Natural killer (NK) cells are a part of the defensive system of our bodies.  They fight with viruses and cancer cells.  Recent reports in the media claim its importance in infertility and recurrent miscarriages without proper scientific evidence.  Therefore, infertility clinics are offering tests to measure peripheral blood NK cell numbers. However, it may not reflect the NK cells inside the uterus, particularly inside the endometrium before, during, and after the implantation of an embryo.

Uterine natural killer cells are the dominant type of maternal immune cell response of the endometrium during embryo implantation and placenta formation.  Uterine NK cells are present in the endometrium of the pregnant and non-pregnant women.  The number of NK cells in the endometrium increases during the luteal phase right after ovulation.  It is believed that NK cells are essential during placental formation on the contrary to be detrimental to the pregnancy.

It is also unknown whether the uterine NK cells are identical to the peripheral blood NK cells.

There is no evidence that NK cells kill the placental cells (Trophoblasts).  and any treatment for the high number of peripheral blood NK cells can help to get pregnant and to increase IVF success or prevent recurrent miscarriages. 

First of all, the tests for NK cells are not conclusive.  When we measure the increase in NK cell number in peripheral blood, it does not mean the concentration in the endometrium also increases.  The peripheral blood NK cell ratio in healthy women varies between 5% and 29%.  Many clinics accept the positive NK cell test when the ratio is over 12%.  Therefore, the normal level and the pathologic level in clinical practice are overlapping.  Besides these drawbacks, the levels of NK cells are affected by many other benign conditions such as viral infections.  Therefore, there is no relevant information about NK cells and pregnancy failure.  These are the main drawbacks to the value of NK cell tests in infertility.

• The uterine NK cells are different from the peripheral blood NK cells.  Therefore, measuring peripheral blood NK cells has no value.
• The function of uterine NK cells is unknown.

Therefore, although there are several clinics testing infertile women for NK cells, the value of the tests and the related treatments have no scientific base.