Definition/General
Introduction:
Placental previa is the abnormal implantation of the placenta in the lower uterine segment
The placenta partially or completely covers the internal cervical os
It occurs in 0.3-0.8% of pregnancies
It is a major cause of antepartum hemorrhage
The condition requires cesarean delivery in most cases.
Origin:
Results from abnormal implantation of the blastocyst in the lower uterine segment
May be due to defective decidualization
Associated with endometrial scarring
Previous uterine surgery increases risk
Maternal age and parity are contributing factors.
Classification:
Classified by relationship to cervical os: Complete previa (placenta covers entire os)
Partial previa (placenta partially covers os)
Marginal previa (placental edge at os margin)
Low-lying placenta (within 2 cm of os)
Modern classification uses distance from os.
Epidemiology:
Incidence: 0.3-0.8% of pregnancies
Risk increases with maternal age
Multiparity increases risk
Previous cesarean section: 2-3x risk
Smoking increases risk by 2x
More common in multiple pregnancies.
Clinical Features
Presentation:
Painless vaginal bleeding (classic triad)
Bleeding typically occurs in third trimester
Bright red blood
Soft, non-tender uterus
Fetal malpresentation common
Recurrent bleeding episodes.
Symptoms:
Painless bleeding (pathognomonic)
Bleeding may be intermittent
No abdominal pain
No uterine contractions initially
May have iron deficiency anemia
Fetal movements usually normal.
Risk Factors:
Previous cesarean section (major risk)
Advanced maternal age (>35 years)
Multiparity (grand multiparous)
Previous placenta previa
Multiple pregnancy
Smoking
Cocaine use
Previous uterine curettage
Assisted reproductive technology.
Screening:
Transvaginal ultrasound (gold standard)
Routine second trimester screening
Serial ultrasounds if diagnosed early
MRI for suspected accreta
Digital examination contraindicated.
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Gross Description
Appearance:
Placenta located in lower uterine segment
Thin, poorly formed lower segment under placenta
Increased placental surface area
Areas of infarction may be present
Adherent placenta in accreta cases.
Characteristics:
Enlarged placental size
Irregular placental margins
Thinned myometrium beneath placenta
Vascular congestion
Areas of hemorrhage and infarction.
Size Location:
Placenta extends to or beyond internal cervical os
May involve anterior or posterior wall
Central location over os in complete previa
Asymmetric implantation common.
Multifocality:
Bilobed placenta may occur
Succenturiate lobes in lower segment
Vasa previa association
Circumvallate placenta more common.
Microscopic Description
Histological Features:
Normal villous architecture in most cases
Increased syncytial knots
Villous infarction in areas of poor perfusion
Intervillous fibrin deposition
Decidual thinning in lower segment.
Cellular Characteristics:
Syncytiotrophoblast shows maturation appropriate for gestational age
Cytotrophoblast may be prominent
Extravillous trophoblast invasion variable
Decidual cells may be sparse.
Architectural Patterns:
Terminal villi appropriate for gestation
Stem villi with normal branching
Intervillous space may show congestion
Maternal vessels may show thrombosis.
Grading Criteria:
Complete previa: Placenta covers entire os
Partial previa: Placenta partially covers os
Marginal previa: Edge at os margin
Low-lying: Within 2 cm of os
Distance measurement from os edge.
Immunohistochemistry
Positive Markers:
Cytokeratin (trophoblast identification)
hPL (human placental lactogen)
Beta-hCG (syncytiotrophoblast)
CD68 (Hofbauer cells)
Factor VIII (endothelial cells).
Negative Markers:
No specific negative markers
Normal trophoblast markers present
Smooth muscle actin in vessel walls
Desmin in muscle fibers.
Diagnostic Utility:
Limited role in routine diagnosis
Trophoblast markers useful in accreta spectrum
Vascular markers assess invasion depth
Clinical imaging is diagnostic.
Molecular Subtypes:
Isolated previa (no other abnormalities)
Previa with accreta (adherent placenta)
Previa with vasa previa
Associated anomalies.
Molecular/Genetic
Genetic Mutations:
No specific mutations identified
Maternal factors more important
Endometrial receptivity genes may be involved
Angiogenesis factors may play role.
Molecular Markers:
VEGF expression (angiogenesis)
PlGF levels (placental growth factor)
AFP elevation possible
Inhibin levels may be altered.
Prognostic Significance:
Associated with accreta spectrum disorders
Risk of hemorrhage
Preterm delivery common
Neonatal outcomes generally good with appropriate management.
Therapeutic Targets:
Antenatal corticosteroids
Iron supplementation
Blood product availability
Planned cesarean delivery
Uterine artery embolization if needed.
Differential Diagnosis
Similar Entities:
Placental abruption (painful bleeding)
Vasa previa (fetal vessel rupture)
Cervical/vaginal causes of bleeding
Uterine rupture
Marginal sinus rupture.
Distinguishing Features:
Previa: Painless bleeding
Soft uterus
Abnormal placental location
Abruption: Painful bleeding
Uterine tenderness
Normal placental location
Vasa previa: Fetal distress
Membrane rupture triggers bleeding.
Diagnostic Challenges:
Early pregnancy diagnosis may resolve
Posterior previa harder to diagnose
Maternal obesity limits visualization
Multiple pregnancy complexity.
Rare Variants:
Circumvallate previa
Bilobed previa
Previa with vasa previa
Previa with accreta spectrum.
Sample Pathology Report
Template Format
Sample Pathology Report
Complete Report: This is an example of how the final pathology report should be structured for this condition.
Specimen Information
Placenta with membranes and cord, weighing [X] grams, from [gestational age] weeks
Diagnosis
Placenta previa, [complete/partial/marginal/low-lying]
Clinical History
Antepartum hemorrhage, placenta previa diagnosed by ultrasound
Gross Findings
Placenta measures [X] cm, located over/near cervical os, [normal/increased] thickness
Microscopic Findings
Villous architecture appropriate for gestational age, [normal/increased] syncytial knots
Placental Location
Placenta [completely covers/partially covers/at margin of] internal cervical os
Associated Findings
Accreta: [present/absent], Infarction: [X]%, Chorioamnionitis: [present/absent]
Complications
Maternal hemorrhage, need for blood transfusion, [other complications]
Final Diagnosis
Placenta previa, [type], delivered at [X] weeks gestation