[Prevention of fetal hemolytic disease: it is time to take action]

Rh D foeto-maternal alloimmunization prophylaxis with anti-D immunoglobulins reviewed in the era of foetal RHD genotyping

In Belgium, prevention of anti-D immunization is currently based on systematic postnatal prophylaxis associated with targeted antenatal injection in high-risk situations of foeto-maternal haemorrhage.The failures of prevention are mainly due to the non-respect of established guidelines for RhlG prophylaxis, and to spontaneous undetected foetal-maternal haemorrhages without any obvious cause during the third trimester of pregnancy. In order to reduce the rate of residual post-pregnancy anti-D immunization, several countries decided to associate the classical prophylaxis to a routine antenatal anti-D prophylaxis (RAADP) during the 28th or 29th week of gestation. Since a few years, the foetal RHD genotyping in maternal plasma enables us to limit the antenatal prophylaxis only to those D- women carrying a D+ foetus. This paper deals with: the advantages of an antenatal prevention in the light of non-invasive foetal RHD genotyping, the rules rendering prevention protocols efficient whatever the algorithm applied, and the recommended immuno-haematology follow-up of women who received RhlG.

Routine fetal RHD genotyping with maternal plasma: a four-year experience in Belgium

BACKGROUND

The objective was to evaluate the diagnostic value of RHD fetal genotyping from the plasma of D- mothers as soon as 10 weeks' gestation in a routine clinical practice in Belgium.

STUDY DESIGN AND METHODS

A prospective study was conducted between November 2002 and December 2006. DNA extraction was performed in an automated closed tube system. Fetal RHD/SRY genotypes were detected in the plasma of 563 pregnant mothers by real-time polymerase chain reaction (PCR) targeting multiple exons 4, 5, and 10 of the RHD gene and targeting an SRY gene sequence. These were compared to the D phenotypes determined in the 581 babies they delivered.

RESULTS

By combining amplification of three exons, the concordance rate of fetal RHD genotypes in maternal plasma and newborn D phenotypes at delivery was 100 percent (99.8% including one unusual false-positive). The presence of nonfunctional RHD genes and the absence of a universal fetal marker, irrespective of fetal sex, did not influence the accuracy of fetal RhD status prediction. The RHD genotyping from 18 twin pregnancies was also assessed. Five weak D women were excluded from the RHD fetal genotyping prediction. Three discrepant results (0.5%) between predicted fetal genotype and cord blood phenotype were not confirmed by the baby phenotypes from venipuncture blood.

CONCLUSION

Prenatal prediction of fetal RHD by targeting multiple exons from the maternal plasma with real-time PCR is highly sensitive and accurate. Over 4 years, this experience has highly modified our management of D- pregnant women.