Natural killer cells have a distinct name and one of their earliest recognised roles was to kill cells infected by a virus hence the name natural killer cells. They are cells that can be found within the blood and commonly float around the body. However, sometimes they can be found in the lining of the uterus.

There has been speculation that having natural killer cells in the uterus attacks the embryo and/or endometrium and that these cells are instrumental in why women have issues with embryos implanting to the uterus (and not falling pregnant) or have miscarriages. As natural killer cells fight infection, immunosuppressed medication is used to try to kill the cells in the uterus. The truth is actually opposite to this speculation. Natural killer (NK) cells and Treg (regulatory T cells) have important roles in facilitating or helping implantation of an ongoing pregnancy.

However, there is no convincing evidence that having natural killer cells (high or low) in the uterus is the reason for implantation failure or miscarriage. Actually, having immune cells is important in facilitating implantation of embryos.

There are multiple tests to look into if you have failed implantation or miscarriage. Some of these include assessing the uterine cavity for fibroids, polyps, inflammation and seeing if the endometrium is inappropriately thin at the time of ovulation or if there is a displaced window of implantation or altered microbiome in the uterus.