Because women postpone parenthood, reproductive failure is a major social and economic issue. Unfortunately, natural fertilisation and pregnancy maintenance decrease with age. Many need ART. Recurrent implantation failure (RIF) occurs in 10% of IVF-embryo transfer patients. Oocyte and sperm quality, parental chromosomal anomalies, embryo genetic or metabolic abnormalities, poor uterine receptivity, immunological disturbances in the implantation site, and some gynecologic pathologies like endometriosis, uterine fibroids, hydrosalpinx, and endometrial polyps can cause RIF. In vitro fertilisation may also harm implantation. Because NK cells dominate the endometrium and interact with allogeneic extravillous trophoblast cells in early pregnancy decidua, several research are examining their involvement in normal and pathologic pregnancy. CD56bright cells predominate.