Placenta Praevia and Placenta Accreta Spectrum

Definitions

• Low-lying placenta: placental edge within 20mm of the internal cervical os but not covering it
• Placenta praevia: placenta partially or completely covers the internal cervical os
• Minor praevia: leading edge in the lower segment but not covering the os
• Major praevia: placenta covers the internal os partially or completely

Placenta Accreta Spectrum (PAS)

• Placenta accreta: abnormal adherence to myometrium (no intervening decidua basalis)
• Placenta increta: invasion into the myometrium
• Placenta percreta: penetration through the myometrium to or beyond the serosa (may invade bladder, bowel)

Risk Factors

Placenta Praevia

• Previous caesarean section (strongest risk factor — risk increases with number)
• Previous uterine surgery (myomectomy, D&C)
• Multiparity
• Multiple pregnancy
• Advanced maternal age
• Smoking
• Previous placenta praevia
• Assisted conception (IVF)

Placenta Accreta Spectrum

• Previous caesarean section (main risk factor):
  • 1 previous CS: ~3% risk of accreta if praevia present
  • 2 previous CS: ~11%
  • 3 previous CS: ~40%
  • 4+ previous CS: ~60%
• Placenta praevia (risk multiplied by previous CS)
• Previous uterine surgery
• Previous endometrial ablation
• Previous manual removal of placenta

Diagnosis

• Routine anomaly scan (18-20+6 weeks) identifies placental position
• If low-lying/praevia at anomaly scan: rescan at 32 weeks (TVUS preferred — safe and more accurate than TAS)
• If remains praevia at 32 weeks: plan for delivery
• MRI may be useful for suspected PAS, particularly posterior placenta or to assess depth of invasion
• Ultrasound features of PAS: loss of clear zone, placental lacunae ("moth-eaten" appearance), bladder wall interruption, bridging vessels

Management of Placenta Praevia

• Asymptomatic praevia: outpatient management if stable, accessible hospital, reliable transport, and companion available
• Symptomatic (bleeding): hospital admission, IV access, cross-match blood
• Corticosteroids: consider from 34-35+6 weeks if preterm delivery anticipated
• Delivery timing:
  • Uncomplicated praevia: planned caesarean at 36-37 weeks
  • If recurrent bleeding: individualise — may need earlier delivery
  • Emergency delivery if massive haemorrhage
• Mode of delivery: caesarean section for placenta praevia (vaginal delivery may be attempted if low-lying placenta >20mm from os)
• Senior obstetrician should perform the caesarean; consultant presence recommended

Management of Placenta Accreta Spectrum

• Multidisciplinary team planning (MDT): obstetrician, anaesthetist, haematologist, urologist, interventional radiologist, neonatologist
• Delivery in a specialist centre with appropriate resources
• Planned caesarean hysterectomy at 35-36+6 weeks for confirmed PAS
• Interventional radiology (uterine artery balloon occlusion/embolisation) may reduce blood loss
• Cell salvage should be available
• Massive transfusion protocol on standby
• Conservative management (leaving placenta in situ): considered in carefully selected cases with extensive counselling about risks (infection, delayed haemorrhage, need for future hysterectomy)

Important Facts for MRCOG

• Previous caesarean section is the most important risk factor for both praevia and PAS
• Risk of accreta with praevia increases dramatically with each caesarean (3% → 11% → 40% → 60%)
• TVUS is safe for diagnosis and more accurate than transabdominal scan
• Planned delivery: 36-37 weeks for uncomplicated praevia, 35-36+6 for PAS
• Placenta percreta can invade the bladder — urology involvement essential
• "Moth-eaten" lacunae on ultrasound are suggestive of accreta
• Vasa praevia (fetal vessels crossing the cervical os) is a separate condition — presents with painless vaginal bleeding at membrane rupture with fetal compromise

Source: RCOG Green-top Guideline No. 27a (2018)

Read the original on rcog.org.uk

Clinically reviewed by NHS obstetricians and gynaecologists · Source last updated 2018

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