You’ve heard the statistics and you may have lived them: about one in five pregnancies ends in miscarriage. If you’ve experienced more than one miscarriage, you’ve probably also heard the phrase that cuts deepest of all: “Sometimes it just happens—at least you know you can get pregnant, now." Maybe a kind doctor patted your hand and said your body would figure it out. Maybe you were told to try again. Maybe you were told that one miscarriage is random and to come back if you have more. Maybe no one ran a single additional test.
But what if the answer wasn’t just “bad luck?” What if your immune system—that same relentless, microscopic army designed to protect you—was quietly and catastrophically doing its job too well?
This is the question at the heart of reproductive immunology, a field that is reshaping how we understand pregnancy loss, recurrent miscarriage, and unexplained infertility. It is not fringe or even alternative science. It is not a last resort for the desperate. It is increasingly one of the most important frontiers in women’s medicine. And it starts with understanding something remarkable about pregnancy itself.
The immunological miracle nobody talks about
Every pregnancy is, at its biological core, a paradox. Your baby carries DNA that is only half yours. The other half comes from the father and is genetically foreign to your body. Under normal immunological logic, your immune system should attack that foreign tissue the same way it would attack a transplanted organ or an invading pathogen. It should reject it.
And yet, in the majority of healthy pregnancies, it doesn’t.
The fact that human pregnancy works at all is (from an immunologist’s perspective) something close to a miracle. A successful pregnancy requires the maternal immune system to perform a breathtaking act of selective tolerance. Meaning, it stands down against the semi-foreign embryo while remaining alert enough to defend against genuine threats like bacteria and viruses. This isn’t passive. It’s an active, highly orchestrated immunological process, and when it goes wrong, pregnancy can fail.
The fact that human pregnancy works at all is (from an immunologist’s perspective) something close to a miracle. A successful pregnancy requires the maternal immune system to perform a breathtaking act of selective tolerance. Meaning, it stands down against the semi-foreign embryo while remaining alert enough to defend against genuine threats like bacteria and viruses.
Comprendre comment it goes wrong is the entire mission of reproductive immunology.
The man who changed everything: Dr. Alan Beer
The story of reproductive immunology as a clinical discipline begins, in large part, with one relentlessly curious physician: Dr. Alan Beer, an immunologist and obstetrician who spent decades asking questions that mainstream medicine was not yet ready to hear.
Working from the late 1970s onward—initially at the University of Michigan and later at the University of Texas—Beer began studying women who were perfectly healthy by every conventional measure, yet couldn’t sustain a pregnancy [1]. These were women who conceived without difficulty but miscarried repeatedly, often after being told their losses were chromosomal flukes or unexplained. Beer and his patients were unconvinced.
What Dr. Beer found, through painstaking laboratory work, was that many of these women had immune system abnormalities that were interfering with implantation and early placental development. Autoantibodies—proteins made by the immune system that mistakenly target the body’s own tissues—were damaging the delicate blood vessels of the early placenta [2]. Natural killer cells, a type of immune cell critical to the body’s defenses, were behaving aberrantly, attacking the embryo rather than protecting it. The body, in short, was treating the pregnancy as a threat to be neutralized. As a virus of sorts threatening the mother’s body.
What Dr. Beer found, through painstaking laboratory work, was that many of these women had immune system abnormalities that were interfering with implantation and early placental development. Autoantibodies—proteins made by the immune system that mistakenly target the body’s own tissues—were damaging the delicate blood vessels of the early placenta.
Beer’s work was groundbreaking and, at the time, deeply controversial. Colleagues pushed back. The field was not yet ready to accept that the immune system could be a root cause of pregnancy loss. But he kept going. He kept publishing prolifically, treating patients, and building a body of evidence that would outshine the skeptics.
By the time he passed away in 2006, Beer had seen reproductive immunology transform from a fringe hypothesis into a legitimate medical subspecialty. His book Is Your Body Baby Friendly?, first published in 2006 and updated in 2019, remains essential reading for anyone navigating recurrent pregnancy loss. He is, without overstatement, widely considered the father of reproductive immunology.
What is actually happening in the body
So what, exactly, goes wrong immunologically when a pregnancy fails? The science is complex, but the core story involves several key players.
Natural Killer Cells (NK Cells)
These are among the most important immune cells at the maternal-fetal interface. Uterine natural killer (uNK) cells are not the same as their blood-circulating counterparts. These cells are specialized, less cytotoxic, and under normal circumstances, they play a utile role in early pregnancy. They support trophoblast invasion (the process by which the embryo anchors into the uterine wall) and remodel the spiral arteries to deliver nutrients and oxygen to the growing placenta [3].
But when uNK cells are elevated or dysfunctional, they can shift from protectors to destroyers. A 2022 systematic review and meta-analysis publié dans Mise à jour sur la reproduction humaine confirmed that elevated uNK cell counts are associated with recurrent miscarriage and recurrent implantation failure, and that inappropriate uNK function has been implicated in reproductive failure [4]. More recent research published in Fertilité et stérilité in 2023 further underscored that uNK cells are “critical in early gestation and implantation” and that their dysregulation may be a significant but underdiagnosed cause of pregnancy loss [3].
Regulatory T Cells (Tregs)
If NK cells are the soldiers, regulatory T cells are the peacekeepers. Tregs are a specialized subset of immune cells whose job, in pregnancy, is to suppress the immune response against paternal antigens. Essentially, they are training the body to tolerate the “foreign” embryo. Recherche publié dans le Journal of Clinical Investigation (en anglais) describes Tregs as “master regulators of pregnancy tolerance,” noting that they operate to inhibit effector immunity, contain inflammation, and support the vascular adaptations needed for the placenta to function [5].
When Treg numbers are low or functionally deficient, this tolerance breaks down. A comprehensive review en Mise à jour sur la reproduction humaine found that inadequate Treg cells are linked with infertility, miscarriage, and pre-eclampsia [6]. Animal studies have gone even further: murine (i.e., mouse) miscarriage can actually be prevented by transferring regulatory T cells from normally pregnant mice. This finding has profound implications for future human therapies [7].
Antiphospholipid Syndrome (APS)
This is the most well-established immunological cause of recurrent pregnancy loss, and the one where mainstream medicine has made the clearest therapeutic progress. Antiphospholipid syndrome is an autoimmune condition in which the body produces antibodies that attack phospholipids (the fatty molecules found in cell membranes). In pregnancy, these antibodies can cause clotting in the placental blood vessels, starving the embryo of the oxygen and nutrients it needs to survive [8].
APS is now recognized as the most important treatable autoimmune cause of recurrent miscarriage, and the combination of low-dose aspirin and heparin (which are two anticoagulant treatments), has a success rate of approximately 70–80% in high-risk patients [8]. This is a remarkable outcome for women who, without testing and diagnosis, might have been sent home and told simply to try again. It’s also a triumph and joy, to know that this available treatment is so simple and accessible.
APS is now recognized as the most important treatable autoimmune cause of recurrent miscarriage, and the combination of low-dose aspirin and heparin (which are two anticoagulant treatments), has a success rate of approximately 70–80% in high-risk patients. This is a remarkable outcome for women who, without testing and diagnosis, might have been sent home and told simply to try again.
What no one is telling women who miscarry
Here is the part that matters most for women sitting in the middle of this grief: the mainstream medical approach to recurrent miscarriage still leaves up to 50% of cases “unexplained.”
A 2025 papier publié dans le Journal international des sciences moléculaires confirmed that recurrent pregnancy loss affects 3–5% of women trying to conceive, and that despite extensive investigation, the causes in a significant proportion of cases “remain unexplained” [9]. Another 2020 review put the figure even higher, noting that more than half of RPL cases still lack a clear diagnosis [10]. It’s important to keep in mind that miscarriage statistics must be taken with some nuance given that many women miscarry without knowing or reporting, so accurate data is almost impossible. Nonetheless, the numbers above are based on reported miscarriages.
Reproductive immunology offers a framework for investigating some of those unexplained cases. But it requires specialized testing that is not part of standard recurrent miscarriage workups in most countries.
What testing might be available?
Women with recurrent pregnancy loss; which is typically defined as two or more consecutive losses, per the current ESHRE (European Society of Human Reproduction and Embryology) 2022 guidelines, may benefit from evaluation that includes antiphospholipid antibody screening, NK cell panels (both peripheral blood and uterine biopsy), cytokine profiling, and assessment of HLA compatibility between partners. Thyroid antibodies, which have been independently associated with pregnancy loss risk, should also be evaluated. Consult the medical professionals available to you in order to determine which testing might be most appropriate.
It is worth noting that this field is not without controversy. While practitioners of la médecine reproductive réparatrice (MRR) take a more thorough approach to repeated miscarriage, mainstream reproductive medicine bodies, including the American Society of Reproductive Medicine (ASRM), currently only formally validate heparin and aspirin for antiphospholipid syndrome-related pregnancy loss, and do not yet recommend broader immunomodulatory treatments as standard care [11]. This does not mean the science is wrong, it means the clinical trials needed to satisfy guideline bodies are still catching up to the immunological findings. Well-powered randomized controlled trials are urgently needed, and researchers in the field are calling for exactly that [3].
What treatments exist?
For women who test positive for immunological abnormalities, treatments currently being studied and used in specialized clinics include:
- Intravenous immunoglobulin (IVIG), which modulates the immune response and has shown promise in improving live birth rates, particularly in women with antiphospholipid antibodies or elevated NK cells. A 2022 systematic review and meta-analysis found that IVIG improves live birth rates among women with underlying immune conditions and recurrent pregnancy loss [12].
- Low-dose prednisolone (corticosteroid), used to suppress uterine NK cell activity. A méta-analyse found that prednisolone therapy improved pregnancy outcomes in women with RPL [13].
- Intralipid infusions, an intravenous fat emulsion that may suppress elevated NK cell cytotoxicity, and which carries a favorable side-effect profile.
- Low-dose aspirin and heparin, the gold standard for APS, with strong evidence behind them [11].
A field still developing
Reproductive immunology is young, contested, and like all newer sciences, it is imperfect. Not every clinic offering immune testing is doing so with the same rigor. Not every test result tells a complete story. And the translation from lab finding to clinical treatment remains, in many areas, an active area of debate. But the direction of science is clear: the immune system est central to pregnancy and it fait go wrong in some women who miscarry—perhaps especially in women with repeat miscarriage. Furthermore, those women deserve better than being handed a statistic and a pamphlet about grief.
Dr. Beer spent decades fighting for that belief. Researchers in labs across the world are building on his foundation. And the women who have lost pregnancies and who have sat in waiting rooms and not received helpful answers, deserve to know that the question is still being asked on their behalf. Because “unexplained” is not the same as “unexplainable.” At least not anymore when it comes to miscarriages.
If you have experienced two or more miscarriages and you are struggling to understand why, speak with a reproductive specialist or maternal-fetal medicine physician about immune testing, particularly antiphospholipid antibody screening. You can also ask your healthcare professional about referral to a reproductive immunologist. The ESHRE guidelines on recurrent pregnancy loss (updated 2022) are a useful reference for understanding what investigations are currently recommended [14]. Take a look and embrace the next stage of your fertility journey with confidence and hope.
Références
[1] Alan E. Beer Medical Center for Reproductive Immunology. Scientific Papers – Dr. Beer. https://repro-med.net/scientific-papers-dr-beer.html
[2] Murvai VR, et al. Antiphospholipid syndrome in pregnancy: a comprehensive literature review. BMC Pregnancy Childbirth. 2025;25:337.
[3] Bequet YLBN, et al. The role of uterine natural killer cells in recurrent pregnancy loss and possible treatment options. Fertil Steril. 2023;120(5):945–947.
[4] Von Woon E, 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.
[5] Robertson SA, et al. Regulatory T cells in embryo implantation and the immune response to pregnancy. J Clin Invest. 2018;128(10):4224–4235.
[6] Robertson SA, Moldenhauer LM. Immunological determinants of implantation success and pregnancy. Hum Reprod Update. 2009;15(5):517–535.
[7] Zenclussen ML, et al. Potentiating maternal immune tolerance in pregnancy: a new challenging role for regulatory T cells. Am J Reprod Immunol. 2014;71(4):287–299.
Références
[8] Bletry O, Piette AM. Recurrent fetal loss and antiphospholipid antibodies: clinical and therapeutic aspects. Infect Dis Obstet Gynecol. 1997;5(3):183–191.
Références suite
[9] Veza A, et al. Exploring the immunological aspects and treatments of recurrent pregnancy loss and recurrent implantation failure. Int J Mol Sci. 2025;26(3):1295.
[10] Von Woon E, 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 Jun 30;28(4):548-582. doi: 10.1093/humupd/dmac006. PMID: 35265977; PMCID: PMC9247428.
[11] Guideline No. 464: Recurrent Pregnancy Loss. J Obstet Gynaecol Can. 2025.
[12] Habets DHJ, et al. Intravenous immunoglobulins improve live birth rate among women with underlying immune conditions and recurrent pregnancy loss: a systematic review and meta-analysis. Allergy Asthma Clin Immunol. 2022;18:23.
[13] Sfakianoudis K, et al. The role of uterine natural killer cells on recurrent miscarriage and recurrent implantation failure: from pathophysiology to treatment. Biomedicines. 2021;9:1425.
[14] The ESHRE Guideline Group on RPL, Ruth Bender Atik, Ole Bjarne Christiansen, Janine Elson, Astrid Marie Kolte, Sheena Lewis, Saskia Middeldorp, Saria Mcheik, Braulio Peramo, Siobhan Quenby, Henriette Svarre Nielsen, Marie-Louise van der Hoorn, Nathalie Vermeulen, Mariëtte Goddijn, ESHRE guideline: recurrent pregnancy loss: an update in 2022, Human Reproduction Open, Volume 2023, Issue 1, 2023, hoad002, https://doi.org/10.1093/hropen/hoad002
