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

Related Guidance

GTG 27a and GTG 27b replaced the combined GTG 27 in 2018. Vasa praevia is covered separately in RCOG Green-top Guideline No. 27b: Vasa Praevia.

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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