The causes of unexplained recurrent miscarriages are always a matter of heated discussion among fertility specialists. The only factors that physicians tend to universally accept as being related to recurrent miscarriages are antiphospholipid syndrome, some uterine malformations such as a septate uterus, and chromosomal abnormalities in the sperm or eggs (which can result from a condition like balanced translocation or simple bad luck).
There are many other unproven causes that some physicians test for because they believe that the treatment will probably or possibly work even though the solid evidence isn't there. Some of these unproven causes are more controversial than others. For example, many physicians will offer treatment with progesterone supplements because they're unlikely to cause harm even though they might not help. The idea that elevated natural killer cells (NK cells) causes recurrent miscarriages in some women is among the more controversial of these unproven causes and treatments.
There is a lot of evidence that NK cells are elevated in women who have had recurrent miscarriages, but not so much evidence that the NK cells themselves are the actual cause of the recurrent miscarriages. Researchers are looking at whether treatments to reduce the levels of NK cells reduces the risk of miscarriage.
Despite the strange sounding name, natural killer cells are not a bad thing to have -- on the contrary. NK cells play a vital role in the functioning of the immune system. They help the body fight off tumors and destroy cells that are infected by viruses.
Because of the finding of elevated NK cells in some women who have had miscarriages, some researchers have speculated that NK cells could also be responsible for a woman's body terminating a pregnancy. Not everyone agrees, however, that the finding of elevated NK cells in women with recurrent miscarriages means that the NK cells themselves are the cause of the miscarriages. There are other reasons why the NK cells could be elevated.