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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
The Bottom Line
- Placenta praevia = placenta overlying the internal cervical os. Low-lying placenta = within 20 mm of os but not covering it
- Identified at 20-week anomaly scan — most low-lying placentas migrate upward by third trimester
- Repeat TVUSS at 32 weeks if low-lying or praevia at anomaly scan. Confirm at 36 weeks
- Presentation: painless, bright red vaginal bleeding (typically from 28 weeks onwards)
- Delivery: elective caesarean section at 36–37 weeks for confirmed praevia. Emergency CS if significant haemorrhage
- Risk factors: previous CS (most important), previous praevia, multiparity, multiple pregnancy, smoking, IVF
Overview
Placenta praevia is defined as a placenta that is inserted wholly or partly into the lower uterine segment, in relation to the internal cervical os. Current classification divides this into placenta praevia (when the placenta lies directly over the internal os) and low-lying placenta (placental edge within 20 mm of the os but not covering it). It is one of the major causes of antepartum haemorrhage, characteristically producing painless, bright red vaginal bleeding. Placenta accreta spectrum (accreta, increta, percreta) is an important associated condition where the placenta invades the myometrium, particularly in women with praevia overlying a previous caesarean scar.
Epidemiology
Placenta praevia at delivery occurs in approximately 0.4% of pregnancies. At the 20-week anomaly scan, a low-lying placenta is found in 5–10% of women, but the vast majority migrate upward as the lower segment develops, and only a small proportion persist. The most significant risk factor is previous caesarean section — the risk increases with each successive CS. Other risk factors include previous praevia, multiparity (≥5), advanced maternal age, multiple pregnancy, assisted reproduction, and smoking.
Clinical Features
Symptoms
Painless bright red vaginal bleeding — the hallmark symptom, typically from 28 weeks
Bleeding often recurrent and progressively heavier
No abdominal pain (unlike abruption)
May present with massive haemorrhage requiring emergency delivery
May be an incidental finding on anomaly scan in asymptomatic women
Signs
Non-tender, soft uterus (in contrast to the hard, tender uterus of abruption)
Malpresentation — breech or transverse lie (common with praevia)
High presenting part — not engaged
Tachycardia, hypotension if significant haemorrhage
Investigations
First-line
Transvaginal ultrasound (TVUSS)Gold standard for placental localisation. Safe and more accurate than transabdominal USS. Confirms relationship of placental edge to internal os
FBC, group and save, coagulationBaseline bloods if presenting with bleeding
Second-line
Repeat TVUSS at 32 weeksIf low-lying or praevia at anomaly scan — most will have resolved. Further scan at 36 weeks if still praevia/low-lying
MRIIf placenta accreta spectrum suspected (praevia overlying previous CS scar) — assess depth of myometrial invasion
Specialist
Multidisciplinary planningIf placenta accreta suspected — MDT including consultant obstetrician, anaesthetist, haematologist, interventional radiologist, urologist
1
Asymptomatic praevia
- Identified at 20-week scan — repeat TVUSS at 32 weeks and 36 weeks to assess for resolution
- If praevia persists: plan elective caesarean section at 36+0–37+0 weeks
- Advise to attend hospital immediately if any vaginal bleeding, contractions, or reduced fetal movements
- Discuss pelvic rest (avoid intercourse) if major praevia
2
Symptomatic — antepartum haemorrhage
- A-E approach. IV access, send FBC, crossmatch (4 units), coagulation screen
- Admit and monitor with continuous CTG
- If <34 weeks: antenatal corticosteroids (betamethasone)
- If preterm, stable, and bleeding settles: conservative inpatient management
- If massive haemorrhage or fetal distress: emergency caesarean section regardless of gestation
3
Delivery
- Elective CS at 36–37 weeks for confirmed praevia
- Consultant obstetrician should be present
- Senior anaesthetist required — higher risk of major haemorrhage
- Crossmatch blood available in theatre
- If placenta accreta: may require classical CS incision, cell salvage, interventional radiology, and potential hysterectomy
Complications
- Massive haemorrhage: Requiring emergency CS, blood transfusion, and potentially hysterectomy
- Placenta accreta spectrum: Placenta invades myometrium (accreta), through to serosa (increta), or beyond into adjacent organs (percreta). Major risk of catastrophic haemorrhage at delivery
- Preterm delivery: Emergency or planned early delivery due to bleeding
- Vasa praevia: Fetal vessels crossing the cervical os — rupture causes fetal exsanguination. Fetal mortality >60% if undiagnosed
UKMLA Exam Tips
- 1Painless bright red bleeding after 28 weeks = think placenta praevia. Pain = think abruption
- 2TVUSS is SAFE in praevia and is gold standard. Digital vaginal examination is DANGEROUS
- 3Most low-lying placentas at 20 weeks will resolve (migrate upward) — only ~10% persist
- 4Previous CS is the strongest risk factor for praevia. Risk increases with each CS
- 5Elective CS at 36–37 weeks for confirmed praevia. Earlier if symptomatic
- 6Placenta praevia overlying a CS scar = high risk of accreta spectrum — needs MDT planning
practicetest your knowledge on placenta praeviaApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — obstetrics and beyond.
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