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Understanding “Natural Killer Cells”

Understanding why people experience recurrent pregnancy loss has proven challenging over the past half century. Throughout this time, many suspects have been identified as possibly causing increased miscarriage rates – but then have been found not to play a meaningful role in this process. One such suspect is natural killer cells.

What are Natural Killer Cells?

In the mid 1990s, so-called “natural killer cells” or “NK cells” were introduced as a possible reason for some recurrent pregnancy loss and/or problems with embryos attaching to the uterine lining. Natural killer cells are simply a part of your immune system, and they are found in all people. Specifically, these cells are a type of large lymphocyte (white blood cell) and are needed for your immune system to function properly.

The initial theory behind natural killer cells was that an overactive immune system may harm an embryo or pregnancy, and so investigators began measuring the activity level of natural killer cells in patients suffering from recurrent pregnancy loss. This practice became relatively widespread in the late 1990s.

Is There a Link Between Natural Killer Cells and Miscarriage?

As more information has been collected on natural killer cells, a link between natural killer cell levels and pregnancy loss remains unclear. In one experiment using mice, natural killer cells were eliminated altogether, and this resulted in the mice losing 100% of their pregnancies. This suggested that natural killer cell function, and immune function in general, is likely a key component of a healthy and normal pregnancy.

Other studies have found natural killer cell activity in women undergoing elective pregnancy termination to be similar to levels found in women with recurrent pregnancy loss. Some investigators in small retrospective trials have published a link between natural killer cells and pregnancy loss or implantation issues; However, the vast majority of large and well constructed, high quality studies fail to find such a link.

Current papers recently published  in the two leading infertility journals, Fertility and Sterility in the United States and Human Reproduction in Europe, state that elevated natural killer cell activity has not been found to be a risk factor for miscarriage or decreased implantation potential and should, at the present time, only be collected for experimental purposes.

Even so, a handful of clinics continue to measure natural killer cells and treat patients. In many instances, such clinics state that they are performing “cutting edge medicine” that other clinics are not providing. However, testing and treating patients for natural killer cells is usually costly for the patient and likely leads to misdiagnosis for many couples.

If you’d like more information about recurrent pregnancy loss, please call our Fertility Associates of Memphis office at 901-747-2229 to make an appointment. Our board certified physicians would be happy to assist you.