Successful implantation is an absolute requirement for the reproduction of species, including humans. The process by which a foreign blastocyst (embryo) is accepted by the maternal endometrium (lining of the womb) is complex and requires interplay of many systems.

Implantation occurs during the specific time of “implantation window” (7-10 days following ovulation), in which the maternal endometrium is ready to accept the blastocyst, which on the other hand, also plays a specific role. It appears that immune cells and in particular, uterine NK cells are pivotal in inducing tolerance to the blastocyst.

uNK cells can secrete an array of cytokines (chemicals) that are important in angiogenesis (development of blood vessels) and thus placental development and the establishment of pregnancy.

Uterine NK (uNK) cells are present in large numbers in the wall of the womb at implantation and in the early months of pregnancy. Maternal uNK cells are adjacent to, and have the ability to interact directly with, fetal placenta. They seem to help the placenta link up with your blood vessels and so set up a healthy supply line to the fetus. However, scientists do not know exactly how they do it. (In mice that lack NK cells in the womb, development of the placenta is abnormal and the young are smaller than usual). Uterine NK cells appear to be essential and very important regulators of successful implantation and pregnancy.

There is no absolute evidence that uterine NK cells are destructive and attack placental or embryonic cells. However, recently it was suggested that abnormal regulation of NK cells in the womb and/or the blood was evident in women prone to recurrent pregnancy loss and implantation failure. In addition studies found that increased numbers of uNK cells have been associated with reproductive failure.

Increased numbers of uNK cells have been associated with reproductive failure. The mechanisms of reproductive failure associated with raised uNK cell density appear to be increased angiogenesis (blood vessel development) and peri-implantation blood flow, which may lead to early maternal circulation and hence pregnancy failure due to excessive oxidative stress.

Although the evidence regarding a link between blood and uterine NK cells is limited, it was recently shown that NK cells in the blood could reflect changes in uterine NK cell levels.