General pathology · Paediatric pathology
Fetal hydrops: immune and nonimmune !
- Mechanism
The fetus builds up fluid in the skin and several body cavities, nearly always because its heart fails, it is very anaemic or its lymph drainage does not work.
Updated 2026-11-01
What it is
Fetal hydrops is the abnormal accumulation of fluid in two or more fetal compartments: skin oedema, ascites, pleural effusion or pericardial effusion. The immune form is due to maternal antibodies that destroy fetal red cells; today the nonimmune form is far more common, grouping cardiac, chromosomal, anaemic and infectious causes.
Structure
- Oedematous fetus with ascites and effusions; a large, pale, oedematous placenta.
- In the immune form, many erythroblasts in the blood (erythroblastosis fetalis) and extramedullary haematopoiesis in the liver and spleen.
- After birth, unconjugated bilirubin can stain the basal ganglia (kernicterus), because the placenta no longer clears it.
Step by step
- Maternal antibodies or a fetal cause (parvovirus, alpha-thalassaemia) produce severe fetal anaemia.
- Anaemia causes hypoxia; the heart raises its output and ends in high-output heart failure.
- Venous and capillary hydrostatic pressure rise and fluid leaks into tissues and cavities.
- The liver fills with foci of extramedullary haematopoiesis, enlarges and makes less albumin.
- Oncotic pressure falls and oedema worsens; hypoxia injures the endothelium and raises permeability.
- In causes such as Turner syndrome, lymphatic obstruction (cystic hygroma of the neck) is the main mechanism.
Causes of fetal hydrops
| Cause | Group | Main mechanism |
|---|---|---|
| Rh alloimmunisation | Immune | Haemolysis by maternal IgG anti-D; anaemia and heart failure |
| ABO incompatibility | Immune | Haemolysis usually mild; rarely causes hydrops |
| Fetal heart defects and arrhythmias | Nonimmune | Heart failure and high venous pressure |
| Turner syndrome and trisomies | Nonimmune | Lymphatic obstruction and heart defects |
| Alpha-thalassaemia major (haemoglobin Bart's) | Nonimmune | No alpha chains; severe anaemia and haemoglobin that does not release oxygen |
| Congenital infections (parvovirus B19, syphilis, TORCH) | Nonimmune | Parvovirus destroys red cell precursors; liver and heart damage |
| Twin-to-twin transfusion | Nonimmune | Volume overload in the recipient twin |
In the lab
- Indirect Coombs test in the mother to detect antibodies against red cell antigens.
- Positive direct Coombs test and rising bilirubin in the newborn with haemolytic disease.
- Ultrasound to diagnose hydrops and middle cerebral artery Doppler to estimate anaemia.
Test yourself
Think of the answer, then tap the question to see it.
Why does the first Rh-positive child of an Rh-negative mother rarely develop immune hydrops?
Because the mother is usually sensitised during that first delivery; the IgG antibodies attack in later pregnancies.
Which virus causes hydrops by destroying red cell precursors?
Parvovirus B19.
Which mechanism explains the oedema of fetal hydrops in Turner syndrome?
Obstruction of lymphatic drainage, seen as a cystic hygroma of the neck.
Why does kernicterus appear after birth and not in the womb?
Because in the womb the placenta clears bilirubin; after birth the immature liver cannot conjugate it fast enough.
Related
- Edema and effusions: the four mechanisms
- Sex chromosome disorders: Klinefelter and Turner
- Trisomies 21, 18 and 13, and the 22q11.2 deletion
Related conditions
- Rh alloimmunisation in pregnancy
- Neonatal jaundice
- Turner syndrome
- Thalassemias (α and β)
- Congenital (TORCH) infections: syphilis, toxoplasmosis, CMV and Zika
- Syphilis
- Stillbirth (intrauterine fetal death)
Elsewhere in DurgiPedia
- ABO and Rh blood groups, hemolytic disease of the newborn
- Type II hypersensitivity (antibody against cells)
- Direct and indirect Coombs test
- Rh incompatibility
- Starling forces at the capillary
- How edema forms
- Twins: dizygotic and monozygotic, chorionicity, twin–twin transfusion
Microorganisms
In Pharmacology
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Hydrops fetalis: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Turner syndrome: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026).
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Representative model, for educational purposes only.