Chronic endometritis is a persistent, low-grade inflammation of the uterine lining caused by bacterial infection, and it is far more common than most people realize. Unlike its acute counterpart, which announces itself with fever, pelvic pain, and obvious infection, chronic endometritis tends to fly under the radar. It is frequently discovered only when someone is being evaluated for infertility, recurrent miscarriage, or repeated IVF failure. The good news is that antibiotics clear most cases, often restoring fertility in the process, but getting there requires knowing it exists in the first place.
What Causes Chronic Endometritis
The uterine cavity was once thought to be sterile, but researchers now know it hosts its own microbial community. Chronic endometritis develops when bacteria colonize the endometrium and trigger a sustained immune response. The culprits are not exotic pathogens. In a study of women with recurrent miscarriage, common bacteria were found in roughly three-quarters of those with chronic endometritis, while Mycoplasma and Ureaplasma appeared in about a quarter of cases and Chlamydia in around one in eight.1PubMed Central. Chronic endometritis due to common bacteria is prevalent in women with recurrent miscarriage as confirmed by improved pregnancy outcome after antibiotic treatment A broader analysis spanning 40 studies over nearly four decades identified Chlamydia, Ureaplasma, Streptococcus, Mycoplasma, and Enterococcus as the five most frequently reported bacterial groups.2PubMed Central. Defining a panel of principal bacteria associated with endometritis
In other words, the bacteria behind chronic endometritis are often the same organisms that live in the genital tract or lower reproductive tract and migrate upward. Some are sexually transmitted, while others are part of the vaginal or gut flora that have simply ended up somewhere they shouldn’t be. Intrauterine devices, prior uterine procedures, and previous pregnancies that involved instrumentation can all create opportunities for bacteria to reach the endometrium. The inflammation that follows is not dramatic enough to cause an obvious infection, but it is enough to subtly alter the environment of the uterine lining.
The Microbiome Connection
Recent research has shifted attention from individual pathogens toward the broader microbial ecosystem of the uterus. A healthy endometrium tends to be dominated by Lactobacillus species, the same protective bacteria associated with a healthy vaginal environment. Women with chronic endometritis consistently show a lower proportion of Lactobacillus in their endometrial microbiome compared to women without the condition. Some studies report a dramatic gap: one found Lactobacillus making up roughly 41% of the endometrial microbiome in women with chronic endometritis versus 64% in unaffected women, while another found a more than 40-fold difference.3PubMed Central. The Role of Endometrial Microbiota in the Pathogenesis of Chronic Endometritis: A Systematic Review and Meta-Analysis
This isn’t just a matter of harmful bacteria showing up. Research in chronic endometritis patients has found reduced overall microbial diversity and a shift in the types of bacteria present, with an increase in certain groups and a decline in others that are normally part of a balanced community.4Scientific Reports. Microbiome dysbiosis in patients with chronic endometritis and Clostridium tyrobutyricum ameliorates chronic endometritis in mice Whether this microbial imbalance is a cause or a consequence of the inflammation remains an active question, but it does suggest that chronic endometritis is part of a broader disruption in the uterine ecosystem rather than a simple one-bug-one-disease scenario.
Why It So Often Goes Unnoticed
Chronic endometritis has been described as a “silent disease,” and the label fits.5PubMed Central. Endometritis – Diagnosis,Treatment and its impact on fertility – A Scoping Review Many people with the condition experience no symptoms at all, or symptoms so mild that they go unattributed. When symptoms do appear, they can include abnormal uterine bleeding, pelvic pain, and unusual discharge. Research has linked the condition to altered patterns of uterine muscle contraction, which may help explain these symptoms, as well as a possible connection to painful periods and even endometriosis.6PubMed. Altered uterine contractility in women with chronic endometritis
The distinction from acute endometritis matters here. Acute endometritis involves a more aggressive immune response with visible pus formation and white blood cell infiltration. It produces clear symptoms and is usually caught quickly. Chronic endometritis, by contrast, involves plasma cells quietly taking up residence in the uterine lining’s connective tissue.7Fertility and Sterility. Chronic Endometritis: Causes, Symptoms, and Treatment The inflammation smolders rather than burns, which is exactly why it tends to be found incidentally during fertility workups rather than during a visit for symptoms.
How Chronic Endometritis Affects Fertility
This is where chronic endometritis has attracted the most clinical attention over the past decade. The condition has been linked to both recurrent miscarriage and repeated failure of embryo implantation during IVF. A 2024 systematic review and meta-analysis found chronic endometritis in about 38% of women with recurrent pregnancy loss, compared to roughly 16% of controls, putting the odds of having the condition at more than three and a half times higher in the recurrent miscarriage group.8PubMed Central. Chronic endometritis and recurrent reproductive failure: a systematic review and meta-analysis
Among women with repeated implantation failure after IVF, one retrospective study found a chronic endometritis prevalence of about 12%, with those affected having a clinical pregnancy rate of 20% compared to roughly 47% in those without the condition.9PubMed. Chronic endometritis impairs embryo implantation in patients with repeated implantation failure: A retrospective study That gap is striking. The sustained inflammation appears to disrupt the delicate process by which an embryo attaches to the uterine wall and establishes a pregnancy.
Part of the explanation involves the immune environment of the uterus. Uterine natural killer cells, a specialized population of immune cells that play a role in implantation and early pregnancy, appear to be present at higher densities in women with chronic endometritis. One study found that nearly 38% of women with chronic endometritis had elevated natural killer cell densities, compared to about 8% of women without it.10PubMed. Association between chronic endometritis and uterine natural killer cell density in women with recurrent miscarriage: clinical implications More recent work has identified distinct subtypes of these natural killer cells with different functional profiles, and the balance between them appears to shift in chronic endometritis in ways that may favor immune rejection over the tolerance needed for successful implantation.11PubMed Central. Uterine NK Cell Polarization Associates with Chronic Endometritis and Predisposition to Recurrent Implantation Failure
How It Is Diagnosed
Diagnosis is one of the trickiest aspects of chronic endometritis, and it is where the field still has the most disagreement. The current gold standard is an endometrial biopsy examined with a staining technique that targets a protein called CD138, which marks plasma cells. Plasma cells are not normally abundant in the uterine lining, so finding them there in meaningful numbers points to chronic inflammation.12PubMed Central. The Role of Plasma Cells as a Marker of Chronic Endometritis: A Systematic Review and Meta-Analysis This immunohistochemistry approach was a real step forward from older staining methods, which made it harder to distinguish plasma cells from other immune cells and produced less consistent results between different pathologists looking at the same slide.
Even with CD138 staining, though, the method is not as standardized as you might expect. The number of plasma cells required for a positive diagnosis, the number of microscope fields examined, and the magnification used all vary between clinics and studies. This inconsistency means that a biopsy read as positive at one lab could conceivably be read as negative at another.13PubMed Central. Plasma Cell Counting Methodology Determines Chronic Endometritis Prevalence The lack of a universally agreed threshold is one reason prevalence estimates for chronic endometritis vary so widely across published studies.
Hysteroscopy, where a thin camera is passed into the uterus, offers another window. Clinicians look for characteristic visual signs including micropolyps, redness, swelling of the lining, and small hemorrhagic spots sometimes described as a “strawberry” appearance.14PubMed. Hysteroscopic features suggestive of chronic endometritis: a systematic review Micropolyps, in particular, show a strong correlation: one large study found that when micropolyps were present during hysteroscopy, the diagnosis was confirmed histologically in about 94% of cases.15PubMed. Endometrial micropolyps at fluid hysteroscopy suggest the existence of chronic endometritis The limitation is that hysteroscopy misses cases that show no visual signs, so it works better as a flag to prompt a biopsy than as a standalone test.
Newer molecular approaches using DNA-based techniques to identify bacteria directly from endometrial samples have shown promise, especially for detecting organisms that do not grow well in standard lab cultures. In one comparison, the molecular method confirmed more than half of the pathogens identified by conventional culture and also picked up additional microorganisms that culture had missed entirely.16American Journal of Obstetrics and Gynecology. Molecular microbiology for diagnosing chronic endometritis These techniques are not yet routine, but they may eventually help guide more targeted antibiotic selection.
Antibiotic Treatment and Cure Rates
Antibiotics are the standard treatment, and they work well for most patients. Doxycycline is the most commonly used first-line drug, typically prescribed for a two-week course. In one study of women with repeated implantation failure, a single course of doxycycline achieved a cure rate of about 92% on follow-up biopsy.17PubMed. Live birth rate following oral antibiotic treatment for chronic endometritis in infertile women with repeated implantation failure Other studies report slightly lower clearance rates with one course, in the range of 68–71%, but most patients who are not cured by the first round clear with a second.18PubMed Central. Chronic endometritis: screening, treatment, and pregnancy outcomes in an academic fertility center
The case for treating chronic endometritis rather than waiting it out is strong. In a controlled study comparing antibiotic treatment to no treatment, over 81% of treated patients achieved resolution after up to three antibiotic courses, compared to just 6% in the untreated group.19Fertility and Sterility. Antibiotic therapy versus no treatment for chronic endometritis: a case-control study Chronic endometritis rarely resolves on its own, which makes treatment not just effective but effectively necessary for anyone hoping to improve their fertility outcomes.
What happens to fertility after treatment? The results are encouraging. In one study of women with unexplained infertility who were treated for chronic endometritis, clinical pregnancy rates reached about 61%, and the rate of taking a baby home was 60%, both significantly higher than in a comparison group that was not screened or treated. Perhaps more striking, the miscarriage rate after treatment dropped to around 2%, compared to 16% in the unscreened group.20PubMed Central. The effect of chronic endometritis and treatment on patients with unexplained infertility
When Standard Antibiotics Are Not Enough
A meaningful minority of patients, roughly one in five, do not clear chronic endometritis with one or even two standard antibiotic courses. These cases, sometimes called multi-drug-resistant chronic endometritis, have become more recognized over the past decade. For these patients, clinicians turn to alternative antibiotics. A pilot study compared oral moxifloxacin (a fluoroquinolone) with azithromycin as third-line options and found cure rates of about 79% and 75%, respectively, with no significant difference between the two.21PubMed Central. Multi-drug-resistant chronic endometritis in infertile women with repeated implantation failure: trend over the decade and pilot study for third-line oral antibiotic treatment
The existence of drug-resistant cases underscores why follow-up biopsies after treatment matter. A negative biopsy after antibiotics is the only reliable way to confirm the condition has cleared. Relying on symptom improvement alone is risky because, as noted earlier, many patients had no symptoms to begin with.
The Emerging Role of Probiotics
Given the link between chronic endometritis and reduced Lactobacillus in the uterine microbiome, researchers have started exploring whether probiotics could help restore a healthier microbial balance. Early results are intriguing but still preliminary. One study found that combining doxycycline with vaginal Lactobacillus produced a slightly higher pregnancy rate than doxycycline alone, though the difference did not reach statistical significance. The combination group did show a significantly lower rate of premature rupture of membranes during subsequent pregnancies.22PubMed Central. Antibiotics combined with vaginal probiotics in the embryo transfer cycle of infertile patients with chronic endometritis
A clinical trial using a specific Lactobacillus crispatus strain alongside antibiotics reported improved progesterone levels and a pregnancy rate of about 87% in the combined group versus 76% with standard treatment alone.23PubMed. The Therapeutic Potential of Lactobacillus crispatus for Chronic Endometritis: A Comprehensive Clinical Trial and Experimental Investigation These numbers are promising, but the studies are small and the field is young. Probiotics are not yet a standard part of treatment protocols, though they are increasingly being discussed as a potential add-on, particularly for patients who relapse or whose endometrial microbiome testing shows low Lactobacillus levels.
Recurrence After Successful Treatment
Clearing chronic endometritis with antibiotics does not guarantee it stays gone. One study tracking recurrence in infertile women found that among those without a history of pregnancy loss, cumulative recurrence rates were about 6% at six months, 14% at twelve months, and 20% at eighteen months after successful treatment. A prior pregnancy loss was a strong predictor of recurrence, roughly quadrupling the odds. Interestingly, having undergone hysteroscopic surgery was associated with a substantially lower recurrence rate.24Wiley Online Library / American Journal of Reproductive Immunology. Analysis of the Predictive Factors for Chronic Endometritis Recurrence in Infertile Women
The possibility of recurrence has practical implications for anyone undergoing fertility treatment. If there is a significant gap between the biopsy confirming clearance and the planned embryo transfer or conception attempt, a repeat biopsy may be worth discussing with your clinician. The timing of that re-check matters, and the recurrence data suggest that the longer you wait after treatment, the more the risk creeps up. For patients with a history of pregnancy loss, staying vigilant about recurrence is especially warranted, since they sit in the highest-risk category for the condition coming back.
Why Diagnosis Still Varies So Much Between Clinics
One of the most frustrating aspects of chronic endometritis for patients and clinicians alike is that reported prevalence rates swing wildly depending on who is doing the testing. Published estimates range from under 10% to over 40% in infertile populations, a span that cannot be explained by genuine biological variation alone. The core issue is methodological: how many plasma cells on a biopsy slide are “too many” has never been universally agreed upon.13PubMed Central. Plasma Cell Counting Methodology Determines Chronic Endometritis Prevalence Some labs require just one plasma cell per high-power field, others require five or more. Some count across ten fields, others across fewer.
This variability has real consequences. If you are seen at a clinic with a low threshold for diagnosis, you are more likely to be told you have chronic endometritis and treated with antibiotics. A clinic with a higher threshold might call the same biopsy normal. Neither is necessarily wrong; they are operating under different definitions that the field has not yet reconciled. If you receive a diagnosis and feel uncertain, asking your clinician which diagnostic criteria they use and how the biopsy was interpreted is a reasonable step. As the field matures, standardized cutoffs will likely emerge, but for now, there is more room for clinical judgment than most patients realize.