NK cell and immune blood tests in fertility and miscarriage: what the evidence shows

Clinical Review · Published:  16 October 2026 · Last reviewed: 15 October 2026

Author: Dr Ciara Wright PhD BSc DipNT, Nutritional Therapist, member of Nutritional Therapists of Ireland. ORCID: 0009-0002-8875-6090

A narrative review of the evidence on NK cell, Th1/Th2 and cytokine testing in fertility and recurrent miscarriage

This review is for women and couples considering immune testing after failed IVF or recurrent miscarriage, and for the practitioners advising them.

 

Key findings

• Blood NK cell tests do not describe NK cells in the uterus, which belong to the same family but have a different role.
• The immune environment of the endometrium does appear to be altered in some women with recurrent loss or implantation failure, but this needs further research.
• The Th1/Th2 ratio on the "Chicago bloods" measures what stimulated T cells can produce in a test tube, not active inflammation. Measuring T helper subsets by flow cytometry directly identifies whether there is a Th1, Th2 or Th17 skew, or a loss of regulatory T cells.
• Serum cytokines reflect the body's overall inflammatory load and are a useful signal of systemic inflammation.
• Systemic inflammation and autoimmune conditions, such as thyroid autoimmunity, endometriosis and rheumatoid arthritis, are linked to reduced fertility.
• Raised immune markers are best read as a prompt to look for the source of inflammation, which is often not obvious. The areas with the most evidence base are thyroid autoimmunity, the gut, vaginal and endometrial microbiomes, and nutrient status.
• Before a further transfer, we focus on low-risk steps: addressing autoimmunity, correcting nutrient deficiencies, supporting the gut and vaginal microbiome, and improving diet quality.

 

 

Summary

Peripheral blood immune tests cannot diagnose what is happening in the uterus, and no guideline recommends them routinely [1]. We measure them for a different reason: systemic inflammation and autoimmunity have their own evidence base in subfertility and pregnancy loss, and they are often modifiable.

In this light, a raised NK cell count, cytokine ratio or Th1 activity is a prompt to look for a source of inflammation in the body. Without an obvious cause, this can sometimes seem like looking for a needle in a haystack, but we investigate the thyroid, gut microbiome, vaginal microbiome and uterus as particularly significant sources of inflammation.

 

The question: why test immune markers in fertility?

In patients with 'unexplained' infertility, recurrent IVF failure, implantation failure or recurrent early miscarriage, repeating the same process over and over can cost a great deal of time and money, and bring repeated heartbreak. Recurrent pregnancy loss is usually defined as two or more pregnancy losses [1], and recurrent implantation failure as repeated transfer of good-quality embryos without a pregnancy. Before another attempt, it is reasonable to ask whether anything can be improved. Immune markers help us look for an underlying, or 'root-cause', factor: a systemic inflammatory or autoimmune load that could be identified and reduced before the next pregnancy.

 

Immune blood tests: what they measure

NK cell testing: what it measures and what it means

NK cells. Natural killer (NK) cells are lymphocytes of the innate immune system and make up roughly 5% to 15% of lymphocytes in blood. Unlike T and B cells, they need no prior exposure to act. They recognise infected, stressed or malignant cells by a loss of the "self" HLA class I molecules that normally hold them in check, together with activating stress signals. They then kill those cells by releasing perforin and granzymes, or shape the wider immune response through cytokines such as IFN-γ and TNF-α. This makes them an early line of defence against viruses and tumours.

Uterine NK cells. These cells were given the name because they carry the defining features of the NK lineage. They express CD56 and NKp46, lack the T cell marker CD3, and contain perforin and granzyme granules, as peripheral NK cells do [2,3]. In other respects they are a distinct population. About 90% of blood NK cells are CD56-dim, CD16-positive cytotoxic cells, whereas uterine NK cells are CD56-bright, CD16-negative and tissue-resident, expressing markers such as CD9 and CD49a that blood NK cells largely lack.

This is not unusual in immunology, where one lineage name often covers cells with quite different tissue roles, as with macrophages in the lung, gut and brain. Despite their granules, uterine NK cells are weakly cytotoxic and act mainly through cytokines, growth factors and vascular remodelling. Blood NK cells are therefore not uterine NK cells: a peripheral cytotoxicity assay measures a different cell doing a different job, and a blood result does not describe the endometrium.

How are NK cells tested?

Peripheral blood NK testing usually combines up to three types of measurement. The first is a count by flow cytometry: NK cells are identified as CD3-negative, CD56-positive lymphocytes and reported as a percentage of lymphocytes, an absolute count, or both, often split into CD56-bright and CD56-dim subsets.

The second is a cytotoxicity assay. The classic method incubates the patient's cells with a standard tumour target cell line (K562) at several effector-to-target ratios and measures how many target cells are killed. This is the NK assay offered by the Rosalind Franklin laboratory in Chicago as part of the "Chicago bloods", which also reports killing after IVIG or Intralipid is added to the sample.

The third, which is the method we use through a Eurofins laboratory in Germany, measures activation. NK cells are stimulated in culture with IL-2, and the increase in the activation markers CD69 and CD25 (the alpha chain of the IL-2 receptor) is measured by flow cytometry. CD69 is an early activation marker and CD25 rises as cells respond to IL-2, so the result reflects how readily the NK cells can be switched on. This measures basal activation levels plus capacity under stimulation, but none of these assays describe uterine NK cells.

Do raised blood NK cells predict IVF failure or miscarriage?

The relevance of peripheral blood NK cell analysis to fertility is not yet clear. A 2014 meta-analysis found higher peripheral NK cell numbers in infertile women than in fertile controls (standardised mean difference, SMD 3.16; 95% confidence interval, CI 1.07 to 5.24) and in women with recurrent miscarriage than in controls (SMD 0.81; 95% CI 0.47 to 1.16), but the differences did not hold when NK cells were expressed as a percentage [4]. An earlier systematic review found too few women in the available studies to say whether high peripheral NK percentage or activity predicts subsequent miscarriage or implantation failure, though the estimates leaned towards higher risk; importantly, none reached statistical significance [5].

A 2019 study combined a case-control analysis with a meta-analysis [6]. In its own cohort of 115 women, peripheral NK cell proportions did not differ between women with recurrent miscarriage or implantation failure and controls, nor between women who went on to miscarry and those who had a live birth. Pooling the wider literature, however, showed higher NK proportions than in controls, by about 3.5 percentage points in recurrent miscarriage (95% CI 2.94 to 4.00) and 1.6 in implantation failure (95% CI 0.82 to 2.45).

Are uterine NK cells different in recurrent loss?

Results from studies of uterine NK cells have been mixed, and depend on how and when the cells are measured. The 2014 review above pooled six uterine studies and found no difference in uterine NK cells, measured as a percentage of stromal cells, between women with recurrent miscarriage and controls [4]. A larger 2022 meta-analysis that separated the type of tissue sampled found higher CD56+ uterine NK levels in the pre-pregnancy endometrium of women with recurrent miscarriage than in controls (SMD 0.49; 95% CI 0.08 to 0.90; 1,100 women), although heterogeneity was very high [7].

In 2019, an Irish group used flow cytometry on endometrial biopsies to compare 612 patients with 35 controls [8]. Women with implantation failure had higher uterine NK proportions than controls or women with recurrent loss (53.2% versus 45.2% and 42.9%, p < 0.0001). All the subfertile groups had more peripheral-type NK cells in the endometrium, with greater CD69 activation, more B cells and a higher CD4:CD8 ratio. The same group found more CD56-dim peripheral-type NK cells in the menstrual blood of 58 patients than in 15 fertile controls, along with more T cells and B cells and a higher CD4:CD8 ratio, which may offer a non-invasive window on the uterine environment [9].

These findings need careful interpretation, and they have several limitations. First, results depend on how the cells are counted. When the same group measured concentration per milligram of tissue in 455 patients, uterine NK cells did not differ between groups, although proportions had differed in the larger study [10]. Second, a higher count is not the same as higher activity. Third, uterine NK cells are only weakly cytotoxic and work mainly through cytokines and vascular remodelling, so it is not established whether more of them is a cause, a consequence or a marker of a disturbed endometrium.

The more consistent finding may be the increase in peripheral-type NK cells, more cytotoxic CD56-dim cells that normally predominate in blood rather than the uterus. These were raised in the endometrium of all the subfertile groups whether measured as a proportion or as a concentration, and in menstrual blood [8–10]. So far, these data come from a single group with small control populations, and have not yet been shown to predict treatment outcome.

What the differences do suggest is that the immune environment of the endometrium is altered in some women with recurrent loss or implantation failure. That calls for more research, and in the meantime it makes sense to ask what might be disturbing the environment of the implanting embryo.

Is the Th1/Th2 ratio on the "Chicago bloods" useful?

The test commonly called the Th1/Th2 ratio, which appears on the "Chicago bloods", is not a Th1/Th2 analysis. It does not identify Th1 or Th2 cells. The laboratory stimulates lymphocytes in culture, counts the CD4 T cells that stain for intracellular TNF-α, IFN-γ or IL-10, and reports TNF-α:IL-10 and IFN-γ:IL-10 ratios, with a reference range for TNF-α:IL-10 of 13.2 to 30.6 [11]. It is a cytokine assay, and a limited one, for three reasons.

First, it measures capacity, not activity. The assay shows what CD4 T cells can produce once stimulated in a test tube. It does not measure circulating cytokines, so a high ratio can occur with an unremarkable serum TNF-α.

Second, the cytokines are not specific to Th1 or Th2 cells, or to CD4 T cells. TNF-α is made by several activated T cell subsets and by innate cells such as monocytes and NK cells. IL-10 is made by regulatory T cells, B cells and monocytes, and by Th1 cells as well as Th2 cells. Counting cytokine-positive CD4 T cells alone misses many of the cells that set cytokine levels in the body.

The panel also has no IL-4, the signature Th2 cytokine, and no IL-17 or regulatory T cell marker, so it cannot separate a Th1 skew from a Th17/Treg imbalance, which is now considered at least as relevant [12].

Third, a ratio has two moving parts. A high TNF-α:IL-10 ratio can mean too much TNF-α, or a normal amount of TNF-α with too little IL-10. The first points towards active inflammation and the second towards weak regulation, and they may call for different responses.

Because IL-10-positive cells are usually a small fraction of CD4 T cells, a small drop in the denominator moves the ratio a long way: if they fall from 2% to 1%, the ratio doubles although nothing pro-inflammatory has changed. A ratio in the reference range can equally hide two values that are both raised or both low. The percentages behind a ratio therefore need to be read alongside it, not just the ratio itself.

In practice, an Irish group used this assay, with the Rosalind Franklin reference ranges of 13.2 to 30.6 for TNF-α:IL-10 and 5.8 to 20.5 for IFN-γ:IL-10, in 337 women with implantation failure or pregnancy loss, and found raised ratios in 44.5% of the participants [13]. Of these, 134 began nutraceuticals, prednisolone and intralipids, and implantation and miscarriage rates improved against their own history. There was no untreated control arm, and the authors called for randomised trials.

A companion paper from the same group shows how strongly the denominator drives the result [14]. At baseline fewer than 1% of cells expressed IL-10, against roughly 50% to 57% for TNF-α, giving TNF-α:IL-10 ratios of about 71. After a 10-week antioxidant and vitamin regime the mean ratio fell to 21.0, and the improvement came primarily from a rise in IL-10 expression, with only a moderate fall in stimulated TNF-α.

T helper subsets measured by flow cytometry

Each T helper subset has a distinct role in pregnancy, and timing matters. Around implantation a tightly controlled Th1, pro-inflammatory response predominates, and it supports the invading trophoblast rather than harming it. Soon after implantation the balance shifts towards Th2, anti-inflammatory responses, which come to dominate at the implantation site, temper Th1 activity and allow the foetus and placenta to develop. Regulatory T cells keep local inflammation in check. Th17 cells defend against extracellular microbes during pregnancy, but in excess they may drive uncontrolled neutrophil infiltration at the maternal-foetal interface [12]. Healthy pregnancy is therefore not simply Th2-dominant, and the balance between subsets matters more than any one of them.

Flow cytometry measures these populations directly. Rather than deriving a ratio from cytokines, immunophenotyping identifies Th1, Th2, Th17 and regulatory T cells by their own markers and reports each as a proportion of total CD4 T cells. This shows which population has actually moved. It separates an expansion of effector cells from a loss of regulatory cells, and allows the true Th1/Th2 balance and Th17/Treg balance to be assessed directly. It does not remove every limitation. The result still describes blood rather than endometrium, and reference ranges for women trying to conceive are not standardised.

The same technique can be applied to the endometrium itself. An Irish group used a 10-colour flow cytometry panel on endometrial biopsies to measure each subset as a proportion and as a concentration per milligram of tissue. Across 612 patients, all subfertile groups had a higher proportion of Th1 CD4 cells and fewer regulatory T cells than controls [8]. In 455 patients, Th1 concentrations were higher in every adverse outcome group, while Th2 and regulatory T cell concentrations were lower but not significantly so; Th17 did not differ [10]. This points towards a more inflammatory immune environment in the uterine lining, although a woman's individual result did not predict whether her next treatment succeeded.

Serum cytokines as markers of systemic inflammation

Cytokines are better measured directly, as circulating proteins in serum or plasma, usually by ELISA or a multiplex immunoassay. This reflects what the body is actually exposed to, rather than what cells can produce once stimulated in a test tube, and the assays are widely available.

What a serum result cannot do is say which cells produced a cytokine, or where in the body. Most cytokines act locally and briefly, so a blood level reflects the overall inflammatory load. Read this way, serum cytokines are markers of systemic inflammation. The local immune response in the uterus is complex and still needs far more research. On current evidence though, systemic inflammation is where the link to fertility is strongest, and it may be the most important thing these tests can tell us.

 

Does inflammation affect fertility?

Active or undiagnosed inflammatory disease reduces fecundity, and the effect largely disappears once the disease is found and controlled. This holds across very different conditions, which suggests that the overall inflammatory load matters, not just the specific disease. Where outcomes have improved, it has been by tackling the source of the inflammation.

In the table and below, OR (odds ratio), HR (hazard ratio) and RR (relative risk) compare outcomes between groups. A value above 1 means higher odds or risk and a value below 1 means lower; a hazard ratio for pregnancy below 1 means a lower chance of conceiving over time.

Condition or marker	Key finding	Ref
Low-grade inflammation (hsCRP 1.95 mg/L or above)	Lowest live birth rate on placebo (44%); preconception low-dose aspirin raised it to 59%, with no effect at lower hsCRP	[15]
Rheumatoid arthritis	About four in ten women take over 12 months to conceive; each point of disease activity lowers the chance of pregnancy (HR 0.81)	[16]
IL-6 in rheumatoid arthritis	Higher preconception IL-6 predicted longer time to pregnancy (HR 0.66, n = 61); TNF-α did not	[17]
Thyroid autoimmunity	Levothyroxine did not improve live birth in 952 euthyroid, antibody-positive women (37.4% versus 37.9%)	[18]
Undiagnosed coeliac disease	About three times more common in infertile women (OR 3.09) and six times in unexplained infertility; no excess risk once diagnosed (OR 0.99)	[19,20]
Endometriosis	Nearly half of affected women experience infertility; peritoneal TNF-α, IL-1β, IL-6 and IL-8 are raised	[21]
PCOS	CRP 96% higher than controls, independent of obesity; IL-6 and TNF-α not different	[22]

 

Aspirin and low-grade inflammation. In a randomised trial of preconception low-dose aspirin in women with prior pregnancy loss, a secondary analysis found the benefit was confined to women with low-grade inflammation [15]. In those with hsCRP of 1.95 mg/L or above, live birth rose from 44% on placebo to 59% on aspirin, with no effect at lower hsCRP. As a secondary analysis, it generates rather than proves the hypothesis, but it is the nearest thing to trial evidence that calming low-grade inflammation before conception changes outcomes.

Thyroid autoimmunity and fertility. Thyroid antibodies are common, found in about one in five women with infertility. A 2024 meta-analysis of 48 studies found antibodies around 1.5 times more prevalent in infertile women than in controls. A further subgroup of studies that included only euthyroid infertile women showed antibodies around 2.8 times more prevalent than in controls [23].

They appear to have relevance even when thyroid function is normal. In a Taiwanese national insurance cohort, women with Hashimoto's disease had an 89% higher risk of amenorrhoea and a 2.4-fold higher risk of infertility due to ovarian failure, with a similar risk in those who were euthyroid [24]. In IVF, euthyroid women with thyroid peroxidase antibodies had lower AMH, fewer good-quality embryos, lower live birth (23% versus 36%) and more miscarriage, with lower live birth at higher titres [25].

In a 2026 study of 4,332 women having IVF, thyroid autoimmunity was an independent risk factor for impaired fertilisation, accompanied by changes in the follicular fluid [26]. Not every IVF study finds an effect, and earlier meta-analyses were mixed, but thyroid antibodies have been detected in follicular fluid, which offers a plausible route to the ovary.

In a large randomised trial, levothyroxine did not improve live birth in 952 euthyroid women with thyroid peroxidase antibodies (37.4% versus 37.9%) [18]. Correcting thyroid hormone did not address the risk, which suggests it lies in the autoimmunity rather than the thyroid itself. Thyroid autoimmunity has been linked to altered NK and T cell immunity in women with recurrent loss or unexplained infertility [27].

An Irish case report describes a 22-year-old woman with normal thyroid function, a strongly positive thyroid peroxidase antibody result (titre 1:25,600 on an older dilution assay; current laboratories report these antibodies in IU/mL) and five failed IVF cycles [28]. The patient had IVIG in her fifth cycle without success. In her sixth, etanercept, a TNF-α inhibitor, was added before stimulation, and she went on to deliver twins at 34½ weeks. A single case proves nothing, and its authors stressed that immunomodulators are inappropriate for unselected IVF populations, but it illustrates the principle that targeting the immune response, not the hormone, may be what matters.

Coeliac disease shows the pattern clearly. In a Swedish national cohort of 11,495 women with biopsy-confirmed coeliac disease, fertility, measured as births, was reduced before diagnosis, matched that of controls after diagnosis, when a gluten-free diet would normally begin, and was slightly higher than controls in the longer term [29]. The reduction tracks active, untreated disease rather than the condition itself. This fits the meta-analyses showing more undiagnosed coeliac disease in infertile women and no excess risk once diagnosed [19,20].

 

Where does inflammation affecting fertility come from?

Raised NK cell counts or cytotoxicity, a marked Th1/Th2 or Th17/Treg skew, or raised inflammatory cytokines are best read as signs of a pro-inflammatory state, not as diseases in their own right. Several of the conditions above would also be expected to raise the Chicago ratio, because it reads pro-inflammatory CD4 activity.

If you have a diagnosed autoimmune or inflammatory condition, that is the obvious place to start. But what if there is no diagnosis? Then the question is where the inflammation is coming from. When blood markers are raised, or the history suggests an inflammatory problem, but no autoimmune disease has been diagnosed, the gut, vagina and uterus are the places with evidence behind them.

Does the gut microbiome affect fertility?

The gut microbiome may influence fertility through its effect on immune balance, although direct fertility evidence is still limited. The gut is the body's largest interface with the immune system, and its bacteria help set the balance between inflammatory and regulatory responses. Short-chain fatty acids such as butyrate, produced when gut bacteria ferment dietary fibre, drive the formation of regulatory T cells, largely shown in animal studies [30,31]. These are the same cells whose balance with Th17 cells appears important in recurrent loss [12].

When the gut barrier is compromised, bacterial lipopolysaccharide can reach the circulation and drive low-grade systemic inflammation. This "gut barrier–endotoxaemia" model was first proposed for PCOS [32], and a pilot study found lower gut microbial diversity and some markers of gut barrier damage in women with PCOS, though not higher serum endotoxin [33].

Studies linking the gut to reproductive outcomes are still few. In one, women with unexplained miscarriage had higher serum IL-2, IL-17A, IL-17F, TNF-α and IFN-γ and reduced gut microbial diversity compared with women having an elective termination, and faecal metabolites tracked the Th1/Th17 cytokines [34]. In healthy adults, ten weeks of a high fermented food diet increased microbial diversity and lowered 19 inflammatory proteins including IL-6 [35]. The first study is cross-sectional and the second is not a fertility trial. Together they make the gut a plausible and modifiable source of a systemic signal.

Does the vaginal microbiome affect IVF and miscarriage?

A Lactobacillus-dominant vaginal microbiome is linked to better outcomes, but treating dysbiosis has not yet been shown to improve pregnancy rates. In 167 women, 93 of whom miscarried, a Lactobacillus-depleted vaginal microbiota was more common when the miscarried pregnancy was chromosomally normal than when it had a chromosomal abnormality, the usual cause of miscarriage [36]. This was associated with higher cervicovaginal IL-1β, IL-8 and IL-6 than in women whose pregnancies continued to term, which suggests that vaginal dysbiosis and local inflammation may contribute to losses that genetics does not explain. Women with recurrent miscarriage and raised peripheral NK cells also carried Gardnerella and Gram-negative anaerobes more often [37].

A Danish trial screened 1,533 IVF patients and randomised 338 with abnormal vaginal microbiota to clindamycin, clindamycin plus a Lactobacillus crispatus product, or placebo [38]. Treatment worked on the microbiota: by the day of embryo transfer, the dysbiosis had cleared in 75% of women given clindamycin alone and in 100% of those given clindamycin plus L. crispatus, compared with 36% on placebo. Despite this, clinical pregnancy rates were the same in all three groups (46%, 42% and 45%). However, in a post hoc analysis, women whose microbiota was Lactobacillus-dominant at baseline had a higher clinical pregnancy rate whatever treatment they received (RR 1.36, 95% CI 1.00 to 1.85).

Is the endometrial microbiome linked to implantation?

Several studies link a non-Lactobacillus-dominant endometrium to poorer implantation, although most are small. In a 35-patient pilot, a non-Lactobacillus-dominant endometrium was associated with lower implantation (23.1% versus 60.7%) and live birth (6.7% versus 58.8%) [39]. A 342-patient multicentre study linked Gardnerella, Atopobium, Streptococcus and Klebsiella to failed outcomes [40]. Chronic endometritis is reported in 14% to 67.5% of implantation failure and 9.3% to 67.6% of recurrent loss, a range that reflects inconsistent diagnostic criteria [41].

Which nutrient deficiencies matter for immune balance and fertility?

Vitamin D, omega-3, iron and zinc all help regulate immune function and are linked to reproductive outcomes. Deficiencies are common, and they are simple to test and correct.

• Vitamin D. Low vitamin D has been associated with higher NK cell levels, NK cytotoxicity and autoantibodies in women with recurrent pregnancy loss [42]. In a meta-analysis of 11 cohort studies including 2,700 women having assisted reproduction, women replete in vitamin D had a higher live birth rate than those who were deficient or insufficient (OR 1.33; 95% CI 1.08 to 1.65) [43].
• Omega-3. Long-chain omega-3 fats have well-established anti-inflammatory effects. In women having IVF, higher serum omega-3 levels were associated with a higher probability of clinical pregnancy and live birth, with an 8% higher chance of live birth for each 1% increase in serum level [44].
• Iron. In more than 18,000 women in the Nurses' Health Study II, those taking iron supplements had a 40% lower risk of ovulatory infertility (RR 0.60; 95% CI 0.39 to 0.92) [45]. Iron should be supplemented only on the basis of test results: Ireland has one of the highest rates of hereditary haemochromatosis in the world, with about 1 in 83 people carrying two copies of the main risk variant, and iron overload is pro-oxidant and pro-inflammatory [46]. Ferritin is also an acute-phase protein that rises with inflammation, so results should be carefully interpreted.
• Zinc. Zinc is needed for T cell development and function. Deficiency weakens cell-mediated immunity, and experimental work suggests zinc favours regulatory T cells over Th17 cells [47]. Human fertility data on zinc are limited to very small studies however.

These are associations, and intervention trials with live birth as the outcome are largely lacking. Correcting a deficiency is nonetheless low risk and worth doing for general health.

 

Is immune testing worth it after failed IVF or recurrent miscarriage?

Not every embryo implants, even when conditions are optimal. A chromosomally normal (euploid) blastocyst implants in no more than 60% to 70% of transfers, and the shortfall has been described as a black box, attributed mainly to uterine factors [48]. Immune and inflammatory problems are only one possible contributor. At critical points, however, such as after repeated failed transfers or recurrent loss, or when only one embryo remains, it seems reasonable to look for anything that could be optimised before the next transfer.

We hold this position with the counter-arguments in view:

• Testing can produce false positives, cost and anxiety.
• Most women with idiopathic recurrent miscarriage will have a live birth without any immune intervention, so apparent responses are easily over-read [49].
• A marker can improve without the outcome improving, as the vaginal microbiota trial showed [38].
• Time spent optimising must be weighed against maternal age.

Current ESHRE guidance does not recommend routine NK cell or cytokine testing in recurrent pregnancy loss, but does recommend testing thyroid function and TPO antibodies [1]. That guidance relates to using these tests as a way to diagnose the cause of pregnancy loss or to guide immune treatment. We use them differently: as one part of a whole-person assessment before IVF or another pregnancy, looking for systemic inflammation that is worth addressing in its own right.

The interventions that follow from this approach are low risk and worth doing for general health. They include identifying and addressing autoimmune conditions such as thyroid autoimmunity, correcting vitamin D and other nutrient deficiencies, assessing and supporting the gut and vaginal microbiome, tailoring supplement protocols and improving diet quality. The evidence threshold for these is lower than for immunosuppressive drugs, where the data remain weak and the decision is a medical one [50].

 

Conclusion

Studies of the endometrium itself suggest that its immune environment is altered in some women with recurrent loss or implantation failure, but there is much still to learn about this complex environment.

Peripheral blood immune markers are not tests of the uterus and implantation environment. Their more useful role is as a signal of systemic inflammation. The data linking inflammatory and autoimmune disease to reduced fertility are considerably stronger. Until well-designed trials are carried out, looking at overall inflammation and autoimmunity, and at the gut, vaginal and uterine sources that may drive them, seems to us a proportionate way to prepare for a pregnancy, particularly after repeated failed transfers or recurrent loss.

 

About this review

About the author. Dr Ciara Wright holds a PhD from the Institute of Immunology at Maynooth University, where her research involved performing flow cytometric analysis of immune cells and cytokine assays. She also holds a Diploma in Medical Laboratory Sciences, covering the principles, methods and interpretation of clinical laboratory testing. She has practised as a Nutritional Therapist since 2014, specialising in fertility and supporting patients attending fertility clinics across Ireland. She lectures regularly to practitioners, including at the Functional Medicine Conference (FMC) and Nutritional Medicine Institute conferences, and has presented at international fertility meetings including the World Congress on Controversies in Obstetrics, Gynaecology and Infertility (COGI). She has published peer-reviewed research and a book chapter on male fertility, including the role of oxidative stress.

Methods. This is a narrative review. PubMed and Google Scholar were searched for studies on peripheral and uterine immune markers, systemic inflammation and reproductive outcomes, prioritising systematic reviews, meta-analyses, randomised trials and large cohort studies, and including Irish data where available. Evidence is described in the text according to study design and consistency of findings.

Conflict of interest. Ciara Wright is the Director of Positive Nutrition, which offers immune and nutritional testing, including NK cell activation testing through a Eurofins laboratory, and provides nutritional support to patients preparing for or undergoing fertility treatment.

 

References

1. ESHRE Guideline Group on RPL; Bender Atik R, Christiansen OB, Elson J, et al. ESHRE guideline: recurrent pregnancy loss: an update in 2022. Hum Reprod Open. 2023;2023(1):hoad002. Link
2. Vento-Tormo R, Efremova M, Botting RA, et al. Single-cell reconstruction of the early maternal-fetal interface in humans. Nature. 2018;563:347-353.
3. Koopman LA, Kopcow HD, Rybalov B, et al. Human decidual natural killer cells are a unique NK cell subset with immunomodulatory potential. J Exp Med. 2003;198(8):1201-1212. Link
4. Seshadri S, Sunkara SK. Natural killer cells in female infertility and recurrent miscarriage: a systematic review and meta-analysis. Hum Reprod Update. 2014;20(3):429-438. Link
5. Tang AW, Alfirevic Z, Quenby S. Natural killer cells and pregnancy outcomes in women with recurrent miscarriage and infertility: a systematic review. Hum Reprod. 2011;26(8):1971-1980. Link
6. Kolanska K, Suner L, Cohen J, et al. Proportion of cytotoxic peripheral blood natural killer cells and T-cell large granular lymphocytes in recurrent miscarriage and repeated implantation failure: case-control study and meta-analysis. Arch Immunol Ther Exp. 2019. doi:10.1007/s00005-019-00546-5.
7. Woon EV, Greer O, Shah N, Nikolaou D, Johnson M, Male V. Number and function of uterine natural killer cells in recurrent miscarriage and implantation failure: a systematic review and meta-analysis. Hum Reprod Update. 2022;28(4):548-582. Link
8. Marron K, Walsh D, Harrity C. Detailed endometrial immune assessment of both normal and adverse reproductive outcome populations. J Assist Reprod Genet. 2019;36(2):199-210. Link
9. Marron K, Harrity C. Potential utility of a non-invasive menstrual blood immunophenotype analysis in reproductive medicine. Reprod Fertil. 2022;3(4). doi:10.1530/RAF-22-0047. Link
10. Marron K, Harrity C. Endometrial lymphocyte concentrations in adverse reproductive outcome populations. J Assist Reprod Genet. 2019;36(5):837-846. Link
11. Rosalind Franklin University Clinical Immunology Laboratory. TH1/TH2 intracellular cytokine ratio: test description. Link
12. Wang W, Sung N, Gilman-Sachs A, Kwak-Kim J. T helper (Th) cell profiles in pregnancy and recurrent pregnancy losses: Th1/Th2/Th9/Th17/Th22/Tfh cells. Front Immunol. 2020;11:2025.
13. Harrity C, Shkrobot L, Walsh D, Marron K. ART implantation failure and miscarriage in patients with elevated intracellular cytokine ratios: response to immune support therapy. Fertil Res Pract. 2018;4:7. Link
14. Marron K, Kennedy JF, Harrity C. Anti-oxidant mediated normalisation of raised intracellular cytokines in patients with reproductive failure. Fertil Res Pract. 2018;4:1. Link
15. Sjaarda LA, Radin RG, Silver RM, et al. Preconception low-dose aspirin restores diminished pregnancy and live birth rates in women with low-grade inflammation: a secondary analysis of a randomized trial. J Clin Endocrinol Metab. 2017;102(5):1495-1504.
16. Brouwer J, Hazes JMW, Laven JSE, Dolhain RJEM. Fertility in women with rheumatoid arthritis: influence of disease activity and medication. Ann Rheum Dis. 2015;74(10):1836-1841. Link
17. Bongenaar M, Smeele HTW, Lubberts E, Dolhain RJEM. IL-6 but not TNFα levels are associated with time to pregnancy in female rheumatoid arthritis patients with a wish to conceive. Front Pharmacol. 2020;11:604866. Link
18. Dhillon-Smith RK, Middleton LJ, Sunner KK, et al. Levothyroxine in women with thyroid peroxidase antibodies before conception. N Engl J Med. 2019;380(14):1316-1325.
19. Lasa JS, Zubiaurre I, Soifer LO. Risk of infertility in patients with celiac disease: a meta-analysis of observational studies. Arq Gastroenterol. 2014;51(2):144-150. Link
20. Singh P, Arora S, Lal S, Strand TA, Makharia GK. Celiac disease in women with infertility: a meta-analysis. J Clin Gastroenterol. 2016;50(1):33-39. Link
21. Miller JE, Ahn SH, Monsanto SP, Khalaj K, Koti M, Tayade C. Implications of immune dysfunction on endometriosis associated infertility. Oncotarget. 2017;8(4):7138-7147. Link
22. Escobar-Morreale HF, Luque-Ramírez M, González F. Circulating inflammatory markers in polycystic ovary syndrome: a systematic review and metaanalysis. Fertil Steril. 2011;95(3):1048-1058. Link
23. Huang Y, Xie B, Li J, et al. Prevalence of thyroid autoantibody positivity in women with infertility: a systematic review and meta-analysis. BMC Womens Health. 2024;24:630. Link
24. Hsieh YT, Ho JYP. Thyroid autoimmunity is associated with higher risk of premature ovarian insufficiency: a nationwide Health Insurance Research Database study. Hum Reprod. 2021;36(6):1621-1629. Link
25. Wei SX, Wang L, Liu YB, Fan QL, Fan Y, et al. TPOAb positivity can impact ovarian reserve, embryo quality, and IVF/ICSI outcomes in euthyroid infertile women. Gynecol Endocrinol. 2023;39(1):2266504. Link
26. Zhong L, Luo Y, Zhu J, et al. Thyroid autoimmunity is associated with fertilization impairment and follicular fluid exosomal miRNA alterations in euthyroid women undergoing IVF-ET. Front Immunol. 2026;17:1870650. Link
27. Kim NY, Cho HJ, Kim HY, et al. Thyroid autoimmunity and its association with cellular and humoral immunity in women with reproductive failures. Am J Reprod Immunol. 2011;65(1):78-87.
28. Sills ES, Walsh DJ, Shkrobot LV, Palermo GD, et al. Clinical experience with intravenous immunoglobulin and TNF-α inhibitor therapies for recurrent pregnancy loss. Ulster Med J. 2009;78(1):57-58. Link
29. Zugna D, Richiardi L, Akre O, Stephansson O, Ludvigsson JF. A nationwide population-based study to determine whether coeliac disease is associated with infertility. Gut. 2010;59(11):1471-1475. Link
30. Furusawa Y, Obata Y, Fukuda S, et al. Commensal microbe-derived butyrate induces the differentiation of colonic regulatory T cells. Nature. 2013;504(7480):446-450. Link
31. Arpaia N, Campbell C, Fan X, et al. Metabolites produced by commensal bacteria promote peripheral regulatory T-cell generation. Nature. 2013;504(7480):451-455. Link
32. Tremellen K, Pearce K. Dysbiosis of Gut Microbiota (DOGMA): a novel theory for the development of polycystic ovarian syndrome. Med Hypotheses. 2012;79(1):104-112. doi:10.1016/j.mehy.2012.04.016.
33. Lindheim L, Bashir M, Münzker J, et al. Alterations in gut microbiome composition and barrier function are associated with reproductive and metabolic defects in women with polycystic ovary syndrome (PCOS): a pilot study. PLoS One. 2017;12(1):e0168390. Link
34. Liu Y, Chen H, Feng L, Zhang J. Interactions between gut microbiota and metabolites modulate cytokine network imbalances in women with unexplained miscarriage. NPJ Biofilms Microbiomes. 2021;7:24. Link
35. Wastyk HC, Fragiadakis GK, Perelman D, et al. Gut-microbiota-targeted diets modulate human immune status. Cell. 2021;184(16):4137-4153.e14. Link
36. Grewal K, Lee YS, Smith A, et al. Chromosomally normal miscarriage is associated with vaginal dysbiosis and local inflammation. BMC Med. 2022;20:38. Link
37. Kuon RJ, Togawa R, Vomstein K, et al. Higher prevalence of colonization with Gardnerella vaginalis and gram-negative anaerobes in patients with recurrent miscarriage and elevated peripheral natural killer cells. J Reprod Immunol. 2017;120:15-19. Link
38. Haahr T, Freiesleben NlC, Jensen MB, et al. Efficacy of clindamycin and LACTIN-V for in vitro fertilization patients with vaginal dysbiosis: a randomised double-blind, placebo-controlled multicentre trial. Nat Commun. 2025;16:5166. Link
39. Moreno I, Codoñer FM, Vilella F, et al. Evidence that the endometrial microbiota has an effect on implantation success or failure. Am J Obstet Gynecol. 2016;215(6):684-703. Link
40. Moreno I, Garcia-Grau I, Perez-Villaroya D, et al. Endometrial microbiota composition is associated with reproductive outcome in infertile patients. Microbiome. 2022;10:1. Link
41. Ticconi C, Inversetti A, Marraffa S, et al. Chronic endometritis and recurrent reproductive failure: a systematic review and meta-analysis. Front Immunol. 2024;15:1427454. Link
42. Ota K, Dambaeva S, Han AR, Beaman K, Gilman-Sachs A, Kwak-Kim J. Vitamin D deficiency may be a risk factor for recurrent pregnancy losses by increasing cellular immunity and autoimmunity. Hum Reprod. 2014;29(2):208-219.
43. Chu J, Gallos I, Tobias A, Tan B, Eapen A, Coomarasamy A. Vitamin D and assisted reproductive treatment outcome: a systematic review and meta-analysis. Hum Reprod. 2018;33(1):65-80. Link
44. Chiu YH, Karmon AE, Gaskins AJ, et al. Serum omega-3 fatty acids and treatment outcomes among women undergoing assisted reproduction. Hum Reprod. 2018;33(1):156-165. Link
45. Chavarro JE, Rich-Edwards JW, Rosner BA, Willett WC. Iron intake and risk of ovulatory infertility. Obstet Gynecol. 2006;108(5):1145-1152. Link
46. Ryan E, Byrnes V, Coughlan B, et al. Underdiagnosis of hereditary haemochromatosis: lack of presentation or penetration? Gut. 2002;51(1):108. doi:10.1136/gut.51.1.108. Link
47. Maywald M, Wessels I, Rink L. Zinc signals and immunity. Int J Mol Sci. 2017;18(10):2222. Link
48. Vitagliano A, Paffoni A, Viganò P. Does maternal age affect assisted reproduction technology success rates after euploid embryo transfer? A systematic review and meta-analysis. Fertil Steril. 2023;120(2):251-265. Link
49. Brigham SA, Conlon C, Farquharson RG. A longitudinal study of pregnancy outcome following idiopathic recurrent miscarriage. Hum Reprod. 1999;14(11):2868-2871.
50. Woon EV, Day A, Bracewell-Milnes T, Male V, Johnson M. Immunotherapy to improve pregnancy outcome in women with abnormal natural killer cell levels/activity and recurrent miscarriage or implantation failure: a systematic review and meta-analysis. J Reprod Immunol. 2020;142. Link