Reproductive · PANCE / PANRE

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

Distinguishing placenta previa from placental abruption.

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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