Fetal anaemia and hydrops fetalis — structured oral station (12 minutes)

Opening prompt — "You are shown a hydropic fetus at 21 weeks with a negative antibody screen. Talk me through your approach."

Model response — say it in this order: ^[1]

• "This is non-immune hydrops — two or more fluid collections with a negative antibody screen. Hydrops is an endpoint, not a diagnosis, so I will work systematically to find the cause before any decision on viability."
• "Several non-immune causes are treatable — arrhythmia, anaemia, hydrothorax — so I will not assume the worst."^[1]

Examiner is listening for: the endpoint-not-diagnosis framing, and the antibody-screen branch. ^[1]

Probe 1 — "What is your differential and how will you investigate it?"

• "The SMFM #7 categories of non-immune hydrops, in order of frequency: cardiovascular (structural and arrhythmia), chromosomal, haematological, structural anomaly, monochorionic twin complications, infection, and placental causes."
• "Workup: detailed anomaly scan and echocardiography, fetal karyotype and chromosomal microarray regardless of anomaly, maternal infection screen (parvovirus B19 IgG/IgM and PCR, CMV, syphilis, toxoplasma), and middle cerebral artery peak systolic velocity to detect anaemia."^[1]

Probe 2 — "Why do you measure MCA-PSV in a hydropic fetus?"

• "Because anaemia of any cause drives much of hydrops. In the anaemic fetus, blood viscosity falls and cardiac output rises, so the velocity in the middle cerebral artery increases."
• "Mari's 2000 New England Journal study showed an increased MCA-PSV detected moderate or severe anaemia with a sensitivity of 100 percent (95 percent confidence interval 86 to 100 percent) and a false-positive rate of 12 percent."
• "The action threshold is an MCA-PSV above 1.5 multiples of the median — that triggers fetal blood sampling with intrauterine transfusion readiness."^[2][3]

Probe 3 — "The fetal heart shows supraventricular tachycardia. How does that change your management?"

• "This is a treatable cause — arrhythmia is one of the leading non-immune categories. I would start transplacental therapy, most commonly digoxin, with sotalol or flecainide as alternatives, to restore sinus rhythm. Hydrops often resolves once the rhythm is controlled."
• "I would also continue the rest of the workup, because more than one cause can coexist."^[1]

Probe 4 — "If this were a parvovirus infection instead, how would the anaemia differ and how would you manage it?"

• "Parvovirus causes anaemia by erythroid aplasia, not haemolysis — the virus has a tropism for erythroid progenitors and arrests erythropoiesis."
• "I would monitor with serial MCA-PSV for weeks after infection, and transfuse intrauterinely for severe anaemia; intravenous immunoglobulin has a role in selected cases."
• "Survival with appropriate intrauterine transfusion is good; untreated hydropic parvovirus has high mortality."^[1][4]

Probe 5 — "How do you counsel the woman about prognosis?"

This is a scored domain — speak to it out loud: ^[1]

• "Prognosis depends entirely on the cause. A treatable arrhythmia or anaemia can resolve completely. A lethal chromosomal cause carries near-total mortality."
• "I would be honest about the uncertainty, explain the plan and the timeline, and offer to involve the fetal medicine and neonatal teams and, where appropriate, palliative care."
• "I would give written information and a named contact, and acknowledge her fear."^[1]