Overview
Placenta Praevia
1. Overview
Placenta Praevia is defined as the placenta being inserted wholly or partially into the lower uterine segment, typically after 32 weeks gestation. It is a major cause of Antepartum Haemorrhage (APH) and significant maternal/fetal morbidity.
Classification (RCOG Guidelines)
The old "Major/Minor" or "Grade I-IV" terms are phasing out in favor of descriptive ultrasound measurements:
- Low-Lying Placenta: Placenta edge is <20mm from the internal os (but not covering it).
- Placenta Praevia: Placenta covers the internal os partially or completely.
The Great Danger
- Bleeding: The lower uterine segment stretches in the 3rd trimester, causing placental separation and massive bleeding.
- Accreta Risk: If there is a prior Caesarean scar, a low placenta may invade it (Placenta Accreta Spectrum), leading to life-threatening hemorrhage at delivery.
Epidemiology
| Factor | Details |
|---|---|
| Incidence | 0.5-1% of pregnancies at term |
| Recurrence | 4-8% risk |
| Migration | 90% of "low lying" placentas at 20 weeks resolve by 36 weeks ("Placental Migration") |
2. Pathophysiology
┌─────────────────────────────────────────────────────────────────────────────┐
│ PLACENTA PRAEVIA PATHOPHYSIOLOGY │
├─────────────────────────────────────────────────────────────────────────────┤
│ │
│ ┌─────────────────────────────────────────────────────────────────────┐ │
│ │ ABNORMAL IMPLANTATION │ │
│ │ • Low implantation of blastocyst │ │
│ │ • Risk Factors: Uterine scarring, Multiparity, Advanced Age │ │
│ │ • Large Placenta (Twins, Smoking) │ │
│ └─────────────────────────────────────────────────────────────────────┘ │
│ ↓ │
│ ┌─────────────────────────────────────────────────────────────────────┐ │
│ │ LOWER UTERINE SEGMENT DEVELOPMENT │ │
│ │ • 3rd Trimester: Lower segment stretches and thins │ │
│ │ • Cervix may begin to efface │ │
│ └─────────────────────────────────────────────────────────────────────┘ │
│ ↓ │
│ ┌─────────────────────────────────────────────────────────────────────┐ │
│ │ SHEARING FORCES & SEPARATION │ │
│ │ • Micro-separations of placental edge from uterine wall │ │
│ │ • Inability of lower segment muscle fibres to contract/constrict │ │
│ │ vessels (unlike the fundus) │ │
│ └─────────────────────────────────────────────────────────────────────┘ │
│ ↓ │
│ ┌─────────────────────────────────────────────────────────────────────┐ │
│ │ ANTEPARTUM HAEMORRHAGE (APH) │ │
│ │ • Maternal blood loss (mostly) │ │
│ │ • Usually PAINLESS (no retroplacental clot/abruption) │ │
│ │ • "Sentinel Bleed": Minor bleed warning of subsequent major bleed │ │
│ └─────────────────────────────────────────────────────────────────────┘ │
│ │
└─────────────────────────────────────────────────────────────────────────────┘
Risk Factors
- Prior Caesarean Section: Increases risk 4-fold.
- Multiparity: Higher parity = scarred uterus.
- Advanced Maternal Age: (>35y).
- Multiple Pregnancy: Larger placental surface area.
- Smoking: Relative placental hypertrophy due to hypoxia.
- Assisted Reproduction (IVF).
3. Clinical Features
History Taking
Classic Presentation:
- Painless vaginal bleeding in the 3rd trimester.
- Often bright red.
- May be triggered by intercourse.
- Often recurrent.
Contrast with Placental Abruption:
- Praevia: Painless, uterus soft non-tender, fetal heart usually normal (unless shock).
- Abruption: Painful, uterus "woody"/tense, fetal distress common.
Physical Examination
- Abdominal Exam:
- Uterus soft and non-tender.
- High presenting part (head not engaged).
- Abnormal Lie (Transverse or Breech common as placenta prevents engagement).
- Vaginal Exam: CONTRAINDICATED! Never perform a digital vaginal exam (VE) in APH until placenta position is confirmed by ultrasound. You can provoke massive hemorrhage.
- Speculum: Gentle speculum is safe to identify cervical/vaginal causes (ectropion, polyp) but ultrasound is priority.
4. Diagnosis
Diagnostic Imaging
Transvaginal Ultrasound (TVUS) is the Gold Standard.
- Safe (probe stops 2-3cm from cervix) and more accurate than abdominal US.
- Measurement: Distance from placental edge to internal os.
- Placenta Accreta Screen: Essential in women with Praevia + Prior C-Section. Look for loss of "clear zone", bladder wall weakness, lacunae.
Follow-Up Protocol (RCOG)
- Low lying at 20 week Anomaly Scan:
- Rescan at 32 weeks:
- If solved (>20mm): Discharge to potential vaginal birth.
- If still low: Rescan at 36 weeks.
- Rescan at 36 weeks: Final decision for delivery mode.
5. Management Algorithm
┌─────────────────────────────────────────────────────────────────────────────┐
│ PLACENTA PRAEVIA MANAGEMENT (RCOG) │
├─────────────────────────────────────────────────────────────────────────────┤
│ │
│ DIAGNOSIS CONFIRMED (TV Ultrasound) │
│ ↓ │
│ ┌─────────────────────────────────────────────────────────────────────┐ │
│ │ ASYMPTOMATIC (No Bleeding) │ │
│ │ • Manage as Outpatient (if close to hospital, 24/7 support) │ │
│ │ • "Pelvic Rest": No intercourse │ │
│ │ • Education: Come in immediately if ANY bleeding │ │
│ │ • Optimize Hb (Iron supps, target Hb >110) │ │
│ │ • Serial Ultrasounds (32, 36 weeks) │ │
│ └─────────────────────────────────────────────────────────────────────┘ │
│ ↓ │
│ ┌──────────────────────────────────────┐ │
│ │ BLEEDING OCCURS (APH) │ │
│ └──────────────────────────────────────┘ │
│ ↓ Minor Bleed ↓ Major/Massive Bleed │
│ ┌──────────────────────────┐ ┌──────────────────────────────────────┐ │
│ │ ADMIT & STABILIZE │ │ EMERGENCY RESUSCITATION │ │
│ │ • IV access, G&S │ │ • Call 2222 (Obs Code Red) │ │
│ │ • Anti-D (if Rh neg) │ │ • ABC approach │ │
│ │ • Steroids (<34w) │ │ • O-Neg blood / Massive Transfusion │ │
│ │ • Mag Sulphate (<30w) │ │ • Immediate C-Section (Category 1) │ │
│ │ • Admit till 24h bleed- │ │ regardless of gestation if │ │
│ │ free (minimum) │ │ maternal life at risk │ │
│ └──────────────────────────┘ └──────────────────────────────────────┘ │
│ ↓ │
│ ┌─────────────────────────────────────────────────────────────────────┐ │
│ │ PLANNED DELIVERY (ELECTIVE C-SECTION) │ │
│ │ • Timing: 36-37 weeks (Balance prematurity vs bleed risk) │ │
│ │ • Consultant Obstetrician & Anesthetist present │ │
│ │ • Cross-match (4 units) │ │
│ │ • Cell Salvage available │ │
│ └─────────────────────────────────────────────────────────────────────┘ │
│ │
└─────────────────────────────────────────────────────────────────────────────┘
Delivery Planning
- Mode: Caesarean Section is mandatory for Placenta Praevia (covering os).
- Low-lying (10-20mm): Vaginal birth may be attempted with caution/consultant approval, but high risk of bleeding/C-section.
- <10mm: Caesarean usually recommended.
- Timing:
- Asymptomatic: 36+0 to 37+0 weeks.
- Symptomatic (Recurrent bleeds): Individualize (may need earlier).
- Anesthesia: Regional (Spinal/Epidural) is safe and preferred over GA (airway risk), unless massive uncontrolled hemorrhage.
6. The Danger: Placenta Accreta Spectrum (PAS)
Definition: Abnormal invasion of placental trophoblasts into the uterine myometrium (Accreta), deeper muscle (Increta), or surrounding organs like bladder (Percreta).
Highest Risk Scenario:
- Placenta Praevia + Previous C-Section
- Risk increases with number of CS: 1 prior CS (11-25% risk), 2 prior (40%), 3 prior (61%).
Management of PAS:
- Planned delivery at 34-36 weeks in specialist center.
- Multidisciplinary team (Urology, Interventional Radiology).
- Correct consent: High risk of Caesarean Hysterectomy (removing womb to stop bleeding).
7. Prognosis
- Maternal: Good with modern care. Main risk is massive hemorrhage and hysterectomy. VTE risk also high (bed rest).
- Fetal: Risks of prematurity (forced delivery), IUGR (rare), and anemia.
- Vasa Praevia: Fetal vessels running over the os. High mortality (60%) if membranes rupture. Often associated with low-lying placenta.
8. Complications
- Postpartum Haemorrhage (PPH):
- Lower segment does not contract well to compress vessels.
- Placenta may be adherent (Accreta).
- Management: Uterotonics, Bakri balloon, B-Lynch suture, Hysterectomy.
- Preterm Birth: Iatrogenic or spontaneous.
- Hysterectomy: 5% risk (higher if Accreta).
- VTE: Due to immobility/hospitalization.
9. Special Considerations
Anti-D Prophylaxis
- Required for all RhD-negative women with ANY bleeding.
- Kleihauer test determines dose (quantifies Fetomaternal Hemorrhage).
- Re-administer every 6 weeks if bleeding continues (RCOG).
Outpatient Management
Only if:
- No bleeding for >48h.
- Live close to hospital (<20 mins).
- Have transport/phone.
- Compliance with advice (no sex, bed rest).
10. Key Clinical Pearls
Exam-Focused Points
- NO VE: Do not put your finger in.
- Painless: Hallmarks are Painless Bleeding, Soft Uterus, High Head.
- The Accreta Link: Praevia + Prior C-Section = Accreta until proven otherwise.
- Transvaginal US: Is SAFE and accurate. Abdominal US misses up to 20%.
- Timing: Deliver 36-37 weeks. Do not wait for term (40w) as risk of labor/bleed increases.
- Migration: 90% of "low placentas" at 20 weeks move up by 36 weeks. Don't panic early. Re-scan.
- Definition: Placenta <20mm from os = Low Lying. Covering os = Praevia.
Common Exam Scenarios
- 34w pregnant woman, prior C-section, painless bleeding. Next step? (Speculum/US - NO VE).
- 20w scan shows placenta reaching os. Advice? (Rescan at 32w - likely to migrate).
- Massive bleed in A&E with known praevia. Immediate management? (Call 2222, ABC, Category 1 CS).
11. Patient Explanation
What is Placenta Praevia?
"Normally, the placenta (afterbirth) attaches high up in the womb. In your case, it has attached very low down and is covering the 'doorway' (cervix) where the baby needs to come out."
Why is there bleeding?
"As you get closer to your due date, the bottom part of the womb stretches to make room for the baby's head. Since your placenta is stuck there, this stretching can cause the placenta to peel away slightly from the wall, causing bleeding. This is usually painless but can be heavy."
What is the plan?
"Because the way out is blocked, you will need a Caesarean section. We usually plan this for 37 weeks - a bit earlier than normal to avoid you going into labor naturally, which could cause dangerous bleeding. If you have heavy bleeding before then, we may need to deliver the baby immediately."
12. Evidence & Guidelines
Key Guidelines
| Guideline | Organization | Year | Key Points |
|---|---|---|---|
| GTG 27a: Placenta Praevia | RCOG | 2018 | Definitions, outpatient care, delivery timing |
| Placenta Accreta | RCOG / FIGO | 2018 | Screening and multidisciplinary management |
Landmark Data
ACA Trial (Recent):
- Confirmed safety of outpatient management for selected stable patients.
- Showed regional anesthesia is safe for placenta praevia CS (previously GA was standard).
Evidence-Based Recommendations
| Recommendation | Evidence Level |
|---|---|
| Transvaginal US for diagnosis | High |
| Corticosteroids if <34w | High |
| Delivery at 36-37w | Moderate |
| Regional Anesthesia | High |
| Screening for Accreta | High |
13. References
- Jauniaux E, et al. Placenta Praevia and Placenta Accreta: Diagnosis and Management. Green-top Guideline No. 27a. BJOG. 2018;125(10):e34-e60.
- Silver RM, et al. Maternal morbidity associated with multiple repeat cesarean deliveries. Obstet Gynecol. 2006;107(6):1226-1232.
- Fonseca A, et al. Placenta Previa and Placenta Accreta Spectrum. Clin Obstet Gynecol. 2023.