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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Classic presentation: painless bright-red vaginal bleeding after 20 weeks
- Do not perform digital cervical examination until previa excluded by ultrasound
- Transvaginal ultrasound is safe and most accurate for placental location
- Risk factors include prior cesarean, prior previa, multiparity, multiple gestation, smoking, and advanced maternal age
- Persistent previa near term requires cesarean delivery
Overview
Placenta previa occurs when placental tissue overlies or approaches the internal cervical os. It is a key cause of second- and third-trimester bleeding. The Step 2 CK safety rule is to avoid digital cervical exam until ultrasound excludes previa.
Epidemiology
Risk increases with prior cesarean delivery and with the number of prior cesareans. Previa over a uterine scar raises concern for placenta accreta spectrum.
Clinical Features
Symptoms
Painless bright-red bleeding after 20 weeks
Recurrent bleeding unrelated to contractions
No abdominal pain or uterine tenderness in typical cases
Heavy bleeding, syncope, or dizziness
Contractions may occur after bleeding
Signs
Soft nontender uterus
Fetal malpresentation is more common
Maternal tachycardia or hypotension if severe
Nonreassuring fetal status with major hemorrhage
Digital exam contraindicated until previa excluded
Investigations
First-line
Transvaginal ultrasoundMost accurate and safe test for placental location
Maternal stabilization labsCBC, type/screen or crossmatch, coags if heavy bleeding
Fetal monitoringContinuous monitoring if viable and active bleeding
Second-line
Transabdominal ultrasoundOften initial but less accurate for lower placental edge
Accreta assessmentPlacental lacunae, loss of clear zone, myometrial thinning, abnormal bladder interface
Rh testingRh-negative unsensitized patients with bleeding require Rh(D) immune globulin
Specialist
MRI pelvisAdjunct if accreta suspected and ultrasound indeterminate
MFM consultationPersistent previa, suspected accreta, recurrent bleeding, or preterm delivery planning
1
Initial bleeding management
- Assess maternal hemodynamics and fetal status
- Avoid digital cervical exam until previa excluded
- Large-bore IV access, CBC, type/crossmatch, blood products if needed
- Give Rh(D) immune globulin if Rh-negative and unsensitized
2
Expectant management
- If bleeding resolves and status reassuring, manage expectantly with precautions
- Antenatal corticosteroids if preterm delivery risk before 34 weeks
- Repeat ultrasound because low-lying placenta may resolve
3
Delivery
- Persistent placenta previa requires cesarean delivery
- Timing is individualized in late preterm/early term window if stable
- Immediate cesarean for uncontrolled hemorrhage or nonreassuring fetal status
- Suspected accreta requires planned delivery at center with blood bank and surgical expertise
Complications
- Maternal hemorrhage: Can be recurrent and severe
- Preterm birth: Often from bleeding or indicated delivery
- Placenta accreta spectrum: Especially previa over prior cesarean scar
- Malpresentation: Breech or transverse lie is more common
- Postpartum hemorrhage: Lower uterine segment contracts poorly and accreta may coexist
USMLE Step 2 CK Exam Tips
- 1Painless third-trimester bleeding = placenta previa until proven otherwise
- 2Painful bleeding + tender uterus = abruption
- 3Never perform digital cervical exam before ultrasound excludes previa
- 4Transvaginal ultrasound is safe and best
- 5Prior cesarean + previa = accreta concern
- 6Persistent previa near term = cesarean delivery
- 7Rh-negative bleeding = Rh(D) immune globulin
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