Introduction: Transfusion medicine is a specific clinical-laboratory and manufacturing activity that unites medicine, biotechnology, pharmacy, science and the community as a whole. Data on the first direct human blood transfusion are related to the name of James Blundell, a British narrator who performed the first successful blood transfusion on December 11, 1818 reported by the Medical Surgery Society in London. The erythrocyte membrane contains 308 different antigens classified into 35 blood group systems. ABO and Rh systems are the most important blood group systems. Blood group Rh antigens are highly immunogenic, which in people who do not have these antigens more often than other antigens stimulate the immune response and create irregular antibodies. Rh antibodies are the most common antibodies caused by pregnancy and transfusion, they are IgGs class and cause hemolytic transfusion reaction and severe forms of hemolytic disease of the fetus and newborn. Objective: The aim of the study is to demonstrate the importance of anti-erythrocyte antibodies for mother and child's health in an example of their frequency in pregnant women. Frequency is presented by comparing positive IAAs with the total number of tests performed in population of preoperative hospital population, pregnant women on the first testing and pregnant women on control testing. Also, it is shown the number of identified types of anti-erythrocyte antibodies among these population. Data sources and methods: All developed countries prescribe the determination of ABO blood group, RhD antigen and anti-erythrocyte antibodies search test in early pregnancy (10th -14th week). Early pregnancy tests reveal antibodies created before the present pregnancy. Due to fetomaternal bleedings occurring throughout pregnancy, a negative test in the first trimester may become positive in RhD negative and RhD positive pregnant women on the third trimester control test. The recommendations of all developed countries provide checking antibodies at the beginning of the third trimester (28th - 30th week) for RhD negative pregnant women. In addition to the aforementioned checking antibodies in the 28th week of pregnancy, in Croatia, according to the standards and guidelines of transfusion medicine, it is also recommended to do search test in the 34th week of pregnancy. Search test in the 28th week of pregnancy is important not only for detection of RhD immunization during pregnancy, but also as the last check before RhD-protection is applied in countries carrying out routine prenatal RhD-protection. Determination of ABO blood groups is done by two methods, test in test tubes and test on the plate. Aglutination or hemolysis are the reaction between antigens and antibodies and indicate a positive reaction. Plate technique and test tube technique with monoclonal test reagent or polyclonal test serum are used to detect D-antigen on the erythrocyte membrane in the assessment of RhD blood group. Anti-erythrocyte antibodies search test is performed in an indirect antiglobulin assay (IAA). It can be done by method in tubes, microstrips and microspheres. Results: Study of the frequency of anti-erythrocyte antibodies in pregnant women was conducted at the Transfusion Medicine Center in Split. The results cover the period from the beginning of 2014 to the 6th of May 2018. The percentage of positive IAAs in population of pregnant women on control testing was 4.66%, while the percentage in population of pregnant women on the first testing was 0.83%. It is evident that a greater number of pregnant women developed primary immunization before the control testing, respectively in the third trimester of pregnancy as a result of fetomaternal bleedings occuring throughout pregnancy. The percentage of positive IAAs obtained for the preoperative hospital population (1.92%) was not significantly higher than the percentage of positive IAAs in the population of pregnant women on the first testing (0.83%), although the total number of IAAs in the preoperative hospital population was twofold higher (63401 versus 27253). The highest frequency of clinically significant anti-erythrocyte antibodies in the pregnant population was observed for anti-D antibody from the Rh system. The second clinically significant antibody by frequency is anti-c. In addition to the Rh system antibodies that are the most significant and at the same time the most common in the pregnant population, among more common clinically significant antibodies are anti-K from Kell system and anti-S from MNS blood group system. Conclusion: Study of anti-erythrocyte antibodies frequency in pregnant women in the region of Split has been shown that the most common antibodies are from Rh system. Anti-D antibody is the most common clinically significant antibody. However, the importance of other antibodies from Rh system or antibodies from other blood group systems, especially those antibodies that cause HDFN, should not be ignored. Immunohaematological examination is a mandatory part of each pregnancy. Abidance of specific process steps and well-run documentation are the basis for accurate, precise and reproducible results.