2.4 Placental Disorders: Previa, Abruption, Vasa Previa & PAS

Comparative Overview of Obstetric Placental Disorders

Third-trimester vaginal bleeding is an obstetric emergency requiring immediate, methodical clinical assessment. The labor and delivery nurse must rapidly differentiate between maternal bleeding disorders (placenta previa, placental abruption, placenta accreta spectrum) and fetal bleeding disorders (vasa previa) to prevent catastrophic maternal-fetal morbidity.

                 [ THIRD-TRIMESTER VAGINAL BLEEDING TRIAGE ]
                                      │
       ┌──────────────────────────────┴──────────────────────────────┐
       ▼                                                             ▼
[ PAINLESS Bright Red Bleeding ]                             [ PAINFUL Dark Red Bleeding ]
• Soft, relaxed, non-tender uterus                           • Rigid, hypertonic "woody" uterus
• Normal baseline resting tone                               • High baseline tone, tachysystole
• FHR usually reassuring initially                           • Recurrent late decels, bradycardia
               │                                                             │
       ┌───────┴───────┐                                                     ▼
       ▼               ▼                                            [ PLACENTAL ABRUPTION ]
[ PLACENTA PREVIA ]  [ VASA PREVIA ]                                • Check Fibrinogen / DIC
• NO DIGITAL EXAM!   • Occurs at ROM                                • 2 large-bore IV lines
• Confirm via US     • Immediate Fetal                              • MTP readiness
• Plan C/S at 36–37w   Bradycardia / Exsanguination                  • Urgent delivery if compromised

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1. Placenta Previa

• Definition & Classifications: Placental tissue implants over or adjacent to the internal cervical os in the lower uterine segment:
  • Complete Previa: The placenta completely covers the internal cervical os.
  • Marginal Previa: The placental edge extends to within <2.0 cm of the internal cervical os but does not cover it.
  • Low-Lying Placenta: The placental edge is located 2.0 to 3.5 cm from the internal cervical os.
• Pathophysiology & "Migration": In early pregnancy, up to 4% of gestations exhibit a low-lying placenta. Due to differential growth and elongation of the muscular lower uterine segment in the late second and third trimesters, >90% of early previas "migrate" away from the cervix (trophotropism) by term.
• Risk Factors: Prior cesarean delivery (scar tissue impairs fundal implantation), prior uterine curettage/myomectomy, multiparity, advanced maternal age ($\ge 35\text{ years}$), cigarette smoking, cocaine use, multifetal gestation (larger placental surface area), and prior placenta previa (4- to 8-fold increased recurrence).
• Clinical Presentation: Classic hallmark is PAINLESS, bright red vaginal bleeding in the second or third trimester. The uterus remains soft, non-tender, and relaxed with normal resting tone. Fetal heart rate is typically reassuring unless severe maternal hypovolemic shock occurs.
• Management Guidelines:
  • STRICT CONTRAINDICATION: No digital vaginal examinations under any circumstance until ultrasound definitively confirms placental location.
  • Pelvic Rest: Strict avoidance of sexual intercourse, vaginal exams, and strenuous physical exertion.
  • Delivery Timing: Scheduled cesarean delivery is recommended at 36 0/7 to 37 6/7 weeks (or earlier if persistent bleeding or hemodynamic instability occurs).

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2. Placental Abruption (Abruptio Placentae)

• Definition: The premature separation of a normally implanted placenta from the uterine decidua basalis prior to the delivery of the fetus.
• Classifications:
  • Revealed (External) Abruption (~80%): Blood dissects downward between the fetal membranes and decidua, exiting through the cervix.
  • Concealed (Internal) Abruption (~20%): Blood remains trapped behind the placenta (retroplacental hematoma) with no visible external bleeding. Represents the highest clinical risk because bleeding severity is masked.
• Risk Factors: Maternal hypertension / preeclampsia (strongest chronic factor), sudden abdominal trauma (MVA, domestic violence), cocaine or methamphetamine use (acute vasospasm), cigarette smoking, sudden uterine decompression (rapid amniotic fluid loss in polyhydramnios or delivery of first twin), and history of prior abruption (10-fold increased risk).
• Clinical Hallmark Signs:
  • PAINFUL, dark red vaginal bleeding (or severe pain with no visible bleeding in concealed abruption).
  • Severe continuous uterine/abdominal pain and back pain.
  • Uterine hypertonicity and board-like rigidity ("woody" abdomen) with high resting tone (>20–25 mmHg on IUPC) and frequent, low-amplitude uterine contractions (uterine tachysystole).
  • Fetal Distress: Recurrent late decelerations, loss of variability, severe prolonged decelerations, and fetal bradycardia.
• Couvelaire Uterus: Massive retroplacental hemorrhage dissects into the myometrial smooth muscle bundles and beneath the uterine serosa, giving the uterus a dark, ecchymotic, purple/copper appearance. Severely impairs myometrial contractility, causing refractory postpartum hemorrhage due to uterine atony.
• Disseminated Intravascular Coagulation (DIC): Decidual trauma releases massive quantities of tissue factor (thromboplastin) into maternal circulation, activating the extrinsic coagulation cascade. Fibrinogen, platelets, and clotting factors are rapidly consumed. A maternal serum fibrinogen level <200 mg/dL in pregnancy is abnormal (normal pregnancy fibrinogen is 400–600 mg/dL) and indicates early consumptive coagulopathy; <100–150 mg/dL reflects severe DIC.

Emergency Nursing Interventions for Abruption

1. Establish large-bore IV access: Two 16- or 18-gauge peripheral IV lines immediately.
2. Stat Laboratory Workup: Type and crossmatch (4–6 units PRBCs), CBC, PT/INR, PTT, Fibrinogen, D-dimer, and Kleihauer-Betke test.
3. Continuous Maternal-Fetal Surveillance: Continuous EFM and continuous maternal vital signs monitoring.
4. Massive Transfusion Protocol (MTP) Readiness: Prepare 1:1:1 balanced resuscitation (PRBCs, FFP, Platelets) and Cryoprecipitate (to restore fibrinogen >150–200 mg/dL).
5. Immediate Delivery Planning: Emergent cesarean delivery for maternal/fetal compromise, or rapid controlled vaginal delivery if fetal demise has occurred and maternal status is stabilized.

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3. Vasa Previa

• Pathophysiology: Fetal blood vessels traverse the fetal membranes across the internal cervical os, unsupported by either the umbilical cord Wharton's jelly or placental tissue.
• Underlying Anatomical Variations:
  • Velamentous Cord Insertion: The umbilical cord inserts into the amniotic membranes rather than the placental mass; vessels travel through membranes to reach the placenta.
  • Succenturiate Placental Lobe: An accessory placental lobe connected to the main placenta by membranous fetal vessels traversing the internal os.
• Clinical Presentation: PAINLESS vaginal bleeding at the EXACT MOMENT of membrane rupture (spontaneous rupture or artificial amniotomy), immediately followed by catastrophic fetal bradycardia, sinusoidal pattern, or rapid fetal exsanguination.
  • Critical Fact: The bleeding is 100% fetal blood. Total fetal-placental blood volume is only ~80–100 mL/kg (~250–300 mL total at term); a loss of even 50 to 100 mL can cause fatal fetal hypovolemic shock within minutes.
• Management: Prenatally diagnosed vasa previa requires elective hospitalization at 30 to 34 weeks, antenatal corticosteroids, and planned scheduled cesarean delivery at 34 0/7 to 35 6/7 weeks (prior to the onset of labor or membrane rupture).

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4. Placenta Accreta Spectrum (PAS)

Placenta Accreta Spectrum refers to the pathologic adherence and invasion of placental trophoblasts into the uterine myometrium due to partial or complete absence of the decidua basalis and Nitabuch's layer.

                  [ PLACENTA ACCRETA SPECTRUM DEPTH OF INVASION ]

  1. Placenta Accreta (~75%)     Placenta attached directly to superficial myometrium;
                                 no muscular invasion.

  2. Placenta Increta (~15%)     Chorionic villi invade deeply into myometrial muscle wall.

  3. Placenta Percreta (~5–10%)  Chorionic villi penetrate COMPLETELY through myometrium,
                                 invading uterine serosa and adjacent pelvic organs (bladder).

• Major Risk Factors: The combination of prior cesarean delivery AND current placenta previa creates the highest risk for PAS:
  • 1 prior CD + previa: ~11% risk
  • 2 prior CDs + previa: ~40% risk
  • 3 prior CDs + previa: ~61% risk
  • $\ge 4$ prior CDs + previa: >67% risk
• Surgical Management Protocol: Multidisciplinary team in a Level III/IV perinatal center (MFM, Gynecologic Oncology, Obstetric Anesthesia, Interventional Radiology, Blood Bank). Scheduled cesarean hysterectomy at 34 0/7 to 35 6/7 weeks. DO NOT attempt manual removal of the placenta upon delivery of the fetus, as forced separation triggers catastrophic, life-threatening maternal hemorrhage.

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Master Differentiation Table of Obstetric Placental Disorders