Fetal rhesus factor test from maternal blood
Non-invasive prenatal fetal rhesus factor (RhD) DNA test using maternal blood
What is rhesus factor (RhD)?
Rhesus factor depends on the presence or absence of a protein—Rhesus D antigen (RhD)—on the surface of erythrocytes (red blood cells). If rhesus D antigen is present on the surface of erythrocytes, the person is rhesus positive: RhD+. If there is no rhesus D antigen on the surface of erythrocytes, the person is rhesus negative: RhD-.
What determines if a person is rhesus positive or negative?
The human rhesus factor is determined by the gene RHD, the product of which is a protein: the RhD antigen. Humans have two sets of chromosomes: one inherited from their mother and one from their father, meaning that each of our cells has two copies of the RHD gene, the so-called alleles. If both copies function and produce the RhD antigen, the human is rhesus positive-homozygous, whereas if only one copy is active: rhesus positive-heterozygous. In rhesus-negative people, both RHD alleles of the gene are non-functional, and the RhD antigen is not produced or is inactive.
What is rhesus factor incompatibility?
Rhesus factor incompatibility occurs when a rhesus-negative person, who does not have the rhesus D antigen, receives erythrocytes of rhesus-positive blood with the rhesus D antigen. As a result, the rhesus D antigen is recognized as a foreign body, and antibodies are formed against it.
Rhesus factor incompatibility can lead to complications both after a blood transfusion and during pregnancy. During pregnancy, rhesus incompatibility may occur when the woman is rhesus-negative, but the biological father of the unborn child is rhesus-positive: there is a possibility that the foetus may be rhesus-positive, resulting in antibodies to the foetal antigen RhD in the pregnant woman’s blood.
During the first pregnancy, antibodies are rare and the baby is usually healthy. The risk of complications increases with each subsequent pregnancy. Rhesus antibodies interfere with foetal development, may lead to premature birth or neonatal haemolytic disease.
When is a rhesus factor (RhD) DNA test recommended?
The test is recommended only for those rhesus-negative pregnant women if the father of the unborn child is rhesus positive and heterozygous, or in situations where complete information about the rhesus factor of the expected child’s biological father is lacking. In our laboratory, this test can only be performed if it is clear that the parents of the unborn child are of European origin. The reason for this limitation is because the cause of rhesus negativity for people of other races and backgrounds is genetically different from what we identify in our test.
What is the probability that the fetus could be rhesus positive?
If the father of the unborn child is homozygous (both the RHD alleles of the gene are functional), then the fetus will definitely be rhesus positive. It also means that all children of this man will be rhesus positive — whether the child’s mother is rhesus positive or not. If the pregnant woman is rhesus negative and is not her first pregnancy, she may need additional treatment.
If the father of the unborn child is heterozygous (one of his two RHD alleles is functional), there is a 50% chance that the foetus may be rhesus positive and a 50% chance that the rhesus factor is negative. In such cases, it would be advisable for a rhesus-negative woman to check the rhesus status of the fetus (positive or negative) during each pregnancy. A rhesus-negative pregnancy does not pose any of the risks described above in a rhesus-negative woman.
Can fetal rhesus factor be detected in a blood sample of a pregnant woman?
Yes. From about the 8th week of pregnancy, extracellular (free) DNA of the fetus (placenta) begins to circulate in the mother’s blood: it can be isolated (purified) from the mother’s blood for fetal rhesus factor testing.
Test description
Non-invasive fetal rhesus factor testing is an absolutely safe procedure for both the fetus and the pregnant woman.
Two samples are taken from the pregnant woman: venous blood and a swab from the oral mucosa, i.e. the “saliva sample.”
The circulating fetal DNA is isolated from the woman’s blood.
The non-invasive prenatal fetal rhesus factor DNA test includes: detection of two RHD fragments of the gene, two fragments of an adjacent gene, the Y chromosome, and the presence of control DNA fragments.
- Gender reporting optional (Y chromosome was or was not detected).