Introduction The introduction explains that blood types are immutable biological characteristics which, according to Mendel's laws, heritage factors, genes, passed from generation to generation (found at: erythrocytes, leukocytes, platelets and plasma proteins). The essence of the division of the blood is to show that the erythrocytes isogenese properties, comprising, many antigens using the antibodies can be demonstrated. In addition to the division of the ABO groups, there is Rhesus factor (Rh factor), which is named after the Rhesus monkey with whom he first discovered. All people are divided into Rh + (Rh positive) and Rh (Rh negative). People with Rh + blood created rhesus protein, while people with Rh blood does not create. In pregnancy, Rh factor is inherited. If the mother is Rh, there is a chance of 50% to 100% that the baby will be Rh + (providing that the father Rh +). During childbirth, the blood of mother may come in contact with blood from the placenta and can be initiated the creation of an immune response in the mother. Mother after giving birth can create antibodies and will be permanent immunized. Objective The objective of this paper is to point out the importance of determining the Rh (D) antigen in pregnant women to prevent serious health consequences for the mother and fetus. Sources of information and methods In each laboratory there is a specific protocol that must be followed in determining the Rh (D) antigen, and it is important to properly and fully comply with the rules. To avoid immunization of pregnant women testing must be carried out at prescribed intervals in accordance with said protocol. If, however, spread immunization, the laboratory is that the same timely diagnosed and when the time comes for the intervention to take the necessary steps to further treatment. The diagnostics have to use the best available equipment and methods all in favour of successful monitoring and treatment of pregnant women and newborns. Conclusion Determination of Rh (D) antigen is one of the imperatives of prenatal diagnosis. The testing is non-invasive and completely safe for the fetus, and the benefits of testing are numerous and large. Diseases caused by Rh (D) immunization of mothers are serious with even potentially fatal to the fetus, and therefore it is important that the Rh (D) antigen mothers determine the time and that further pregnancies are conducted in accordance with the Rhesus factor mother. The only way we can react if there is an increase in the antibody during pregnancy, and so you might as well save the life of a child and enable secure subsequent pregnancies of the patient.