Placenta Previa
Placenta covering or near the internal cervical os — painless bright red bleeding in second/third trimester.
Also known as: placenta previa, low-lying placenta, marginal previa, complete previa
Overview
Placenta implanted over (complete previa) or within 2 cm of (low-lying placenta) the internal cervical os in the late second or third trimester. The 2012 Society of Maternal-Fetal Medicine simplified terminology eliminated 'partial' and 'marginal' in favor of complete vs low-lying.
Epidemiology
Affects ~1 in 200 deliveries at term (many low-lying placentas identified earlier migrate as pregnancy progresses). Risk rises with cesarean delivery history.
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Risk factors
- Prior cesarean delivery (linear with number — 1% after one, ~3% after four)
- Prior placenta previa
- Advanced maternal age (>35)
- Multiparity
- Multifetal gestation
- Smoking, cocaine use
- IVF and assisted reproduction
- Prior uterine surgery (myomectomy, D&C)
Pathophysiology
Implantation in the lower uterine segment, near or over the cervical os, makes the placenta vulnerable to disruption as the lower segment forms and the cervix effaces in late pregnancy. The thin lower segment cannot contract effectively to control bleeding from disrupted placental vessels.
Clinical presentation
Symptoms
- Painless bright red vaginal bleeding in the second or third trimester (classic — sentinel bleed)
- Often unprovoked; can follow intercourse
- Bleeding may resolve spontaneously, recur, or be massive
- Some are asymptomatic and detected on routine ultrasound
Signs / physical exam
- Bleeding without abdominal pain or uterine tenderness
- Soft, non-tender uterus
- Fetal heart tones usually normal initially
- Hemodynamic instability in massive bleeding
Classic findings
Third-trimester woman with painless bright red vaginal bleeding, soft non-tender uterus, and placenta over or near the cervical os on TVUS.
Differential diagnosis
- Placental abruption — Painful bleeding, uterine tenderness, hypertonus; often dark blood; can be concealed; usually NOT preceded by sentinel painless bleed
- Vasa previa — Fetal vessels traversing membranes over cervical os; rupture of membranes → fetal exsanguination; consider with velamentous cord insertion or accessory lobe
- Cervical/vaginal lesion — Polyp, ectropion, cancer, lacerations; speculum exam (gentle, after previa excluded)
- Labor (bloody show) — Small amount of blood-tinged mucus with cervical change
- Uterine rupture — Severe pain, loss of fetal station, fetal distress; prior cesarean scar history
Diagnostic workup
Diagnostic criteria
AVOID digital cervical exam in any patient with third-trimester bleeding until placenta previa is excluded by ultrasound.
Labs
- CBC, blood type and crossmatch, coagulation studies
- Kleihauer-Betke if fetomaternal hemorrhage suspected and mother is Rh-negative
Imaging
- Transabdominal ultrasound first, then transvaginal ultrasound — safe in placenta previa and more accurate; gold standard for diagnosis
- Routine anatomy scan at 18-22 weeks identifies most cases; many low-lying placentas resolve by third trimester
- Repeat ultrasound at 32 weeks for previa identified earlier
- MRI if placenta accreta spectrum suspected (placenta previa + prior cesarean is high-risk)
Diagnostic algorithm
| Feature | Placenta Previa | Placental Abruption |
|---|---|---|
| Bleeding character | Painless, bright red | Often painful, dark |
| Onset | Often sentinel bleed, may resolve | Sudden, may be concealed |
| Uterine tone | Soft, non-tender | Firm, tender, hypertonic |
| Fetal status (initially) | Often reassuring | Often nonreassuring (placental loss) |
| Diagnostic test | TVUS (safe in previa) | Clinical; CT/US may show retroplacental clot (low sensitivity) |
| Coagulopathy | Rare unless massive bleed | Common (especially concealed/severe) |
| Risk factors | Prior cesarean, prior previa, ART, smoking | Hypertension, trauma, cocaine, PROM, smoking |
| Delivery | Cesarean | Vaginal or cesarean based on stability |
Treatment
First-line
- Avoid digital cervical exam, intercourse, vaginal tampons
- Activity restriction (modified — strict bedrest no longer recommended)
- Hemodynamic stabilization with IV access, type and screen, transfusion as needed
- Inpatient management for active bleeding; outpatient with strict precautions and proximity to hospital for asymptomatic patients
- Antenatal corticosteroids (betamethasone) for fetal lung maturity if 23-34 weeks with bleeding
- Anti-D immunoglobulin for Rh-negative women with any bleeding
Delivery planning
- Cesarean delivery is required for placenta previa (complete or with edge <1-2 cm from os in late pregnancy)
- Scheduled cesarean at 36 0/7 - 37 6/7 weeks for uncomplicated previa
- Earlier delivery for recurrent bleeding, signs of preterm labor, or placenta accreta spectrum
- Tertiary center delivery with blood products, anesthesia, and surgical backup
Placenta accreta spectrum (PAS) — when previa + prior cesarean
- Multidisciplinary planning: gyn-onc, MFM, anesthesia, urology, interventional radiology
- Scheduled cesarean hysterectomy at 34 0/7 - 35 6/7 weeks
- Preoperative cell saver, balloon occlusion catheters in select centers
- Massive transfusion protocol availability
Acute hemorrhage
- ABCs, large-bore IV access, type and crossmatch 4+ units
- Continuous fetal monitoring
- If fetus <34 weeks: stabilize, give corticosteroids, expectant management if bleeding stops
- If fetus ≥34 weeks or bleeding uncontrollable: emergent cesarean delivery
- Massive transfusion as needed
Complications
- Antepartum hemorrhage requiring transfusion
- Preterm birth (often iatrogenic for hemorrhage control)
- Placenta accreta spectrum (accreta, increta, percreta) — especially with prior cesarean — peripartum hysterectomy may be required
- Postpartum hemorrhage from poorly contractile lower segment
- Maternal mortality from hemorrhage
- Vasa previa coexistence (rare)
- Cord prolapse
PANCE pearls
- NEVER perform a digital cervical exam in third-trimester bleeding until placenta previa is excluded by ultrasound.
- Transvaginal ultrasound is safe in placenta previa and more accurate than transabdominal scanning.
- Most low-lying placentas identified before 24 weeks 'migrate' as the uterus grows — repeat ultrasound at 32 weeks before final classification.
- Placenta previa + prior cesarean delivery significantly raises placenta accreta spectrum risk (~25-67% with multiple cesareans + previa) — plan for cesarean hysterectomy.
- Cesarean delivery is mandatory for complete placenta previa or low-lying placenta with placental edge within 1-2 cm of the os in late pregnancy.
- Painless bright red bleeding is classic for previa; painful dark bleeding with uterine tenderness suggests abruption.
References
- ACOG CO 764 — ACOG Committee Opinion 764: Medically Indicated Late-Preterm and Early-Term Deliveries
- SMFM 2018 — SMFM Consult Series: Diagnosis and Management of Placenta Accreta Spectrum (Am J Obstet Gynecol 2018)
- ACOG PB 234 — ACOG Practice Bulletin No. 234: Prediction and Prevention of Spontaneous Preterm Birth (relevant for management)
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