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Placentomegaly causes

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Placentomegaly — Causes

Definition: Placentomegaly refers to an abnormally large or heavy placenta — defined as >95th percentile for gestational age by weight, or >4 cm in thickness on ultrasound.

Major Causes

1. Hydrops Fetalis (Immune & Nonimmune)

The most significant cause. Placentomegaly is a hallmark finding in both forms of hydrops:
  • Immune hydrops — Rh(D) incompatibility or other blood group antigens (Kell, Duffy, Kidd, etc.). Maternal IgG crosses the placenta → fetal hemolytic anemia → high-output cardiac failure → placental edema and enlargement.
  • Nonimmune hydrops (NIHF) — now the more common cause since Rh prophylaxis. Main subcategories:
    • Fetal anemia — parvovirus B19 (lytic infection of erythroid progenitors), alpha-thalassemia, fetal-maternal hemorrhage
    • Cardiovascular defects — structural anomalies, fetal arrhythmias (SVT, complete heart block)
    • Chromosomal abnormalities — Turner syndrome (45,X → cystic hygroma + hydrops), trisomy 21, trisomy 18
    • Lymphatic dysplasia — mutations (e.g., hCALCRL)
    • Infections — CMV, syphilis, toxoplasmosis

2. Maternal Diabetes Mellitus

Poorly controlled diabetes leads to fetal macrosomia with placental overgrowth. Villous edema and increased vascularity from chronic hyperglycemia contribute to the enlarged placenta.

3. Congenital Infections (TORCH + Syphilis)

  • Syphilis — classically causes a large, pale placenta with villous edema and placentitis; the ratio of placental weight to birth weight can approach 1:3 (normal ~1:6)
  • CMV — symptomatic fetuses may show placentomegaly alongside growth restriction, microcephaly, echogenic bowel, and intracerebral calcifications
  • Parvovirus B19 — via severe fetal anemia → hydrops and placentomegaly
  • Other TORCH infections (toxoplasmosis, rubella, HSV)

4. Sacrococcygeal Teratoma (SCT)

Large SCTs cause high-output cardiac failure through arteriovenous shunting within the tumor mass → placentomegaly with or without hydrops. Placentomegaly in this context is a poor prognostic sign (survival 0/9 in one series — Creasy & Resnik's).

5. Gestational Trophoblastic Disease

  • Partial hydatidiform mole — enlarged placenta with anechoic/cystic lesions on ultrasound
  • Complete hydatidiform mole — gross placental enlargement with hydropic villous change

6. Beckwith-Wiedemann Syndrome

Overgrowth syndrome associated with placental enlargement, often with mesenchymal dysplasia of the placenta.

7. Maternal Anemia / Fetal Anemia (Non-Immune)

Severe maternal or fetal anemia (e.g., alpha-thalassemia major) causes compensatory placental hypertrophy.

8. Twin-to-Twin Transfusion Syndrome (TTTS)

Vascular anastomoses in monochorionic twins lead to discordant growth; the recipient twin's placental share may show enlargement associated with volume overload and cardiac failure.

9. Placental Mesenchymal Dysplasia

A rare condition mimicking partial mole on imaging, with stem villous enlargement and abnormal vasculature; often associated with Beckwith-Wiedemann syndrome or androgenetic/biparental mosaicism.

10. Congenital Nephrotic Syndrome (Finnish Type)

Loss of plasma proteins in utero leads to hypoalbuminemia → fetal edema + large placenta (placental weight can be 25–40% of birth weight).

Summary Table

CategoryExamples
Immune hydropsRh incompatibility, Kell, Duffy
Nonimmune hydropsChromosomal anomalies, cardiac defects, fetal arrhythmia
Fetal anemiaParvovirus B19, alpha-thalassemia, fetal-maternal hemorrhage
InfectionsSyphilis, CMV, parvovirus B19
Maternal diseaseDiabetes mellitus
Fetal tumorsSacrococcygeal teratoma
Trophoblastic diseaseComplete/partial hydatidiform mole
Genetic syndromesBeckwith-Wiedemann, congenital nephrotic syndrome
Multiple gestationTTTS (recipient twin side)
Placental pathologyPlacental mesenchymal dysplasia, chorangioma
Ultrasound threshold: Placentomegaly is generally defined as placental thickness >4 cm on second/third trimester ultrasound.
Key association: Placentomegaly occurring alongside fetal hydrops carries a particularly poor prognosis — prompt evaluation for the underlying cause is essential as some (e.g., SVT, parvovirus B19-related anemia) are treatable with fetal intervention.
— Creasy & Resnik's Maternal-Fetal Medicine: Principles and Practice; Robbins & Kumar Basic Pathology
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