NK cells are one of the main cellular populations involved in the alloimmune termination of pregnancy. Due to the fact that NK cells play a significant role in damaging of trophoblast cells, diagnostic approaches to alloimmune disorders are reduced to research of almost only NK cells. That is why detection of disorders related to these cells in peripheral blood in early reproductive losses is increasingly being used as a marker of hidden alloimmune disorders. To evaluate the role of NK cells in the pathogenesis of early reproductive losses (habitual miscarriage (HM), multiple IVF failures). Determination of subpopulation of peripheral blood lymphocytes, relative content of T-regulatory lymphocytes, NK cell activity (NK cells expressing CD107a), induced NK cell activity (CD107a after activation). To determine the cytotoxic activity in relation to trophoblast cells, mononuclear cells were isolated from the peripheral blood of patients using standard centrifugation method. They were then incubated with JEG-3 cells. 168 women of the Caucasian race were examined (18-39 years). The main group included 101 women, whith 2 subgroups: 1 - women with 2 or more consecutive losses of clinical pregnancy up to 10 weeks from one partner (n=60); 2 - women with infertility, 2 or more ART failures (n=40). The comparison group included 67 healthy fertile women. The study included patients with normal karyotype, without antiphospholipid syndrome, high-risk thrombophilia, hypertensive disease, diabetes and obesity. The NK cells above 18% was detected at the same frequency in the HM group and in the control group (20% and 22% respectively), while in the infertility group - only 12%. NK in the range of 12-18% were most often detected in the HM group (40%), in the infertility group (30.3%) and in control 25.8%. When assessing the activity of NK cells, the differences were revealed by the expression of CD107a - significantly more in the HM group (p < 0.05), while the induced activity was greater in the infertility group (p < 0.01). Thus, NK cells can be one of those immunological mechanisms that prevents the normal development of pregnancy. As a result, the survey plan for women with a habitual miscarriage of unclear etiology should be expanded and necessarily include an analysis of the numbers and activity of NK cells. Correction issues of the revealed violations and methods of effectiveness control over the ongoing measures remain open until today.