Fetal anaemia and hydrops fetalis

It is the 20-week morphology scan. The registrar calls you to the room: the fetus has ascites, a pericardial effusion, and skin oedema. "Is this viable?" The honest answer is that you do not know yet — because hydrops is a final common pathway, and the prognosis depends entirely on what is driving it. A supraventricular tachycardia can resolve with transplacental digoxin; a parvovirus aplastic crisis can be transfused; an anti-D titre at the critical level can be managed with intrauterine transfusion. This topic teaches you to find the cause before you answer the question.^[2][3]

Overview and definition

Hydrops fetalis is defined by what you see, not why it happens: two or more abnormal fetal extravascular fluid collections — ascites, pleural or pericardial effusion, skin oedema, or a combination — and it is always a downstream effect of a cause.^[3]

The cause sits in one of two great buckets, and the antibody screen splits them:^[2][3]

• Immune hydrops — maternal IgG crosses the placenta and haemolyses fetal red cells. The classic cause was anti-D; now that routine anti-D prophylaxis has driven RhD disease down, immune hydrops is the minority.^[4][14]
• Non-immune hydrops — everything else, and the modern majority (roughly 90% of hydrops in well-resourced settings).^[3][9]

Fetal anaemia is the engine under much of hydrops, whether the cause is haemolysis (immune), erythroid aplasia (parvovirus), or ineffective erythropoiesis (alpha-thalassaemia). Anaemia drives high-output cardiac failure, extramedullary haematopoiesis, hepatic dysfunction, hypoalbuminaemia, and fluid transudation — the hydropic cascade.^[3][4]

^{[1] [4]}

References14ShowHide

1. [1]Mari G, Deter RL, Carpenter RL, et al. Noninvasive diagnosis by Doppler ultrasonography of fetal anemia due to maternal red-cell alloimmunization. Collaborative Group for Doppler Assessment of the Blood Velocity in Anemic Fetuses N Engl J Med, 2000.PMID 10620643
2. [2]Mari G, Norton ME, Stone J, et al. Society for Maternal-Fetal Medicine (SMFM) Clinical Guideline #8: the fetus at risk for anemia—diagnosis and management Am J Obstet Gynecol, 2015.PMID 25824811
3. [3]Norton ME, Chauhan SP, Dashe JS Society for Maternal-Fetal Medicine (SMFM) Clinical Guideline #7: nonimmune hydrops fetalis Am J Obstet Gynecol, 2015.PMID 25557883
4. [4]Moise KJ Jr Management of rhesus alloimmunization in pregnancy Obstet Gynecol, 2008.PMID 18591322
5. [5]de Winter DP, Kaminski A, Tjoa ML, Oepkes D Hemolytic disease of the fetus and newborn: systematic literature review of the antenatal landscape BMC Pregnancy Childbirth, 2023.PMID 36611144
6. [6]Savoia HF, Parakh A, Kane SC How I manage pregnant patients who are alloimmunized to RBC antigens Blood, 2025.PMID 38743880
7. [7]Enders M, Weidner A, Zoellner I, Searle K, Enders G Fetal morbidity and mortality after acute human parvovirus B19 infection in pregnancy: prospective evaluation of 1018 cases Prenat Diagn, 2004.PMID 15300741
8. [8]Lamont RF, Sobel JD, Vaisbuch E, et al. Parvovirus B19 infection in human pregnancy BJOG, 2011.PMID 21040396
9. [9]Bellini C, Donarini G, Paladini D, et al. Etiology of non-immune hydrops fetalis: An update Am J Med Genet A, 2015.PMID 25712632
10. [10]Miller RS, Miller JL, Monson MA, et al. Society for Maternal-Fetal Medicine Consult Series #72: Twin-twin transfusion syndrome and twin anemia-polycythemia sequence Am J Obstet Gynecol, 2024.PMID 39029545
11. [11]Parent M, Maurice P, Van den Eede E, et al. Perinatal outcomes following in-utero transfusion for hydrops fetalis associated with parvovirus B19 infection Ultrasound Obstet Gynecol, 2026.PMID 41630156
12. [12]Devlieger R, Vergote S, Van den Eede E, Haenen K, Lewi L Intrauterine transfusion: Best practices, techniques, and evolving trends Best Pract Res Clin Obstet Gynaecol, 2026.PMID 41289715
13. [13]Al Hassan J, Yared G, El Hajjar C, et al. Non-immune hydrops fetalis (NIHF): diagnostic evaluation and lessons learned from a case series Future Sci OA, 2026.PMID 42233396
14. [14]Practice Bulletin No. 181: Prevention of Rh D Alloimmunization Obstet Gynecol, 2017.PMID 28742673