The homeostasis is maintained by the immune system which is constituted a network of organs, cells and molecules that act to combating the assaults affecting the body. The immune function has been conceptually divided into innate immunity and acquired immunity. Among the effector cells of innate immunity are the natural killer cells (NK), they play an important role in the reproductive immunology in the establishment and maintenance of pregnancy and fetus. The study of the biological mechanisms involved in the maintenance of pregnancy contributes to increase knowledge about immune tolerance. The way in which the immune system is modulated, and the study of the recognition systems maternal innate and adaptive occurring during pregnancy, allow to understand the survival of the fetus. The aim of this review was to present the main functions of NK cells and describe their role in the process of trophoblastic invasion in the deployment process, in the maternal-fetal interaction and development of the fetus. The knowledge of the precise role of NK cells is necessary, because these cells may be responsible for reactions which lead to embryonic and fetal loss during the organogenesis process.