Placenta praevia

Overview

Placenta implants in the lower uterine segment, partially or completely covering the internal cervical os - a major cause of antepartum haemorrhage with significant anaesthetic and surgical implications.

Risk Factors

Previous caesarean section
Multiparity
Prior uterine surgery/curettage
Advancing maternal age
Multiple pregnancy
Smoking
Previous placenta praevia
Assisted conception

Classification

•Major praevia - placenta covers the internal os at term; caesarean delivery required
•Minor (low-lying) - placental edge within 20 mm of the internal os but not covering it
•Placental edge >20 mm from internal os - generally compatible with trial of vaginal delivery
•Up to 90% of apparent low-lying placentae at 20-week scan resolve by term ('placental migration')

Presentation

•Painless bright-red vaginal bleeding - sudden onset, second or third trimester; the hallmark feature
•Soft, non-tender uterus - key distinction from placental abruption (tense, woody uterus)
•Malpresentation or high presenting part - placenta displaces fetal head; transverse/oblique lie common
•Sentinel bleed - first episode often modest but heralds risk of severe haemorrhage
•Haemorrhagic shock - tachycardia, hypotension, pallor in severe haemorrhage
•Incidental antenatal finding - detected on 20-week anomaly scan before any bleeding
🚨
Never perform a digital vaginal examination in any woman with antepartum haemorrhage until placenta praevia has been excluded by ultrasound - a finger through a partially dilated os into a praevia will trigger catastrophic, potentially fatal haemorrhage.

Investigations

🥇 First-line

•Transabdominal ultrasound - initial placental localisation
•Transvaginal ultrasound (TVS) - more accurate; measures distance from placental edge to internal os; safe in praevia

🏆 Gold standard

•MRI pelvis - reserved for suspected placenta accreta spectrum (PAS) when ultrasound equivocal; superior for assessing myometrial invasion and bladder involvement
•FBC - assess anaemia
•Group and save / crossmatch - crossmatch 4 units packed red cells if haemodynamically compromised
•Coagulation screen - DIC may complicate major haemorrhage
•Kleihauer-Betke test - quantifies feto-maternal haemorrhage; guides anti-D immunoglobulin dosing in RhD-negative women
•CTG - continuous fetal monitoring to detect compromise

Management

Acute haemorrhage
  1. 1Activate major haemorrhage protocol
  2. 2Two large-bore IV cannulae; resuscitate with O-negative blood while awaiting crossmatch
  3. 3Alert obstetric, anaesthetic, and haematology teams
  4. 4CTG for fetal monitoring
  5. 5Anti-D immunoglobulin + Kleihauer-Betke in all RhD-negative women
  6. 6Betamethasone (single course) if <34 weeks for fetal lung maturity
Haemodynamically stable
Planned elective caesarean at specialist centre; optimise maternal Hb; regional anaesthesia (spinal or CSE) preferred
Haemodynamically unstable / urgent delivery
Emergency caesarean; general anaesthesia if unstable; cell salvage available intraoperatively
Anaesthetic considerations
  1. 1Regional anaesthesia preferred for elective caesarean - avoids airway instrumentation, mother remains awake
  2. 2General anaesthesia for urgent cases or haemodynamic instability
  3. 3Cell salvage should be available
  4. 4If PAS suspected: deliver at specialist centre with interventional radiology; plan for possible caesarean hysterectomy
⚠️
When praevia overlies a previous caesarean scar, actively exclude placenta accreta spectrum with ultrasound +/- MRI. Risk of PAS is up to 25% after one prior caesarean, rising with each additional scar. Undiagnosed PAS at caesarean is a leading cause of perioperative catastrophe.

Complications

•Massive obstetric haemorrhage - may require transfusion, uterine artery embolisation, or hysterectomy
•Placenta accreta spectrum - abnormal trophoblastic invasion into/through myometrium when praevia overlies scar
•Preterm delivery - neonatal prematurity is the primary fetal concern
•Feto-maternal haemorrhage - critical in RhD-negative women
•Malpresentation and cord prolapse - placenta prevents normal cephalic engagement
•Maternal death - placenta praevia remains a leading cause of direct maternal mortality in MBRRACE-UK reports