What is a low-lying anterior placenta?

Low-Lying Anterior Placenta: Definition and Clinical Implications

A low-lying anterior placenta refers to a placenta that is located in the lower uterine segment on the front wall of the uterus, with the placental edge positioned within 2 cm of the internal cervical os but not covering it. 1

Diagnosis and Classification

• Low-lying placenta is diagnosed when the placental edge is within 2 cm of the internal cervical os but not covering it, while placenta previa occurs when the placenta completely overlies the internal os 1, 2
• Transvaginal ultrasound is the gold standard for accurate diagnosis and assessment of the distance between the placental edge and internal cervical os (IOD) 1, 2
• Digital pelvic examination should be avoided until placenta previa has been excluded to prevent triggering hemorrhage 2
• Transabdominal ultrasound should be performed initially, followed by transvaginal ultrasound for more precise measurement 1
• Doppler ultrasound is essential to identify any vessels overlying the internal cervical os and assess placental blood flow patterns 1

Natural History and Resolution

• Most low-lying placentas identified in the second trimester will resolve (move away from the cervix) by the third trimester due to differential growth of the uterus 3, 4
• The resolution rate depends on:
  • Distance from internal os: 80% resolution rate when placenta overlaps the os (IOD < 0 mm), 90% resolution rate when IOD < 10 mm 3
  • Placental location: anterior placentas resolve more frequently than posterior placentas (anterior: 97.3% vs posterior: 93.6%) 4
  • Prior cesarean delivery: women without prior cesarean have higher resolution rates except for complete previa (IOD < 0 mm) 3, 4

Clinical Implications and Risks

• Even when resolved in the third trimester, a previously low-lying placenta still carries increased risks:
  • Higher rate of postpartum hemorrhage (9.8% vs 4.4% in controls) 5
  • Increased likelihood of antepartum bleeding episodes 5
  • 2.5 times higher risk of bleeding complications during delivery 5
• For anterior low-lying placentas specifically:
  • Lower risk of persistence compared to posterior placentas 4
  • Different follow-up threshold recommended: 5 mm from internal os (vs 20 mm for posterior placentas) 4
  • Must be evaluated for placenta accreta spectrum disorder if there is a history of prior cesarean delivery 1

Management Recommendations

• Follow-up ultrasound in the third trimester (28-32 weeks) is essential to reassess placental position 1, 2
• For anterior placentas specifically, follow-up is recommended if the IOD is ≤5 mm in the second trimester 4
• If the placenta is ≥2 cm from the internal cervical os at 18-20 weeks, follow-up evaluation is not necessary 1
• Women with resolved low-lying placenta should still be considered at increased risk for hemorrhage during delivery 5
• Delivery planning should include:
  • Notification of blood bank due to increased hemorrhage risk 2
  • Availability of adequate blood products 2
  • Multidisciplinary team approach if placenta accreta spectrum is suspected 1, 2

Special Considerations

• Women with prior cesarean deliveries and anterior low-lying placenta require careful evaluation for placenta accreta spectrum disorder 1, 2
• Risk of placenta accreta increases 7-fold after one prior cesarean delivery and up to 56-fold after three cesarean deliveries 1, 2
• MRI may be helpful in difficult cases or when placenta accreta is suspected based on ultrasound findings 1, 2
• Vaginal delivery may be possible with a low-lying placenta (IOD 0-10 mm: 43% success rate; IOD 11-20 mm: 85% success rate) but carries higher risk of emergency cesarean section 6

Common Pitfalls to Avoid

• Failure to perform transvaginal ultrasound for accurate measurement of IOD 1
• Digital examination before excluding placenta previa, which can trigger hemorrhage 2
• Assuming no risk once a low-lying placenta has resolved - these patients still have 2.5 times higher risk of postpartum hemorrhage 5
• Missing placenta accreta spectrum in women with anterior low-lying placenta and prior cesarean deliveries 1
• Using the same follow-up threshold (20 mm) for both anterior and posterior placentas - anterior placentas can use a lower threshold (5 mm) 4