The blood group factor RhD (aka Rhesus factor D) is one of many known blood group factors. RhD, however, is highly immunogenic, which means that carriers of the RhD-negative factor often form antibodies against RhD when they come into contact with red blood cells (erythrocytes) that have the RhD blood group factor.
During pregnancy and childbirth, small amounts of erythrocytes in the fetus can enter the maternal bloodstream. If the fetus has the blood group factor RhD-positive and the mother is RhD-negative, the mother may be “sensitized” or “immunized”. The antibodies formed against Rhesus factor D are called “anti-D”.
RhD-positive red blood cells can trigger the formation of anti-D in RhD-negative people.
Rh prophylaxis
The Rh prophylaxis treatment is very safe. Even so, pregnant women occasionally express concern since Rh prophylaxis is a blood product. It is produced from the blood of donors who have previously immunized themselves against the Rhesus-D factor. Although anti-D immunoglobin is an exceptionally infection-proof blood product, transmission of infection cannot be ruled out for all batches and for all pathogens. Human anti-D immunoglobin can also rarely (frequency between 1:1000 and 1:10,000) cause hypersensitivity reactions.
Targeted Rh prophylaxis after fetal blood group testing protects just as well as the treatment of all. RhD-negative pregnant women.
It has recently become possible to non-invasively test the unborn child’s (fetal) Rh factor from the mother’s blood. Similar to other tests for non-invasive prenatal diagnosis (e.g. the Harmony® Test), cell-free fetal DNA in the pregnant woman’s blood is used to examine the fetal RHD gene by using a method called PCR. Statistically, about 40% of the children of RhD-negative women are also RhD- negative; therefore, an RhD-positive fetus can be expected in 60% of the tests. On average, non- invasive fetal blood group testing can avoid unnecessary Rh prophylaxis treatment in approximately 40% of the subsequent pregnancies.
With the determination of the fetal Rh factor, Rh prophylaxis only needs to be administered to those pregnant women who are actually expecting an RhD-positive child.
The non-invasive testing has no effect on the unborn child as blood is only taken from the mother. It should be emphasized that there are no health implications for an individual whether they are RhD- positive or RhD-negative, except for in the case of maternal/fetal Rh incompatibility as described previously.
Likewise, the purpose of Rh prophylaxis only for pregnant women expecting an RhD-positive child works as well as the administration of Rh prophylaxis for all pregnant women.
Test Reliability
As with other medical tests, Rhesus NIPT non-invasive fetal blood group testing can produce so- called “false positives” and “false negatives” results.
False positive findings are possible due to “silent alleles”, which are genes without a real function. In such cases, although the fetus was actually RhD-negative, the test results would mean that Rh prophylaxis would be given unnecessarily. This occurs in the approx. 0.4% of cases. Without the use of non-invasive fetal blood group testing, however, Rh prophylaxis would have been given anyway.
False negative results are possible, due to factors such as insufficient cell-free fetal DNA in the mother’s blood. Since the amount of cell-free fetal DNA increases as the pregnancy progresses, it is recommended that the Rhesus NIPT is carried out only after the 19th week of pregnancy. If another non-invasive prenatal test is performed (e.g. the Harmony® Test), and the fetal fraction of the blood is determined to be at least 4%, the fetal Rh status can also be determined from the 12th week of pregnancy.
Overall, the false negative rate is approximately 1 out of 2000 tests. In this case, a required prenatal Rh prophylaxis would not be administered. Given the generally low immunization risk during pregnancy (no more than 1-2% risk per pregnancy with an RhD-positive fetus), this proportion of false negative results is considered acceptable.
In the opinion of the German “Institute for Quality and Efficiency in Health Care” (IQWiG), the standard administration of Rh prophylaxis and targeted prophylaxis after testing with Rhesus NIPT give equivalent protective effects, because false negative results are also occasionally found in blood testing during childbirth.