delivery: Risk increases 7-fold after one cesarean and up to 56-fold after three cesarean deliveries 1, 5
• Prior uterine surgery: Including pregnancy termination and other intrauterine procedures 5
• Advanced maternal age: Increasing age correlates with higher risk 1, 5
• Multiparity: Higher number of previous pregnancies increases risk 5
• Multiple gestations: Twin or higher-order pregnancies have increased risk 6, 5

Additional Risk Factors

• Smoking: Increases risk through vascular mechanisms 5
• In vitro fertilization: Associated with abnormal placentation 1
• Prior postpartum hemorrhage: History of bleeding complications 1
• Asherman syndrome: Uterine scarring can affect placentation 1
• Uterine anomalies: Structural abnormalities of the uterus 1
• Hypertension: Vascular disease may influence placentation 1

Clinical Implications Based on Classification

• Complete placenta previa:

  • Highest risk of antepartum hemorrhage (59.1% vs 17.6% in incomplete previa) 3
  • Significantly higher risk of preterm delivery (45.1% vs 8.8%) 3
  • Increased risk of placenta accreta spectrum disorders, particularly with anterior placentation 1, 3
  • Always requires cesarean delivery 1, 5

• Anterior vs. posterior placentation:

  • In complete placenta previa, anterior location is associated with earlier bleeding onset and higher preterm delivery rates (76.2% vs 32.0%) 3
  • Anterior placentation with prior cesarean delivery significantly increases risk of placenta accreta spectrum 1

• Low-lying placenta:

  • When placental edge is >2 cm from internal os, vaginal delivery may be possible 5, 4
  • Lower risk of hemorrhage and preterm birth compared to other types 2, 3

Diagnostic Approach

• Transabdominal ultrasound should be performed initially as a screening tool 1, 7
• Transvaginal ultrasound should follow for more precise assessment of placental location relative to the cervical os 1, 7
• Doppler ultrasound is essential to identify vasa previa and assess placental blood flow patterns 7
• Digital pelvic examination must be avoided until placenta previa has been excluded to prevent triggering hemorrhage 1, 7
• MRI may be helpful in cases where ultrasound findings are concerning for placenta accreta spectrum, especially with posterior placentation 1

Management Considerations

• Women with placenta previa and prior cesarean deliveries require evaluation for placenta accreta spectrum disorder 1
• Delivery should take place at an institution with adequate blood banking facilities and multidisciplinary expertise 1, 5
• For uncomplicated placenta previa, delivery at 34-35 weeks is generally recommended 1
• Antenatal corticosteroids should be administered when delivery is anticipated before 37 weeks 1

Common Pitfalls to Avoid

• Failure to diagnose placenta accreta spectrum in women with placenta previa and prior cesarean deliveries 1
• Digital examination without first excluding placenta previa can trigger significant hemorrhage 1, 6
• Attempting to manually remove the placenta when placenta accreta spectrum is present 1
• Delaying delivery beyond 36 weeks in cases of placenta accreta spectrum, as approximately 50% require emergent delivery for hemorrhage 1