What is the management for a low-lying placenta (placenta previa)?

Management of Low-Lying Placenta

For a low-lying placenta diagnosed in the second trimester, perform transvaginal ultrasound to measure the distance from the placental edge to the internal cervical os, and if ≥2 cm at 18-20 weeks, no further imaging is needed; if <2 cm, obtain follow-up ultrasound at 28-32 weeks to reassess placental position and screen for placenta accreta spectrum disorder, especially in women with prior cesarean deliveries. 1, 2

Initial Diagnostic Approach

• Perform transabdominal ultrasound first to identify placental location and assess for low-lying position 2, 1
• Follow with transvaginal ultrasound for precise measurement of the distance between the placental edge and internal cervical os, as this provides superior visualization of the lower uterine segment 2, 1
• Add Doppler ultrasound to identify any vessels overlying the internal cervical os and assess placental blood flow patterns, which is essential for detecting vasa previa 2, 1
• Avoid digital pelvic examination until placenta previa has been definitively excluded, as this can trigger catastrophic hemorrhage 1, 3

Risk Stratification Based on Distance

Low-Risk: Placenta ≥2 cm from Internal Os at 18-20 Weeks

• No follow-up imaging is required if the placental edge is ≥2 cm from the internal cervical os at the mid-trimester anatomy scan 2, 1
• These women can be managed per routine obstetric guidelines without additional restrictions 2

Intermediate-Risk: Placenta <2 cm from Internal Os

• Schedule follow-up ultrasound at 28-32 weeks to reassess placental position, as most low-lying placentas will migrate away from the cervix as the lower uterine segment develops 2, 1
• Maintain normal activity levels before 28 weeks, including moderate-to-vigorous physical activity 3
• After 28 weeks, restrict moderate-to-vigorous physical activity but allow activities of daily living and low-intensity walking 3

Screening for Placenta Accreta Spectrum Disorder

This is the critical pitfall to avoid: Women with an anterior low-lying placenta and prior cesarean deliveries require meticulous evaluation for placenta accreta spectrum disorder, as the risk increases 7-fold after one cesarean and up to 56-fold after three cesareans 1, 3

Ultrasound Findings Suggesting Accreta

• Intraplacental lacunae (sonolucent spaces with turbulent flow) 2
• Loss of retroplacental clear zone (<1 cm myometrial thickness) 2
• Placental bulging into the bladder 2
• Bladder wall irregularity or interruption of the uterine serosa-bladder interface 2
• Abnormal subplacental vascularity on Doppler imaging 2

When to Order MRI

• Consider MRI after 24 weeks (optimal timing 30-35 weeks) if ultrasound findings are equivocal, severely abnormal, or limited by maternal obesity 2
• MRI without contrast is preferred, as gadolinium-based contrast agents are category C and should only be used if benefits clearly outweigh fetal risks 2
• MRI helps with surgical planning by defining the precise topography and depth of placental invasion 2

Management of Persistent Low-Lying Placenta or Previa at Term

Delivery Planning

• Coordinate with a multidisciplinary team including anesthesiology, maternal-fetal medicine, neonatology, and expert pelvic surgeons if accreta is suspected 1, 3
• Notify the blood bank in advance due to increased risk of hemorrhage requiring large-volume transfusion 1, 3
• Optimize hemoglobin during pregnancy by treating anemia with oral or intravenous iron 3
• Administer antenatal corticosteroids if delivery is anticipated before 37 weeks 3

Timing of Delivery

• For uncomplicated placenta previa, deliver at 34-36 weeks after confirming fetal lung maturity 3, 4
• For suspected placenta accreta spectrum, deliver at 34 0/7 to 35 6/7 weeks in stable patients, as waiting beyond 36 weeks results in approximately 50% requiring emergent delivery for hemorrhage 3
• Deliver earlier if there is persistent bleeding, preeclampsia, labor, rupture of membranes, or fetal compromise 3

Mode of Delivery

• Cesarean delivery is mandatory for complete placenta previa 5, 4
• Vaginal delivery may be considered if the placental edge is >2 cm from the internal os at term 5
• Regional anesthesia is safe for cesarean delivery in women with placenta previa 5

Intraoperative Considerations for Accreta

• Position patient in dorsal lithotomy to allow vaginal access and optimal surgical visualization 3
• Inspect the uterus after peritoneal entry to determine placental location and plan the uterine incision away from the placenta when possible 3
• Leave the placenta in situ if there is evidence of abnormal attachment, as forced removal causes profuse hemorrhage 3, 5
• Proceed directly to cesarean hysterectomy in most cases of significant placenta accreta, as this is the safest approach 3, 5
• Consider ureteric stent placement preoperatively if bladder involvement is suspected 3

Postpartum Management

• Maintain intensive hemodynamic monitoring in the early postoperative period, often in an ICU setting 3
• Monitor for postpartum hemorrhage, as women with low-lying placentas in the second trimester have a 1.8-fold increased risk even if the placenta migrates 6
• Carefully manage the third stage of labor with active management techniques 6
• Ensure adequate iron supplementation and follow-up of hemoglobin levels if significant blood loss occurred 3

Common Pitfalls to Avoid

• Failing to perform transvaginal ultrasound for accurate measurement of the internal os distance, relying only on transabdominal views 1
• Missing placenta accreta spectrum in women with anterior low-lying placenta and prior cesarean deliveries—this is the most dangerous oversight 1, 3
• Performing digital examination before excluding placenta previa, which can trigger life-threatening hemorrhage 1, 3
• Delaying delivery beyond 36 weeks in women with suspected accreta, as emergency hemorrhage becomes increasingly likely 3