Non-invasive prenatal DNA test of the fetal rhesus factor (RhD) using maternal blood
What is rhesus factor (RhD)?
The rhesus factor is defined by the presence or absence of a protein – rhesus D antigen (RhD) on the surface of erythrocytes (red blood cells). When there is rhesus D antigen on the surface of erythrocytes, the person is rhesus positive: RhD+. When there is no rhesus D antigen on the surface of erythrocytes, the person is rhesus negative: RhD-.
What are the reasons why a person is rhesus positive or negative?
The human rhesus factor is determined by the gene RHD, which produces a protein – the antigen RhD. Humans have two sets of chromosomes – one inherited from the mother, and the other – from the father. Thus, each of our cells contains two copies of the gene RHD, known as alleles. If both copies function and produce the antigen RhD, then the person is rhesus positive – homozygote. In turn, if only one copy is active, the person is rhesus positive – heterozygote. Both alleles of the gene RHD are non-functional in rhesus negative persons, thus the antigen RhD is not formed or is inactive.
What is the incompatibility of rhesus factors?
An incompatibility of rhesus factors arises in the cases when rhesus positive blood with rhesus D antigen enters the bloodstream of a rhesus negative person who has erythrocytes without the rhesus D antigen. As a result, the rhesus D antigen is recognized as a foreign body, and antibodies are formed against it.
An incompatibility of rhesus factors may cause complications after blood transfusion, as well as during pregnancy. The incompatibility of rhesus factors during pregnancy may arise in cases when the expectant mother is rhesus negative, while the biological father of the child is rhesus positive. In this case there is a possibility that the fetus may be rhesus positive, resulting in the formation of antibodies in maternal blood against the fetal RhD antigen.
Antibodies seldom form during the first pregnancy, and a healthy child is usually born. The risk of complications increases with every subsequent pregnancy. The rhesus antibodies interfere with fetal development, may cause premature birth or the occurrence of hemolytic disease in the newborn child.
Should all pregnant women have the “Noninvasive prenatal DNA test of the fetal rhesus factor (RhD) using maternal blood”?
No. This test is recommended only for those rhesus negative women, who arepregnant with a child from a rhesus positive – heterozygote father, or in situations, when complete information is not available regarding the rhesus factor of the biological father of the child.
Why is it recommended that pregnant women who are rhesus negative should have the rhesus factor of fetus determined?
Because there is a possibility that the fetus may be rhesus positive, and antibodies may form in the maternal blood against the fetal antigen RhD. These antibodies seldom form during the first pregnancy, and usually a healthy child is born. The risk of complications increases with every subsequent pregnancy. The rhesus antibodies interfere with fetal development, may cause premature birth or the occurrence of hemolytic disease in the newborn child.
What is the probability that the fetus is rhesus positive?
- When the father of the child is homozygote (both alleles of the gene RHD are functioning), the fetus will definitely be rhesus positive. This also means that all offspring of this man will be rhesus positive, regardless of whether the mother of his child is rhesus positive or negative. When the expectant mother is rhesus negative, she will need an additional treatment.
- When the father of the child is heterozygote (one of the two alleles of his gene RHD is functioning), there is a 50% probability that the fetus may be rhesus positive, and a 50% probability that the fetus may be rhesus negative. In this case it is recommended that the rhesus status of the fetus (positive or negative) is determined during each pregnancy.
Is it possible to determine the rhesus factor of fetus from a sample of maternal blood?
Yes, it is. Starting from approximately the 10th week of pregnancy, the fetal extracellular (free) DNA starts to circulate in the maternal bloodstream. This DNA can be isolated (purified) from maternal blood and used for the determination of fetal rhesus factor.
Description of the test
Non-invasive determination of the fetal rhesus factor is an absolutely harmless procedure both for the fetus and the expectant mother. Two samples are collected from the mother: venous blood and a swab or „saliva sample” from the mucous membranes of the mouth.
The circulating fetal DNA is isolated from the maternal blood.
The non-invasive prenatal DNA test for the fetal rhesus factor includes the determination of the presence of two DNA fragments from the gene RHD, two fragments from an adjacent gene, the Y chromosome, and additional DNA fragments for control.
To make a laboratory appointment and to submit a sample for performing any test, please call: +37167425241 or write to firstname.lastname@example.org, specifying the requested test, name and surname, date of birth of the person to be tested, the preferred date and time of visit, and the phone number.