Placenta Previa

Definition

• Creasy & Resnik's Maternal-Fetal Medicine, p. 1018
• Grainger & Allison's Diagnostic Radiology, p. 915

Incidence

• Affects ~0.5% of all third-trimester pregnancies
• Diagnosed in ~10% of mid-trimester sonograms
• At least 90% of cases identified as low-lying or previa in early pregnancy resolve by the third trimester - a phenomenon called "placental migration" (trophotropism toward the fundus with regression of the lower portion)

Risk Factors

Pathophysiology

1. The lower uterine wall elongates with advancing gestation
2. The cervix effaces and dilates in prelabor/labor
3. Intercourse or vaginal examination disrupts the cervical-placental junction

Clinical Features

• Classic presentation: Painless, bright-red vaginal bleeding in the second or third trimester - often sudden and without preceding trauma
• Several small "herald bleeds" may precede major hemorrhage
• In ~20% of cases, mild uterine irritability is present
• In up to 10% of cases, there is no bleeding until labor onset
• NEVER perform digital or speculum vaginal examination until placenta previa is excluded by ultrasound - this can precipitate catastrophic hemorrhage

Diagnosis

• Safe, accurate, and provides precise measurement of the placental edge-to-os distance
• The bladder must be emptied first (a full bladder can compress the lower segment and mimic or exaggerate previa)
• Abdominal US at mid-trimester may suggest previa but TVUS is required for definitive diagnosis

Fig. 27.23 - Suspected placenta previa at 17 weeks. Abdominal image. The internal os relationship requires TVUS for confirmation. (Creasy & Resnik's MFM)

Fig. 35.52 - Placenta previa on MRI. Coronal T2 image: low-lying placenta () covering the internal os. MRI is used in equivocal cases or when placenta accreta syndrome is suspected. (Grainger & Allison's Diagnostic Radiology)*

Differential Diagnosis

• Low-lying placenta (not covering the os)
• Placental abruption (typically painful bleeding, board-like uterus)
• Vasa previa
• Uterine contraction mimicking low-lying placenta
• Succenturiate lobe
• Local cervical/vaginal lesions (hemorrhoids, cervical ectropion)

Associated Conditions

• Placenta accreta spectrum - always screen for this when previa is present, especially with prior cesarean scars
• Vasa previa - umbilical vessels in the membranes overlying the os; risk of fetal exsanguination at membrane rupture

Management

Acute/Emergency (active bleeding)

1. Two large-bore IV lines + IV fluid resuscitation
2. Labs: CBC, coagulation studies (PT/PTT, fibrinogen, platelets), type and crossmatch
   • Normal fibrinogen in pregnancy is 400-450 mg/dL; values below 300 mg/dL indicate significant coagulopathy
3. 4 units type-specific or O-negative blood made immediately available; 2 units FFP thawed
4. Continuous fetal monitoring
5. Rh immune globulin (300 mcg) if Rh-negative and not yet administered
6. Obstetric consultation immediately; transfer to appropriate facility if needed

Expectant Management (stable, preterm)

• Hospital admission until bleeding-free for at least 48 hours; then possible outpatient management in carefully selected, stable patients
• Betamethasone for fetal lung maturity if <34 weeks
• Pelvic rest (avoid intercourse, vigorous activity)
• Tocolytics (beta-mimetics or magnesium sulfate) can be used if uterine contractions are present - associated with pregnancy prolongation by ~4 weeks after initial bleed
• Cervical cerclage is NOT recommended (no clear benefit)
• A second significant bleed usually warrants readmission until delivery

Delivery

• Cesarean section is indicated for all women with placenta within 2 cm of the cervical os at term
• A woman whose placenta is more than 2 cm from the os may labor safely
• Delivery should be performed by the most experienced team available given the high risk of intraoperative hemorrhage
• Lower segment transverse incision is standard; if the placenta is anterior, the cord should be clamped immediately after uterine entry; a vertical incision may be preferable in some anterior cases
• Planned delivery at 36-37 weeks is typical for stable previa (risk of hemorrhage vs. prematurity balanced)

Complications

• Antepartum hemorrhage - leading cause
• Postpartum hemorrhage (lower segment atony even without intrapartum bleeding)
• Placenta accreta spectrum (especially with prior uterine scars)
• Preterm birth
• Malpresentation (transverse lie, breech) - placenta occupies the lower segment
• Maternal death (rare in modern obstetrics)
• Perinatal mortality from preterm delivery or fetal exsanguination

Key Clinical Pearls

• Never do a digital vaginal exam in unexplained second/third trimester bleeding until US excludes previa
• TVUS is safe and more accurate than transabdominal US - empty the bladder first
• Most mid-trimester previas resolve - only ~0.5% persist to the third trimester
• Placenta previa + prior cesarean = high risk for placenta accreta - plan delivery at a center with blood bank and surgical expertise
• Low-lying placenta still increases postpartum hemorrhage risk due to lower segment atony