Key Facts

Placenta praevia is defined as a placenta that lies wholly or partially in the lower uterine segment, covering or reaching the internal cervical os. It is an important cause of antepartum haemorrhage and necessitates planned delivery by caesarean section.

Epidemiology

• Low-lying placenta at 20-week anomaly scan: 5-10%
• Placenta praevia at term: 0.5% (most migrate as lower segment forms)
• Incidence increasing due to rising caesarean section rates
• Previous CS increases risk: 1 CS = 0.65%, 2 CS = 1.5%, 3 CS = 2.2%, ≥4 CS = 10%

Aetiology

• Previous caesarean section (strongest risk factor)
• Previous uterine surgery (myomectomy, curettage)
• Multiparity
• Advanced maternal age (>40 years)
• Multiple pregnancy
• Assisted reproduction (IVF/ICSI)
• Smoking
• Previous placenta praevia (recurrence rate 4-8%)

Pathophysiology

• Placenta implants in the lower uterine segment rather than the fundus
• As the lower segment develops and thins in the third trimester, the placenta separates from the uterine wall, causing haemorrhage
• The lower segment cannot contract effectively to control bleeding (unlike the upper segment)
• Placenta accreta spectrum may develop where the placenta abnormally invades the myometrium

Typical Presentation

• Painless bright red vaginal bleeding (classically recurrent episodes of increasing severity)
• First episode usually occurs in late second or third trimester
• Soft, non-tender, relaxed uterus
• Malpresentation common: breech (15%), transverse lie (15%), unstable lie
• High presenting part / non-engaged head at term

Incidental Finding

• Many cases now diagnosed at 20-week anomaly scan before symptoms
• Repeat scan at 32 weeks to assess placental migration
• Further scan at 36 weeks to plan mode of delivery

Red Flags

• Major haemorrhage with haemodynamic compromise
• Signs of concealed bleeding (tachycardia, hypotension despite minimal visible loss)
• Confirmed praevia with contractions (risk of massive haemorrhage)
• Symptoms suggesting placenta accreta (painless haematuria, bladder symptoms)

Bedside

• ABCDE assessment if active bleeding
• Speculum examination only (NO digital VE)
• CTG for fetal monitoring
• Blood pressure and pulse monitoring

Bloods

• FBC, group and save (crossmatch if bleeding)
• Coagulation screen
• Kleihauer test (RhD-negative women)
• U&Es

Imaging

• Transvaginal ultrasound (TVS): gold standard for diagnosis - measures distance from placental edge to internal os
• Transabdominal ultrasound: screening tool at anomaly scan
• MRI: may be useful to assess placenta accreta spectrum (particularly posterior placenta)
• Colour Doppler: assess for placental invasion/accreta and vasa praevia

Special Tests

• If placenta accreta suspected: MRI pelvis, multidisciplinary planning
• Serial ultrasound for growth and Dopplers if recurrent bleeds

Non-pharmacological

• If asymptomatic: outpatient management if no bleeding episodes and lives close to hospital
• If symptomatic: inpatient admission after any significant bleed
• Avoid intercourse, strenuous activity
• Patient education regarding when to attend hospital
• Ensure blood group known and antibodies checked

Pharmacological

• Anti-D immunoglobulin for RhD-negative women after any bleeding episode
• Antenatal corticosteroids: betamethasone 12mg IM × 2 if preterm delivery anticipated
• Iron supplementation if anaemic (ferrous sulphate 200mg TDS)
• Tocolysis: may be considered for acute tocolysis (nifedipine) to allow steroid administration

Surgical/Interventional

• Elective caesarean section at 36-37 weeks for confirmed placenta praevia (RCOG GTG 27a)
• Performed by experienced consultant obstetrician
• If placenta accreta suspected: planned delivery in specialist centre with interventional radiology, cell salvage, and senior surgical team
• Emergency CS at any gestation if life-threatening haemorrhage
• B-Lynch suture, uterine artery ligation, internal iliac artery ligation, or peripartum hysterectomy for uncontrolled haemorrhage

Referral Criteria

• Placenta praevia confirmed at 32-week scan: refer to consultant-led unit
• Suspected placenta accreta: refer to specialist centre (regional fetal medicine unit)
• Any bleeding episode: immediate hospital assessment

• Most low-lying placentas at 20 weeks will migrate by term (>90%)
• Anterior placenta praevia with previous CS scar: higher risk of accreta spectrum
• Maternal mortality: rare in UK (<1 per 100,000 with praevia alone)
• Perinatal mortality: approximately 1-2% (mainly from prematurity)
• Risk of massive haemorrhage requiring hysterectomy: 5-8% for praevia, up to 40% for accreta
• Recurrence rate: 4-8% in subsequent pregnancies

Classification

Placenta Accreta Spectrum

Risk of Accreta by CS Number with Praevia