Noninvasive fetal RHD genotyping from maternal plasma

Abstract

Alloimmunization to antigens of Rh blood group system is of clinical relevance during pregnancy. Despite the use of antenatal anti-D immunoglobulin prophylaxis, some proportion of RhD-negative pregnant women still become immunized. RhD-negative pregnant women with a heterozygous partner can be reassured and managed less intensively if RhD-negative status of the fetus was confirmed. The conventional techniques used for prenatal testing of fetal RhD status are mainly invasive such as chorionic villus sampling, amniocentesis and cordocentesis, and carry risk of transplacental hemorrhage and pregnancy loss. The discovery of cell-free fetal DNA (cffDNA) in maternal plasma in 1997 has opened up new possibilities for noninvasive prenatal diagnosis. With the use of real time polymerase chain reaction technology, circulating fetal DNA has been detected robustly in the plasma of pregnant women, even in the first trimester of pregnancy. Various assays have been developed to confirm the fetal RHD status by targeting the cffDNA using a combination of multiple RHD exons. The commonly used exons are 4, 5, 7, and 10. Common causes leading to discordant results in fetal RHD typing are low fetal DNA concentration due to extraction inefficiency, lack of fetal DNA control, silent maternal RHD alleles, and manual error. False negative results can prove critical in case of alloimmunized pregnancies and hence use of appropriate controls and strict reporting criteria is important to increase the sensitivity of the assay. Owing to the wide genetic variation of the Rh blood group system, the development of fetal genotyping strategies according to ethnic origin of the patients would further increase the sensitivity. Fetal blood group genotyping by noninvasive method is safe, and numerous groups have reported fetal RHD genotyping in D-negative mothers with close to 100% accuracy. Noninvasive fetal RHD typing can be performed in RhD-negative alloimmunized and nonimmunized pregnancies to decide on clinical management and to restrict the anti-D immunoglobulin prophylaxis, respectively. Thus, the introduction of this test for screening all RHD-negative pregnant women is highly desirable.

Keywords

• Antenatal prophylaxis
• cell-free fetal DNA
• maternal plasma
• noninvasive fetal RHD genotyping