Placenta Previa
Diagnosis, hemorrhage readiness & delivery planning
Chukwuma Onyeije, MD, MFM | OpenMFM Clinical Education
What Is Placenta Previa?
The placenta implants over or adjacent to the internal cervical os, obstructing the birth canal.
Incidence is rising in parallel with global cesarean delivery rates.
Normal Placenta vs. Previa
Left: cervix unobstructed. Right: placenta covers the internal os, preventing safe vaginal delivery.
Risk Factors
⚠ Highest Risk
Prior cesarean delivery — dose-dependent; each additional scar increases risk substantially.
Significant Risk
Advanced maternal age · Multiparity · Multiple gestation · ART conception
Uterine History
Prior myomectomy, uterine curettage, or endometrial ablation
Lifestyle Factors
Maternal smoking · Cocaine use
Classic Presentation
Sudden · Painless · Bright Red
Vaginal bleeding in the 2nd or 3rd trimester. No uterine contractions. No abdominal pain.
Before Any Cervical Exam
ACOG SMFM Guideline-informed clinical safety principle
Transvaginal Ultrasound (TVUS)
- Gold-standard diagnostic modality — safe and highly accurate
- Measures distance from placental edge to internal os
- Placenta previa that overlies the os requires planned cesarean delivery
- Low-lying placenta: mode of birth depends on internal-os distance, bleeding history, and local protocol
- Transabdominal US may miss low-lying placentas — TVUS preferred
TVUS
Sensitivity & specificity approach 99% for placenta previa diagnosis
Follow-Up Imaging Timeline
Many early previas resolve via trophotropism (lower uterine segment elongation).
Shortened cervical length in complete previa correlates with increased hemorrhage risk and emergent preterm delivery.
Screening for PAS
High-Risk Profile
Anterior placenta previa + history of prior cesarean delivery
Sonographic features suggesting PAS:
- Loss of retroplacental clear zone
- Placental lacunae (Swiss-cheese appearance)
- Abnormal color Doppler at bladder-uterine interface
- Placental bulge into bladder wall
Additional Evaluation
Assess cord insertion for velamentous insertion and vasa previa — highly prevalent with abnormal placentation and ART.
Maternal Risks
Antepartum Hemorrhage
May necessitate hospitalization and blood transfusion
Postpartum Hemorrhage
Poor lower uterine segment contractility impairs hemostasis
Placenta Accreta Spectrum
Abnormal trophoblastic invasion; risk increases with each prior cesarean
Peripartum Hysterectomy
May be required for uncontrolled hemorrhage or PAS
Perinatal & Neonatal Risks
Preterm Birth
Iatrogenic or spontaneous; primary driver of neonatal morbidity
Growth Surveillance
Assess fetal growth when additional placental or maternal risk factors are present
Neonatal Morbidity
Lower APGAR scores; complications related to prematurity
Care Before Delivery
Exam Safety
Avoid digital cervical examination until previa is excluded; individualize pelvic-rest counseling after bleeding
💊 Iron Optimization
Maximize maternal hemoglobin to buffer anticipated surgical blood loss at delivery
Antenatal Corticosteroids
Administer when preterm delivery is anticipated and the patient meets current gestational-age criteria
🏥 Hospitalization
Indicated for multiple bleeding episodes, hemodynamic instability, or poor access to emergency care
Timing & Mode of Delivery
Scheduled Cesarean Delivery
For asymptomatic, uncomplicated placenta previa
ACOG SMFM
⚡ Deliver Immediately If:
Active uncontrolled hemorrhage · Non-reassuring fetal status · Onset of labor
Surgery must be performed at a center with a massive transfusion protocol and immediate availability of pRBC, FFP, and platelets.
Hemorrhage Control Interventions
For placenta previa complicated by PAS — multidisciplinary planning is critical.
Endovascular Procedures
Balloon occlusion is not routine; consider only within experienced PAS teams after case-specific review
Hemorrhage Escalation
Embolization may be an adjunct in selected cases when resources, anatomy, and clinical stability permit
Antepartum Bleeding: Comparison
| Feature | Placenta Previa | Placental Abruption | Vasa Previa | Accreta Spectrum |
|---|---|---|---|---|
| Bleeding | Painless, bright red | Painful, dark red, continuous | Painless, on membrane rupture | Painless, bright red |
| Uterine Tone | Soft, non-tender | Rigid, tetanic | Soft, non-tender | Soft, non-tender |
| Fetal HR | Normal until severe shock | Often non-reassuring | Acute severe bradycardia | Normal until maternal shock |
| Delivery | Cesarean | Vaginal or cesarean | Emergent cesarean | Cesarean ± hysterectomy |
What Patients Need to Know
✅ Reassurance
Many low-lying placentas identified at 18–22 weeks resolve by the third trimester.
📞 When to Call
Any vaginal bleeding, cramping, or contractions — go to labor & delivery immediately.
🚫 Activity Restrictions
Avoid digital examination until your care team confirms placental location; ask for individualized activity guidance.
🤝 Partnership
Close monitoring, scheduled delivery, and a prepared surgical team optimize outcomes for mother and baby.
Key Clinical Takeaways
- TVUS is the gold-standard diagnostic tool — safe and highly accurate
- Never perform a digital cervical exam before ruling out previa with ultrasound
- Schedule cesarean delivery at 36 0/7 – 37 0/7 weeks for uncomplicated cases
- Screen all anterior previas with prior cesarean for Placenta Accreta Spectrum
- Deliver at a center prepared for massive hemorrhage with multidisciplinary support
- Use antenatal corticosteroids when anticipated preterm delivery meets current eligibility criteria
Clinical framework informed by ACOG and SMFM guidance; apply current institutional protocols and patient-specific judgment.