Fetal anaemia and hydrops fetalis — structured SAQ (15 marks)

(a) Diagnostic criterion (2 marks)

• Hydrops fetalis is defined as excess fluid in two or more fetal extravascular compartments (ascites, pleural or pericardial effusion, skin oedema).^[1]
• Ascites alone does not meet the definition — a single fluid collection demands explanation and follow-up but is not hydrops.^[1]

(b) Investigations structured by the first branch point (5 marks)

The antibody screen is the first branch — immune versus non-immune. ^[1]

• Maternal antibody screen (indirect Coombs) — negative here, so this is non-immune hydrops and the workup proceeds through the non-immune categories.^[1]
• Maternal infection screen — parvovirus B19 IgG and IgM with PCR, plus CMV, syphilis, toxoplasma.^[1]
• Detailed fetal anomaly scan and echocardiography — to exclude structural cardiac disease and arrhythmia, the leading non-immune category.^[1]
• Fetal karyotype and chromosomal microarray — regardless of whether a structural anomaly is identified, to exclude chromosomal causes.^[1]
• Middle cerebral artery peak systolic velocity (MCA-PSV) — to detect anaemia; above 1.5 multiples of the median triggers fetal blood sampling with intrauterine transfusion readiness.^[2]

(c) Parvovirus B19 pathogenesis and management (5 marks)

One mark for the mechanism, the rest for the management pathway. ^[3]

• Pathogenesis: parvovirus B19 has a tropism for erythroid progenitors and arrests erythropoiesis, producing an aplastic (not haemolytic) crisis at a gestation when red-cell lifespan is short and demand is high; a coexisting myocarditis worsens the picture in some.^[3]
• The MCA-PSV at 1.6 multiples of the median is above the 1.5 threshold — refer to fetal medicine for fetal blood sampling with intrauterine transfusion ready.^[2]
• Intrauterine transfusion technique: intravascular via cordocentesis into the umbilical vein at the placental insertion; O-negative, CMV-negative, irradiated, leucodepleted, crossmatch-compatible packed cells; volume to a post-transfusion haemoglobin around 14 to 15 g/dL.^[4]
• Repeat transfusion guided by the predicted haemoglobin decline (about 1 g/dL per week) and serial MCA-PSV; intravenous immunoglobulin has a role in selected cases.^[3]
• Counsel on prognosis — survival with appropriate intrauterine transfusion is good; untreated hydropic parvovirus has high mortality.^[3]

(d) Recurrence counselling (3 marks)

One mark per point. ^[3]

• Parvovirus is sporadic and does not recur in subsequent pregnancies by the same mechanism — once immune, the woman is protected.^[3]
• Serostatus matters: confirm she has now seroconverted (IgG positive), which protects against future parvovirus infection.^[3]
• Other hydrops causes are cause-specific — genetic causes recur (with counselling and possible prenatal testing), but a single parvovirus event does not predict hydrops next time. Reassure appropriately.^[1]