Do Elevated Natural Killer Cells Cause Recurrent Miscarriages?

It's a controversial topic that has split sides among fertility specialists

A natural killer cell of the innate immune system. The yellow particles inside the cell represents the toxin
A natural killer cell of the innate immune system. The yellow particles inside the cell represents the toxin. Getty Images/Stocktrek Images

Whether or not natural killer cells (NK cells) cause miscarriage is a widely debated topic among fertility specialists. There is a lot of evidence that natural killer cells are elevated in women with recurrent miscarriages, but not so much evidence that the NK cells themselves are causing these miscarriages.

Despite the strange sounding name, natural killer cells are not a bad thing to have. 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 elevated NK cells levels in women with recurrent miscarriages, some researchers believe that NK cells may also be responsible for a woman's body terminating a pregnancy. However, there are other reasons why the NK cells could be elevated.

Case For NK Cells

It's hard to dispute that NK cells are elevated in women who have recurrent miscarriages since numerous studies have found this to be true. Scientists have fleshed out a few mechanisms in which elevated NK cells could terminate an otherwise viable pregnancy, usually promoting the idea that a disordered immune system causes the cells to attack the pregnancy. At least one study has found evidence that NK cells in the uterus attack cells from the pregnancy in cases of spontaneous miscarriage, although it's unclear what those results mean.

Some researchers have even looked at the chromosomes of the miscarried pregnancy to determine whether the NK cells were elevated because of the body's natural response to a chromosomally abnormal pregnancy.

The researchers discovered that women with elevated NK cells were potentially more likely to miscarry a chromosomally normal baby in their next pregnancy.

If true, elevated NK cells are causing viable pregnancies to miscarry, reducing elevated NK cells should lead to reduced risk of miscarriage. Proponents of this theory sometimes prescribe immune-suppressing medications such as prednisone to women with unexplained recurrent miscarriages and elevated NK cells –– an approach that has been shown in at least one study to reduce the levels of NK cells.

One widely reported study from 2008 claimed that this approach led 75 percent of the study's participants to carry their next pregnancies to term. The researchers suggested that elevated NK cells could be behind as many as one-third of all cases of unexplained miscarriages.

Case Against NK Cells

There are plenty of alternative explanations for the finding of elevated NK cells in women with recurrent miscarriages. There is evidence that stress can cause fluctuations in NK cells. A study from 2006 found that women with recurrent miscarriages could have elevated NK cells in a first of two blood draws -– but then in a second blood draw 20 minutes later, they could show no elevation of NK cells as compared to women with no history of miscarriage. The researchers speculated that there was a chance that some women tended to have an immune system that responded more readily to stress, which might tie in with some other research showing correlations between stress and miscarriage.

There are many gaps in understanding how NK cells affect pregnancy.

Researchers have yet to find compelling evidence that any kind of immune-suppressing therapy actually works to reduce miscarriage rates. Although many proponents of these therapies point to some successful findings, there's a lot to be said for the placebo effect given. Especially since research exists showing that about that same number of women will go on to have successful pregnancies merely with supportive care and no specific treatment.

Getting NK Cell Related Therapies

Despite the occasional headlines, no medical organization formally recommends testing for and treating NK cells in women with recurrent miscarriages. There is a lot more research that needs to be done before anyone can truly recommend steroids or immune suppressants as a recurrent miscarriage treatment. Any treatment for elevated NK cells should be considered experimental at this point.

That being said, when you're having miscarriage after miscarriage, you may be willing to try just about anything whenever for the chance that it might work. If you are pursuing immune system therapies for elevated NK cells, make sure that you understand the uncertainties in the research and work with a practitioner who doesn't ensure unreasonable promises. You should also think carefully before spending a lot of money on this kind of treatment, especially if you have limited financial resources.


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