Obstetrics

Placenta Praevia

Placental tissue lying over the internal cervical os shears as the lower uterine segment forms, opening maternal vessels and causing painless recurrent bleeding from a segment that then contracts poorly.

In a nutshell

Placenta praevia is placental tissue over the internal cervical os; a low-lying placenta has its edge less than 20 mm away. Never perform a digital vaginal examination before praevia is excluded, and plan caesarean birth where on-site blood transfusion is available.

Classic presentation

A woman at 33 weeks with a previous caesarean has a second episode of painless bright-red bleeding, a soft non-tender uterus and a high presenting part.

Key points

Placenta praevia at the end of pregnancy affects about 1 in 200 births.
Placental position at the 18+0 to 20+6 week anomaly scan is good clinical practice, not part of the fetal anomaly screening programme itself.
Risk factors: previous caesarean birth, previous praevia, other uterine surgery, in vitro fertilisation (IVF), multiple pregnancy, older maternal age and smoking.
Placenta accreta spectrum (PAS) also follows curettage, hysteroscopic surgery, endometrial ablation, uterine artery embolisation and myomectomy, not only caesarean scars.
Caesarean scar ectopic pregnancy is a precursor of both praevia and accreta spectrum, and is new in the fifth edition of Green-top 27a.
Transvaginal ultrasound also measures cervical length, an emphasis new in the fifth edition.
Red cells are almost always needed below haemoglobin 60 g/L and rarely above 100 g/L; an acutely bleeding woman can have a normal haemoglobin.
Regional anaesthesia is preferred to general anaesthesia for caesarean birth, including in placenta praevia.

First-line investigation

Transvaginal ultrasound to measure the placental edge against the internal cervical os, with continuous cardiotocography (CTG) and group and crossmatch after any bleed.

Management

Stabilise the bleed

ABCDE, two large-bore cannulae, group and crossmatch, continuous cardiotocography (CTG). Activate the major obstetric haemorrhage response for loss over 1000 mL or any sign of clinical shock.^4,5
Never perform a digital vaginal examination while praevia is possible. A speculum can safely assess bleeding and local causes before ultrasound exclusion; abdominal palpation remains safe.^4,5,1,10

Confirm the position and cover the immediate risks

Transvaginal ultrasound measures the placental edge against the internal os. Praevia is over the os; low-lying is an edge less than 20 mm away.^1,2,3
Anti-D immunoglobulin 500 units deep intramuscularly after 20+0 weeks, or 250 units before, within 72 hours. Add 100 to 125 units per mL if fetomaternal haemorrhage exceeds 4 mL.^9,4
Offer maternal corticosteroids at 24+0 to 33+6 weeks and consider them at 34+0 to 35+6 weeks. Do not delay a life-saving birth for a course.^11,12

Surveillance for the stable low placenta

Rescan at about 32 weeks if the placenta was low at the anomaly scan: about 9 in 10 are no longer low by then.^2,7
Praevia or a low placenta with a previous caesarean or uterine scar: greyscale ultrasound with colour Doppler around 28 weeks, no later than 29 weeks.^3,6
Written plan covering bleeding, distance from hospital, admission and blood availability. Advise avoiding penetrative intercourse and reporting any bleeding at once.^4,1

Plan the birth

At the 36-week scan, caesarean is safest if the placenta covers the os or its edge is under 20 mm; above 20 mm, vaginal birth is an option. Ensure on-site transfusion.^3,10
Without heavy or recurrent bleeding, plan caesarean between 36+0 and 37+0 weeks. Bleeding may require earlier birth under the obstetric team.^1,6,10
Suspected accreta spectrum: specialist centre, consultant obstetrician, gynaecologist and anaesthetist in theatre, neonatal cover, haematology contactable, cross-matched blood, critical care bed.³
Cell salvage is beneficial and safe here. Give at least 1500 units anti-D after reinfusion in RhD-negative women, with fetomaternal haemorrhage testing 30 to 40 minutes later.^8,6,9

Exam traps

Grades I to IV and major or minor praevia are obsolete labels. Current UK guidance describes the placental edge relative to the internal os and nothing else.
NICE NG192 recommendation 1.2.6 still carries the words 'minor or major placenta praevia' in brackets. That is legacy wording from 2011, not a live classification.
The 2018 fourth edition of Green-top 27a is mirrored all over the internet. The current guideline is the fifth edition, published 30 June 2026.
Painless bleeding over a soft uterus favours praevia; a tense, tender uterus favours abruption. A normal CTG does not tell you the mother is safe.
Transvaginal ultrasound is safe in praevia. It is the digital vaginal examination that can cause catastrophic haemorrhage.
'Major haemorrhage' antepartum means 50 to 1000 mL without shock. The postpartum definitions use entirely different volumes, so do not carry the numbers across.
Do not delay a life-saving birth to complete a corticosteroid course.
Praevia does not end when the baby is delivered: the lower segment contracts poorly, so postpartum haemorrhage should be expected and prepared for.

Key sources

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.