Orris

Placetal pervia

Reading File
Finding Sources
Searching PubMed

"placenta previa"[MeSH Terms] AND management

Reading File
Reading File
Reading File
Reading File
Loading Image
Loading Image
Here is a thorough overview of Placenta Previa (note the correct spelling: previa, not pervia), synthesized from multiple authoritative sources.

Placenta Previa

Definition

Placenta previa occurs when the placenta implants over or covers the internal cervical os. It is a leading cause of antepartum hemorrhage.
  • Placenta previa: placenta covers the internal os
  • Low-lying placenta: placental edge is within 2 cm of the internal os but does not cover it
  • If the placental edge is >20 mm from the internal os, position is reported as "normal"
The older classification system (complete, partial, marginal previa) is no longer recommended. Modern practice uses only "previa" vs. "low-lying."
  • Creasy & Resnik's Maternal-Fetal Medicine, p. 1018

Incidence

  • Affects approximately 0.5% of all third-trimester pregnancies
  • Identified on 10% of mid-trimester ultrasounds
  • At least 90% of low-lying placentas identified early in pregnancy resolve by the third trimester ("placental migration" - technically trophotropism, not true migration)

Pathogenesis & Risk Factors

The underlying cause is unknown but involves implantation in the lower uterine segment, often associated with prior endometrial damage or uterine scarring.
Risk FactorIncreased Risk
Previous placenta previa8×
Previous cesarean section1.5-15×
Age >35 years4.73×
Age >40 years9×
Previous suction curettage for abortion1.33×
Multiparity1.1-1.73×
SmokingIncreased
Bleeding mechanism: as the lower uterine segment elongates and the cervix effaces/dilates with advancing gestation or labor onset, marginal placental vessels are torn, causing hemorrhage.
  • Creasy & Resnik's Maternal-Fetal Medicine, p. 1018
  • Rosen's Emergency Medicine, p. 3355

Clinical Features

  • Cardinal symptom: Painless, bright-red vaginal bleeding (classically in the third trimester)
  • "Herald bleeds" - small warning bleeds may precede major hemorrhage
  • In ~10% of cases, no bleeding occurs until onset of labor
  • ~20% of cases have some degree of uterine irritability (usually minor)
  • Critical rule: All patients with painless second-trimester vaginal bleeding should be assumed to have placenta previa until proven otherwise
Do NOT perform digital or instrumental cervical examination before ruling out placenta previa by ultrasound - this can precipitate severe hemorrhage.

Diagnosis

Transvaginal ultrasound (TVUS) is the gold standard - it is safe, accurate, and provides precise measurement of the placenta-to-os distance. The bladder should be emptied before examination to avoid overdiagnosis.
Figure: Suspected placenta previa at 17 weeks (abdominal ultrasound - placenta centrally located over the cervix):
Ultrasound showing suspected placenta previa at 17 weeks - placenta centrally located over the probable cervical location
Figure: MRI of placenta previa - coronal T2-weighted MRI showing low-lying placenta (asterisk) covering the internal os:
MRI coronal T2 image showing placenta previa with low-lying placenta marked with asterisk
MRI is used in equivocal cases or when placenta accreta syndrome is suspected.

Differential Diagnosis

  • Low-lying placenta
  • Placental abruption (painful bleeding vs. painless)
  • Localized uterine contraction mimicking low-lying placenta
  • Placenta accreta
  • Vasa previa
  • Succenturiate lobe
  • Vaginal/cervical lesions (hemorrhoids, polyps, cervicitis)

Associated Conditions

  • Placenta accreta spectrum (abnormal invasion of placental tissue into the myometrium) - risk increases significantly with prior cesarean + anterior previa
  • Vasa previa - umbilical cord vessels traverse fetal membranes overlying the os; rupture can cause rapid fetal exsanguination

Management

Acute hemorrhage (Emergency Department/Initial stabilization)

  1. Two large-bore IV lines + fluid resuscitation
  2. Continuous fetal monitoring
  3. Baseline CBC (hemoglobin), type and crossmatch
  4. Coagulation studies: platelet count, PT, aPTT, fibrinogen (normal in pregnancy: 400-450 mg/dL; <300 mg/dL = significant coagulopathy)
  5. Fresh-frozen plasma or fresh whole blood if coagulopathy develops
  6. Rh-negative patients: 300 µg Rh immune globulin within 72 hours if not yet given at 28 weeks
  7. Urgent obstetric consultation; transfer to obstetric unit

Ongoing management (Obstetric care)

ScenarioManagement
Placental edge >20 mm from osNormal vaginal delivery may be possible
Placenta ≤1 cm from os or covering osCesarean section recommended
Unstable/major hemorrhageEmergency cesarean delivery
Mid-trimester previa, stableExpectant management; repeat ultrasound in third trimester
  • Rosen's Emergency Medicine, pp. 3355-3356
  • Creasy & Resnik's Maternal-Fetal Medicine, p. 1018
  • Grainger & Allison's Diagnostic Radiology, p. 915

Key Points to Remember

  • Painless third-trimester bleeding = placenta previa until proven otherwise
  • Never do digital vaginal exam before ultrasound
  • TVUS is the diagnostic method of choice - it is safe even in previa
  • Old terminology (complete/partial/marginal) is obsolete; use "previa" or "low-lying"
  • Most mid-trimester previas resolve by term (trophotropism)
  • Always check for associated placenta accreta, especially with prior uterine surgery
Recent evidence note: A 2025 meta-analysis (PMID: 40472373) confirmed significantly elevated risk of postpartum hemorrhage in patients with low-lying placenta, reinforcing current guideline recommendations for cesarean delivery when the placental edge is ≤1 cm from the os.
This is a shared conversation. Sign in to Orris to start your own chat.