Haemolytic disease of the fetus and newborn — formative SAQs

SAQ 1 — Prevention, surveillance and the fetal anaemia threshold (10 marks)

A 30-year-old Rh-negative woman at 20 weeks' gestation is found to have anti-D antibody on her booking antibody screen, with a titre of 1 in 32. She did not receive anti-D after a miscarriage two years ago. ^{[1] [4]}

Questions

1. Outline how this sensitisation could have been prevented, and state the evidence for antenatal and postnatal anti-D prophylaxis. (5 marks) ^[1]
2. Describe the antenatal surveillance strategy from this point, including the validated non-invasive test for fetal anaemia and its intervention threshold. (5 marks) ^{[3] [4]}

Model answer

Prevention (5). This woman was sensitised by fetomaternal haemorrhage at the time of her miscarriage, and the sensitisation would very likely have been prevented by anti-D immunoglobulin given after that event. Routine prevention rests on two Crowther Cochrane reviews: antenatal anti-D at 28 weeks to all Rh-negative women, and postnatal anti-D within 72 hours of delivering an Rh-positive infant, together suppress the maternal immune response to fetal RhD-positive cells and dramatically reduce RhD alloimmunisation. Anti-D is also indicated after any sensitising event in pregnancy — antepartum haemorrhage, trauma, miscarriage, ectopic pregnancy, and procedures such as external cephalic version or abdominal trauma — and ABO incompatibility between mother and fetus is naturally protective because maternal anti-A and anti-B clear fetal cells before they sensitise the Rh system. ^[1]

Surveillance (5). Once a clinically significant antibody is identified, the strategy is serial antibody titres or quantitative anti-D levels every two to four weeks, and when the titre or level reaches the laboratory-specific critical threshold, serial middle cerebral artery peak systolic velocity (MCA-PSV) Doppler from about 18 weeks, performed every one to two weeks. Mari's landmark NEJM study established that MCA-PSV above 1.5 multiples of the median for gestational age detects moderate-to-severe fetal anaemia non-invasively with high sensitivity, and the SMFM Clinical Guideline #8 codified this threshold, retiring serial amniocentesis with delta-OD450. When the MCA-PSV exceeds 1.5 multiples of the median or hydrops develops, fetal blood sampling is performed with preparation for intrauterine transfusion, in a tertiary maternal-fetal-medicine centre. ^{[3] [4]}