Fetal Hydrops

1. Definition

Fetal hydrops is accumulation of edema fluid in the fetus during intrauterine growth.

• Localized edema — may be compatible with life.
• Generalized edema (hydrops fetalis) — fluid in ≥2 compartments (ascites, pleural/pericardial effusion, skin edema, ± placental edema); often lethal.

2. Two categories

Immune hydrops	Non-immune hydrops (more common today)
Antibody-induced hemolytic disease (Rh, ABO incompatibility)	Cardiac failure/arrhythmia, severe anemia, hypoproteinemia, chromosomal, infection

3. Immune hydrops (hemolytic disease of the newborn)

• Rh incompatibility — Rh⁺ fetus, Rh⁻ mother; fetal RBCs sensitize mother → maternal IgG crosses placenta → fetal RBC destruction → progressive anemia → ischemia/high-output cardiac failure → edema. Prevented by anti-D (RhoGAM) after delivery of an Rh⁺ baby.
• ABO incompatibility — milder; group A/B infant of group O mother; usually IgM (can’t cross) but some IgG → mild hemolysis (even in first pregnancy).

4. Non-immune hydrops (most common)

• Cardiovascular — malformations, tachyarrhythmia → high-output failure.
• Chromosomal — Turner syndrome, trisomy 21/18.
• Fetal anemia — parvovirus B19 (infects RBC precursors → aplastic anemia), homozygous α-thalassemia.
• Twin-to-twin transfusion; infection (CMV, syphilis, toxoplasmosis); tumors; metabolic disorders.

5. Morphology & clinical course

• Pale fetus/placenta (anemia); hepatosplenomegaly (cardiac failure + extramedullary hematopoiesis); immature erythroid precursors in circulation (erythroblastosis fetalis).
• Hemolysis → hyperbilirubinemia → CNS deposition → kernicterus (yellow basal ganglia).
• Diagnosis: prenatal ultrasound (effusions/edema), MCA Doppler for fetal anemia, positive antiglobulin (Coombs) test; severe anemia → intrauterine transfusion.

💡 High-yield: Hydrops = fluid in ≥2 compartments. Immune = Rh/ABO hemolytic disease → IgG crosses placenta → anemia + kernicterus; prevent with anti-D. Non-immune (most common) = cardiac, parvovirus B19, Turner, twin-twin transfusion. Workup: MCA Doppler, Coombs; treat severe anemia with intrauterine transfusion.