If you have had recurrent miscarriage and every standard test has come back normal this post may be the one that finally gives you an answer.
Because there is a category of immune dysfunction that is present in a significant proportion of women with unexplained recurrent miscarriage one that is not tested in standard investigation, is not widely known outside specialist reproductive immunology, and yet has a growing body of evidence behind both its role in pregnancy loss and its treatment.
It involves a type of immune cell called natural killer cells.
And understanding them may change everything about how you approach your next pregnancy.
WHAT ARE NATURAL KILLER CELLS?
Natural killer cells commonly called NK cells are a type of white blood cell that forms part of the innate immune system. They are so named because of their original characterisation as cells that could kill tumour cells and virus-infected cells without prior sensitisation unlike T cells and B cells, which require prior exposure to an antigen before mounting a response.
NK cells are found throughout the body. But the NK cells that matter most in the context of miscarriage are a specialised population found in the uterus called uterine natural killer cells, or uNK cells.
Uterine NK cells are not the same as peripheral blood NK cells the ones circulating in your bloodstream. They are a distinct population, found specifically in the endometrium, with a unique profile of surface receptors and a very different function from their peripheral counterparts.
And their function in pregnancy is not to attack. It is to facilitate.
WHAT UTERINE NK CELLS ACTUALLY DO
This is where most people's understanding of NK cells in miscarriage goes wrong.
Uterine NK cells are not simply killers. In the context of a healthy pregnancy, they are essential facilitators of implantation and placental development.
Their key functions include:
Facilitating trophoblast invasion. The trophoblast the cells that form the placenta must invade the uterine wall to a precise depth to establish the placental blood supply. uNK cells guide and regulate this invasion, producing cytokines and growth factors that direct trophoblast migration. Too little invasion means an inadequate placenta. Too much invasion is associated with conditions like placenta accreta.
Remodelling the uterine spiral arteries. The spiral arteries the maternal blood vessels that supply the placental bed must undergo dramatic remodelling in early pregnancy to become wide, low-resistance vessels capable of delivering sufficient blood to the growing foetus. uNK cells are the primary drivers of this remodelling. Inadequate spiral artery remodelling is associated with miscarriage, preeclampsia, foetal growth restriction, and placental abruption.
Regulating immune tolerance. uNK cells help establish and maintain the immunological tolerance that allows the semi-foreign embryo to implant and survive. They interact with the trophoblast through specific receptor-ligand interactions and produce regulatory cytokines that create a tolerogenic environment at the implantation site.
Producing angiogenic factors. uNK cells produce vascular endothelial growth factor (VEGF) and other angiogenic factors that stimulate the development of new blood vessels in the decidua the specialised uterine lining of pregnancy.
When uNK cells are functioning normally, all of these processes work in concert to create the ideal environment for a healthy pregnancy.
When they are dysfunctional either too numerous, too active, or producing the wrong profile of cytokines implantation fails or early pregnancy is lost.
HOW NK CELL DYSFUNCTION CAUSES MISCARRIAGE
The relationship between uNK cell abnormalities and miscarriage is complex and it is important to understand that the problem is not simply having too many NK cells.
Elevated uNK cell numbers. In some women with recurrent miscarriage, uNK cell density in the endometrium is significantly higher than normal. Elevated uNK cell numbers are associated with impaired trophoblast invasion, inadequate spiral artery remodelling, and increased inflammatory cytokine production at the implantation site all of which can result in early pregnancy loss.
Abnormal uNK cell activation. Even when numbers are normal, abnormal activation patterns producing an excess of cytotoxic or inflammatory signals relative to tolerogenic ones can create a hostile environment for the developing embryo.
Impaired uNK cell function. In other cases, uNK cells are present in normal numbers but fail to perform their remodelling and tolerance functions adequately resulting in poor placental development despite an apparently receptive endometrium.
NK cell receptor abnormalities. The interaction between uNK cells and trophoblast cells is mediated through specific receptor-ligand pairs including killer immunoglobulin-like receptors (KIRs) on NK cells and HLA-C molecules on the trophoblast. Mismatches between maternal KIR genotype and paternal HLA-C type are associated with significantly higher rates of miscarriage and preeclampsia a finding that has emerged from large population studies and represents one of the most intriguing areas of current reproductive immunology research.
PERIPHERAL BLOOD NK CELLS VERSUS UTERINE NK CELLS
This distinction is critical and is a common source of confusion.
Peripheral blood NK cells are those circulating in the bloodstream. They are relatively easy to test a simple blood draw allows their number and activity to be assessed.
Uterine NK cells are those in the endometrium. They can only be assessed through an endometrial biopsy a minor procedure typically performed in the luteal phase of the cycle, when uNK cell numbers are at their peak.
The relationship between peripheral and uterine NK cells is imperfect. Elevated peripheral NK cell numbers and activity correlate with elevated uNK cell numbers in many but not all women. Peripheral NK cell testing is therefore a useful screening tool but not a definitive assessment of the uterine NK cell environment.
Some clinics offer peripheral NK cell testing as a first step, with endometrial biopsy reserved for women with elevated peripheral results or persistent unexplained miscarriage despite normal peripheral results.
A thorough assessment in a specialist reproductive immunology clinic will typically include both.
WHO SHOULD BE INVESTIGATED?
NK cell investigation is most relevant for women with:
● Three or more unexplained miscarriages with no identified cause on standard investigation
● Recurrent implantation failure in IVF particularly with good quality embryos
● A history of miscarriage combined with failed IVF cycles
●Miscarriages where products of conception testing showed chromosomally normal embryos
— A personal or family history of autoimmune conditions
— Elevated inflammatory markers on blood testing — hsCRP, fibrinogen — without a clear identified cause
The last two points are important. NK cell dysfunction does not occur in isolation — it is frequently part of a broader picture of immune dysregulation that may include thyroid autoimmunity, antiphospholipid antibodies, and elevated systemic inflammation. Investigating one without the others gives an incomplete picture.
THE TREATMENT OPTIONS
This is an evolving area of medicine and treatment protocols vary between specialist centres. It is important to work with a reproductive immunologist or specialist who has experience in this field rather than attempting to self-treat.
That said, the main treatment approaches with evidence behind them include:
Intralipid infusions. Intralipid a fat emulsion originally developed for intravenous nutrition has been shown in several studies to suppress elevated NK cell activity. The mechanism is not fully understood but is thought to involve the immunomodulatory effects of the lipid components on NK cell receptor signalling. Intralipid is administered intravenously, typically before embryo transfer in IVF or around the time of ovulation in natural conception cycles, and may be repeated in early pregnancy. It is generally well tolerated and has a reasonable safety profile.
Intravenous immunoglobulin (IVIg). IVIg a preparation of pooled human immunoglobulin has broader immunomodulatory effects and has been used in recurrent miscarriage associated with immune dysfunction including elevated NK cell activity. The evidence base is less consistent than for intralipid and the treatment is significantly more expensive. It is typically reserved for cases where intralipid has been insufficient or where there is broader immune dysregulation.
Prednisolone. Low-dose corticosteroid therapy with prednisolone has been used to suppress NK cell activity and reduce endometrial inflammation in women with elevated uNK cells. It is typically started before conception or at the time of embryo transfer and continued into early pregnancy. Evidence from several studies supports its use in this context, particularly in combination with other immunomodulatory approaches.
Hydroxychloroquine. As discussed in the antiphospholipid syndrome post in this series, hydroxychloroquine has broad immunomodulatory effects and is increasingly used as an adjunct treatment in immune-mediated recurrent miscarriage including cases involving NK cell dysfunction.
Progesterone. Progesterone has immunomodulatory effects in the endometrium, shifting the immune environment toward tolerance. Luteal phase progesterone support vaginal progesterone from ovulation or embryo transfer is a low-risk intervention with emerging evidence for benefit in immune-mediated miscarriage beyond its established role in luteal phase support.
Lifestyle and nutritional interventions. Vitamin D has direct immunomodulatory effects on NK cells and optimising vitamin D to 100–150 nmol/L is a standard component of managing immune dysfunction in recurrent miscarriage. Omega-3 fatty acids have anti-inflammatory effects that may modulate NK cell activity. Reducing systemic inflammation through dietary and lifestyle intervention supports the broader immune environment.
THE INVESTIGATION GAP WHY THIS IS NOT ROUTINELY OFFERED
NK cell testing and treatment is not part of the standard recurrent miscarriage protocol in most health systems. In the UK, Australia, Canada, and the United States, it is offered primarily through private specialist reproductive immunology clinics not through standard public health pathways.
This reflects a combination of factors: the relative novelty of the field, ongoing debate about optimal testing and treatment protocols, the cost of investigation and treatment, and the complexity of the biology involved.
It does not reflect an absence of evidence. The evidence for NK cell involvement in recurrent miscarriage is substantial and growing. What is lacking is the clinical infrastructure to deliver this investigation universally.
If you have had recurrent miscarriage with no identified cause and you have the means to access a specialist reproductive immunology consultation, it is worth pursuing. The investigation is not definitive for everyone NK cell abnormalities are not present in all cases of unexplained recurrent miscarriage but for the proportion of women in whom they are present, understanding this aspect of their immune picture can be genuinely transformative.
HOW TO FIND SPECIALIST CARE
In Canada, the UK, Australia, and internationally, specialist reproductive immunology clinics exist in most major cities. Key names to search for include:
• Reproductive immunologist
•Recurrent miscarriage clinic
• Implantation failure specialist
• Reproductive medicine specialist with immunology focus
When seeking a referral, ask your GP specifically for a referral to a recurrent miscarriage or reproductive immunology specialist not a general obstetrician or fertility clinic, unless that clinic has a dedicated immunology program.
WHAT TO SAY TO YOUR DOCTOR
"I have had recurrent miscarriage and my standard investigation has not identified a cause. I would like a referral to a reproductive immunologist or recurrent miscarriage specialist to investigate whether NK cell abnormalities or broader immune dysfunction may be contributing to my losses. I understand that peripheral blood NK cell testing and endometrial biopsy for uterine NK cell assessment are available and would like to discuss whether these investigations are appropriate in my case."
THE LAB INTERPRETATION GUIDE
While NK cell testing itself goes beyond the standard blood panel covered in the Lab Interpretation Guide for Fertility Health, the inflammatory markers, thyroid autoimmunity markers, and vitamin D assessment covered in the guide are directly relevant to the immune picture in women with NK cell dysfunction and are often the first step in identifying that broader immune investigation is warranted.
→ Get the Lab Interpretation Guide here $47
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A FINAL WORD
Unexplained recurrent miscarriage is not the same as unexplainable recurrent miscarriage.
For a significant proportion of women told there is nothing to find, the answer lies in an area of reproductive medicine that standard investigation simply does not reach.
NK cell dysfunction is one of those areas.
You deserve to know it exists. You deserve to know it can be investigated. And you deserve to know that for many women, understanding and treating this aspect of their immune picture has been the turning point in
Dr. Rose Ngandalo is a medical doctor with specialist training in functional and integrative medicine, with a focus on metabolic and reproductive health. The information in this post is for educational purposes only and does not constitute medical advice. Always consult a qualified healthcare practitioner regarding your individual health.
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