Infertility Blogger
Lee Trask is an advocate for women dealing with issues of infertility and miscarriage. Having struggled through more than six years of infertility, three miscarriages, and high-risk pregnancy, she is now happy raising her two…
What are "natural killer" cells (NK cells) and why do they cause miscarriage?
Posted in Miscarriage by Lee Trask on Aug 18, 2010
"I have a healthy son, age 4. But over the past two years, I have had three miscarriages in a row. My doctor ran tests and told me a have a high number of "natural killer" cells, and that could be causing my miscarriages. What are NK cells?"

I am sorry to hear about your miscarriages, but am glad that your doctor has found a possible cause. I actually had this diagnosis after three miscarriages, (along with anitphospholipid syndrome), and this is what I learned from my doctors about NK (natural killer) cells:

Every person’s immune system contains white blood cells, and those cells make antibodies. Those antibodies are generally manufactured when the body is being "attacked," and their job is to fight off infection, viruses, bacteria, etc... anything the body sees as an invader. Sometimes those antibodies go astray, and attack normal anatomy, and that is called an autoimmune disorder. (Lupus and Multiple Sclerosis are two examples of the immune system mistakenly attacking healthy body tissue).

When a sperm and an egg create a fetus, the DNA from the father is obviously not the same as the DNA belonging to the mother and the eggs she produces. Normally the genetic material from the father would not cause a reaction in the mother’s system. However, in some cases, the immune system of the pregnant mother mistakes that DNA from the sperm of the father as a “foreign substance,” and will increase production of natural killer cells. These cells cause a hostile and toxic environment for the fetus, and can disrupt the development of the placenta: a miscarriage follows, and the immune system thinks it has done it’s job by “getting rid” of the invading genetic substance.

Autoimmune diseases are not fully understood, and part of their mystery is that they can actually come and go. And because autoimmune disorders can “switch on and off,” it is possible to get pregnant and have a healthy and normal pregnancy, and then have an autoimmune reaction in subsequent pregnancies, with the same father. (I had a perfectly normal pregnancy with my first boy, then three miscarriages, so my body had apparently “switched on” its defenses against my husband’s DNA.)

There are drugs that can suppress the immune system during pregnancy, and can increase the success rate of carrying full term. Prednisone, a steroid, can be taken trimester while the mother’s body would be resisting the presence of the father’s DNA. As the fetus and the placenta develop and the pregnancy progresses, some women can be weaning from the steroid, while others may need to stay on the medication until delivery. My RE took me off prednisone after the first trimester, while my friend Bonnie, who was pregnant through IVF, remained on it her entire pregnancy. Talk with your doctor about whether or not he/she believes that prednisone could be used in your case to increase your chances of maintaining your next pregnancy.


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