Parvovirus B19 in pregnancy with a hydropic fetus — structured SAQ (15 marks)

(a) Mechanism of fetal anaemia and hydrops (2 marks)

One mark for the cellular target, one for the downstream cascade. ^[4]

• Cellular target: parvovirus B19 binds the globoside (P blood group) antigen on the surface of erythroid progenitor cells in the fetal bone marrow and replicates in those precursors, arresting erythropoiesis.^[4]
• Downstream cascade: arrested erythropoiesis causes fetal anaemia; the fetus mounts a high-output cardiac response; at the limit of compensation it develops congestive cardiac failure and hydrops — ascites, skin oedema, pleural and pericardial effusions, placentomegaly.^[2][4]

(b) Fetal surveillance strategy (4 marks)

One mark for the modality, one for the threshold, one for the latency principle, one for the interval. ^[3]

• Modality: the middle cerebral artery peak systolic velocity (MCA-PSV) Doppler, the primary technique to detect fetal anaemia per SMFM Clinical Guideline #8.^[3]
• Threshold: fetal blood sampling is performed — with preparation for intrauterine transfusion — when the MCA-PSV is greater than 1.5 multiples of the median, or when the fetus is hydropic.^[3]
• Latency principle: maternal infection and fetal hydrops are separated by a multi-week latency, so surveillance must continue for several weeks, not end at a single normal scan.^[2][4]
• Interval: serial MCA-PSV Dopplers over the several weeks after the exposure or infection, in a fetal medicine setting.^[3]

(c) Management of the hydropic fetus and intrauterine transfusion (4 marks)

One mark each for the referral, the transfusion indication, the technique target, and the gestational-age decision. ^[2][3]

• Referral: refer to a fetal medicine unit for assessment and management. This fetus is already hydropic, so the threshold for fetal blood sampling with transfusion readiness is met.^[3]
• Indication: the fetus is hydropic and the MCA-PSV is presumed elevated, so fetal blood sampling is performed with intrauterine transfusion ready.^[3]
• Technique: the intrauterine transfusion raises the fetal haematocrit toward the normal range; repeat transfusions may be required until the fetal marrow recovers.^[3]
• Gestational-age decision: the decision between intrauterine transfusion and delivery turns on gestational age — below the gestational age at which delivery risks outweigh the procedure, transfuse; near term, deliver.^[3]

(d) Prognosis with and without intervention, and surveillance interval (5 marks)

Two marks for prognosis with IUT, two for prognosis without IUT, one for the counselling/surveillance principle. ^[1][2]

• Prognosis with intrauterine transfusion: in the Enders cohort, 11 of 13 fetuses with severe hydrops who received intrauterine transfusion survived (84.6%). The systematic review and meta-analysis confirms high perinatal morbidity but the hydrops is largely reversible with transfusion.^[2][1]
• Prognosis without intervention: all of the non-transfused fetuses with severe hydrops died in the Enders cohort. Untreated severe parvovirus hydrops carries a high mortality.^[2]
• Overall figures: the overall fetal death rate among maternal infections was 6.3% in the Enders cohort (11% for infection in the first 20 weeks); among fetal infections the intrauterine demise rate was 38.7% in the meta-analysis.^[2][1]
• Counselling principle: reassure the woman that the treatment (intrauterine transfusion) is effective, while being honest that untreated severe hydrops is life-threatening. Arrange serial surveillance for several weeks because of the latency interval.^[2][4]