Chances of Natural Birth with a Low-Lying Placenta
The chances of vaginal delivery with a low-lying placenta depend primarily on the distance between the placental edge and the internal cervical os, with approximately 85% success rate when the distance is 11-20 mm, but only 43% success when the distance is 0-10 mm. 1
Classification and Diagnosis
- Low-lying placenta is diagnosed when the placenta is positioned close to, but not completely covering, the internal cervical os 2
- Transvaginal ultrasound is the diagnostic modality of choice for accurate assessment of placental position 2
- Digital pelvic examination should be avoided until placenta previa has been excluded to prevent triggering hemorrhage 2
Success Rates by Distance from Internal Cervical Os
- For placental edge to internal os distance (IOD) of 0-10 mm: 43% chance of successful vaginal delivery 1
- For IOD of 11-20 mm: 85% chance of successful vaginal delivery 1
- For IOD >20 mm: 82% chance of successful vaginal delivery 1
Recent Evidence on Delivery Outcomes
- A 2022 study found that among women with low-lying placenta (IOD ≤20 mm) who attempted vaginal delivery:
- 50% success rate with IOD of 11-20 mm
- Only 18.5% success rate with IOD of 1-10 mm 3
- The rate of severe postpartum hemorrhage was similar between planned trial of labor (22.9%) and elective cesarean delivery (23.0%) 3
- Severe maternal and neonatal morbidity rates were also similar between trial of labor and elective cesarean delivery 3
Risk Assessment and Management
- Women with placenta previa and prior cesarean deliveries should be evaluated for placenta accreta spectrum disorder, which significantly increases morbidity 2
- The risk of placenta accreta increases 7-fold after one prior cesarean delivery and up to 56-fold after 3 cesarean deliveries 2
- Delivery should take place at an institution with adequate blood banking facilities and multidisciplinary expertise 2
Considerations for Trial of Labor
- A low-lying placenta is not an absolute contraindication for a trial of labor, especially with IOD >10 mm 1
- Women with a low-lying placenta have a higher chance of emergency cesarean section compared to women with normally positioned placentas 1
- Postpartum hemorrhage risk is higher in women who had a low-lying placenta in the second trimester, even if it resolved later in pregnancy 4
Important Caveats
- The closer the placenta is to the internal os, the higher the risk of antepartum hemorrhage 1
- Women with IOD of 1-10 mm have significantly reduced chances of successful vaginal delivery compared to those with IOD of 11-20 mm 3
- Maternal anemia (hemoglobin <8.0 g/dL) is the most common morbidity associated with low-lying placenta 5
- Careful management of the third stage of labor is essential due to increased risk of postpartum hemorrhage 4