Introduction

With the established use of postnatal anti-D prophylaxis for D-negative women, together with its increasing use for routine antenatal prophylaxis, the incidence of sensitization to D has markedly fallen. This is reflected by the reduction in the overall number of cases of HDFN (Section 5.3.2). However, women with low anti-body levels or in their first sensitized pregnancies continue to be referred to tertiary fetal medicine units for specialized management. Considerable effort is, therefore, spent by fetal medicine specialists in monitoring the disease course, predicting its progression and deciding on the need and timing of intrauterine transfusions.

The most accurate test to assess the degree of fetal anaemia, and thus the need for transfusion, is direct fetal haemoglobin measurement by fetal blood sampling. However, cordocentesis can cause an increase in the antibody concentration and is associated with about a 1% fetal loss rate and so is not an appropriate first-line investigation. Fetal blood sampling is best reserved for cases where severe fetal anaemia is already suspected and so intrauterine transfusion is likely to follow. At amniocentesis, the needle is rarely passed transplacentally and, therefore, the risk of boosting the antibody level is less than with cordocentesis, although the procedure still has small but significant risks of causing miscarriage or preterm labour. In recent years, several noninvasive methods for the prediction of fetal anaemia have been assessed and these are now allowing the use of invasive testing to be deferred, usually until transfusion is necessary.