Have you been diagnosed with unexplained infertility by your GP, or had two or more miscarriages? Do you have healthy embryos that fail to implant during your IVF cycle, or have you conceived naturally before, but are now struggling with fertility, without any explanation?

In clinic, we are seeing an increasingly large amount of women being diagnosed with Natural Killer (NK) cells. NK cells is a debatable topic amongst specialists with varying degrees of opinions. Today, I am here to present to you an understanding of the role of NK cells and how they can affect your fertility.


Our lifestyle and effects on our health

 If you took a snapshot of your current ‘healthy’ lifestyle, a typical day might look like:

  • Wake up at 6am, check your phone, and download emails.
  • Go the gym for an hour, shower, get ready for work, make a smoothie, and drink it in your car on the way.
  • Arrive at work for an 8.30am start, eat lunch at your desk (often consisting of last night’s left overs, or a sandwich/sushi purchased from the cafeteria downstairs).
  • Finish work 6:00pm and drive home in peak-hour traffic.
  • Muster up the effort to make dinner, sit on the couch, watch TV, and check your phone.
  • Go to bed. Repeat.

Sadly, the combination of the environment, accelerated lifestyle, and pursuit to ‘have it all’ dictates the state of our health. Gone are the days of living within the laws of nature. Our body is at war with itself and our immune system is suffering.


Our immune system and infertility

 Our immune system is our natural defence system and is there to protect and fight against any bacteria, viruses and other harmful pathogenic invaders.

When a foreign pathogen invades the body, its first response is to attack and protect the body against disease or any other potentially damaging foreign bodies. Our immune system lines the entire epithelium of small intestine and ovaries, which protects and fights against invades and bacteria.

 While our immune system has evolved over the years, today our bodies are under a lot of stress from the lifestyle we lead and environment we live in, and it is not coping. The combination of high stress levels with a toxic lifestyle, is where we slowly start to see immunological dysfunction in the body, and where NK cells may affect fertility.

 What are Natural Killer cells?

Natural killer (NK) cells are a diverse group of cells that can be found in the blood and other areas of the body including the uterus. They earned the name “killer” cells because it was found that the type of NK cells circulating in the blood were able to bind to and kill certain cancer cells and virus infected cells.

There are several types of NK cells and some have a negative impact on reproduction and some not. NK cells are the most abundant immunological cells in the uterine lining of the uterus and come into contact with an implanting embryo, which that mostly cause repeated failed implantation or recurrent miscarriages.

Blood NK cells

Blood NK cells are quite different from uterine NK cells. They look different under a microscope, they react differently, and they function differently.

Uterine NK cells

Uterine NK cells are found in the uterine lining (endometrium) of all women, and are the dominant maternal immune cells during the formation of your placenta.

NK cells are an essential part of the body’s defence mechanism and are one of several types of lymphocytes in the immune system protecting us from infections. They form an important part of our defence system and protect us from bacteria, viruses and possible cancer cells.

After ovulation, uterine NK cells activity increases, and for those who have elevated NK cells, become highly active during this phase of your cycle (luteal or secretory) and subsequently attack the embryo during the secretory phase (know as our luteal phase).

They are most active after you have ovulated and during implantation. When a woman’s hormone production changes during her menstrual cycle, Uterine NK cells will change. Levels remain low before ovulation and increase dramatically after, day by day.

NK cells and fertility 

NK cells are present in every woman’s uterus and are the dominant maternal immune cells during the formation of the placenta of pregnant women. NK cells play an important role during the implantation and growth of an embryo.

Uterine NK cells are also an important mediator between the placenta and the uterus. They grow in large numbers to protect an embryo and ensure its development. They are there to attack anything that may try and harm the embryo, including viruses. If the NK cells detect that there is anything to attack in the pelvic cavity or uterus, your immune system sends an abundance of NK cells to the site to get to work.

Uterine NK cells accumulate and grow in large numbers at the implantation site. Their job is to to protect the embryo against any infection and ensure its successful development. After ovulation and during early pregnancy, NK cells comprise of more than 80% of the white blood cells population seen in the uterine lining.

Failed implantation and pregnancy loss

After ovulation, a fertilised egg and implantation begins the specialised embryonic cells called trophoblasts, which later forms the placenta, and begins to grow in the uterine lining (endometrium). This transformation is essential to ensure a normal blood supply to the foetus and placenta throughout pregnancy.

Dr Alan Beer, a pioneer in the field of reproductive medicine, states that there are four types of immune problems in women with recurrent miscarriages, with one of them being NK cells, of which can lead to poor blood flow, interference with implantation, early miscarriage and lack of nutrition to the foetus, causing the body to mistakenly attack normal cells as if they were invaders.

When fertilisation and implantation occurs, the body’s normal response is to protect the growing embryo. In auto-immune infertility the body begins to turn against itself.  When an embryo tries to implant, the body’s same defence system recognises it as a foreign threat, sending NK cells to attack, causing failed implantation, no implantation at all or a miscarriage.

‘Unexplained infertility’ explained

What is frustrating for couples when actively trying, is that on paper everything looks normal. You are having sex at all the correct times in your cycle, hormone levels are normal, semen analysis parameters all point to his sperm being capable of fertilising an egg – yet nothing is happening. When this occurs your fertility specialist will normally diagnose you with “Unexplained Infertility”. In clinic, there is always a reason why we are not falling pregnant. 

If you are having continual failed implantation, reoccurring miscarriages, and no pregnancy further testing will often result in the diagnosis of NK cell.

It is also entirely plausible that a ‘fertile’ woman may have future secondary reproductive failure. Woman who conceived the first time and have difficulty with their second child who undergo further testing, often have NK cells being present. I am seeing a lot of this in clinic.

Testing NK cell levels

Natural Killer cells can only be tested through a biopsy sample from the endometrium.

During menstruation we shed the NK cells with our endometrial lining, and the biopsy is performed on day 21 of your menstrual cycle, when the NK cell count is at its highest.

The procedure is done at your fertility specialist and results take about two weeks to receive. If your reading is greater than 14% on day 21, then you are diagnosed with NK cells. Most women say the test is similar to having a pap smear. You can have the procedure done by your fertility specialist.

Is it time to slow down?

With over 10 years of clinical experience and 90% of my patients being diagnosed with NK cells, it is in my opinion that this condition does exist.  With our lifestyle being what it is today, my findings have indicated a need for us to take the time to evaluate our diary, relax and take time out when we can. We really need to practice and indulge in our ‘down time’. We need to bring our bodies back into a state of balance, out of this constant flight or fight into REST & NEST, so important when it comes to our fertility.