Uterus Inflammation: Causes, Symptoms, and Diagnosis

Uterus inflammation, most commonly called endometritis, is a condition in which the inner lining of the uterus becomes inflamed, usually because bacteria that do not belong there have gained a foothold. The condition comes in two broad forms: an acute version that tends to flare after childbirth or uterine procedures, and a quieter chronic version that can linger for months without obvious symptoms. The distinction between the two matters because they differ in how they feel, how they are found, and what they do to fertility if left untreated.

How Bacteria Get Into the Uterus

The uterine cavity is normally a low-biomass environment, meaning it hosts far fewer microorganisms than, say, the vagina. Infection-driven endometritis typically starts when bacteria from the lower genital tract travel upward through the cervix. Anything that opens the cervical canal can facilitate that journey: vaginal delivery, cesarean section, dilation and curettage, hysteroscopy, or the insertion of an intrauterine device (IUD).

A systematic review identifying the bacteria most frequently linked to endometritis found five genera appearing most often: Chlamydia and Ureaplasma (each accounting for about 11% of reported cases), followed by Streptococcus, Mycoplasma, and Enterococcus.¹ That list overlaps heavily with organisms already present in the vagina and cervix in smaller numbers. In chronic endometritis specifically, Staphylococcus, Escherichia coli, Enterococcus faecalis, and even Mycobacterium tuberculosis have been implicated alongside the usual suspects.²

Chlamydia trachomatis deserves a special mention. Animal studies have shown that the organism can ascend from the vagina to the upper genital tract within 24 hours of infection, and that hormonal phase affects how readily it reaches the uterus and fallopian tubes.³ In humans, chlamydial endometritis often produces no dramatic symptoms, which is part of why it can cause damage silently over time.

Bacterial Vaginosis as a Risk Factor

Bacterial vaginosis (BV) is not a classic sexually transmitted infection but rather an imbalance of the vaginal microbiome, with an overgrowth of anaerobic bacteria. Research has found a strong link between BV and endometritis. In one study, nearly half of women with symptomatic BV had plasma cell endometritis on biopsy, compared with about 5% of controls. BV-associated organisms were also cultured directly from the uterine lining in a high proportion of those cases, suggesting that the vaginal imbalance extends upward into the uterus in many women.⁴ This finding has practical implications: treating BV may reduce the risk of endometritis developing or recurring, though the connection does not mean every woman with BV will develop uterine inflammation.

Non-Infectious Causes

Not all uterine inflammation is driven by bacteria. Copper-containing IUDs trigger what is sometimes called aseptic or sterile inflammation. The device acts as a foreign body, provoking the immune system to send lymphocytes, monocytes, and macrophages into the endometrial tissue. This inflammatory infiltrate alters the local immune environment and even changes how the endometrium responds to hormones like estrogen and progesterone.⁵ Interestingly, this sterile inflammation appears to be distinct from the kind caused by infection, and some evidence suggests it may even protect against endometrial overgrowth. Early generations of IUDs were pulled from the market partly because they caused more severe endometritis and increased the risk of pelvic inflammatory disease.⁶ Modern devices cause much less trouble, but low-grade inflammation around the device is still a recognized feature, not a sign of failure.

Radiation therapy for pelvic cancers, retained products of conception after miscarriage, and submucous fibroids are other non-infectious triggers that can produce a chronic inflammatory state in the endometrium. In these cases, the inflammation is a secondary response to tissue damage, abnormal growths, or foreign material rather than to a living pathogen.

Acute Versus Chronic Endometritis

The acute form usually announces itself clearly. It most often follows childbirth (postpartum endometritis) or a uterine procedure and produces fever, lower abdominal pain, foul-smelling vaginal discharge, and uterine tenderness. Postpartum endometritis is more common after cesarean delivery than after vaginal birth, and symptoms typically appear within the first week. In rare but extreme cases, untreated infection can lead to the accumulation of pus inside the uterus, a condition known as pyometra. Case reports document pyometra progressing to uterine rupture, peritonitis, and life-threatening sepsis.⁷

Chronic endometritis is a different animal. Symptoms, when they exist at all, tend to be subtle: irregular bleeding, spotting between periods, mild pelvic discomfort, or increased vaginal discharge. Many women with chronic endometritis have no symptoms and discover the condition only during a fertility workup. Because the inflammation is low-grade and persistent rather than acute and dramatic, it often goes undiagnosed for months or years. In studies of women experiencing repeated implantation failure during IVF, chronic endometritis has been detected in roughly a third of cases.⁸

How It Is Diagnosed

Diagnosing acute endometritis is usually straightforward: fever, pain, and tenderness after delivery or a procedure raise the suspicion, and clinical judgment often suffices to start antibiotics. Chronic endometritis, however, requires more detective work because its symptoms overlap with dozens of other gynecological conditions, and many women are asymptomatic.

Endometrial Biopsy and CD138 Staining

The gold standard for diagnosing chronic endometritis is an endometrial biopsy examined under the microscope. The hallmark finding is the presence of plasma cells in the endometrial stroma. Plasma cells are a type of immune cell that does not normally reside in the uterine lining, so finding them there signals chronic inflammation. Because plasma cells can be hard to spot on standard staining, pathologists increasingly use immunohistochemistry for a protein called CD138 (also known as syndecan-1), which lights up plasma cells specifically. Research has established a significant association between CD138 positivity and confirmed chronic endometritis, making it the most reliable single marker.⁹ One limitation is that the uterine cavity is a low-biomass site where standard bacterial culture often fails to grow anything, even when infection is present.¹⁰ This means a biopsy might confirm the inflammation without identifying the specific culprit.

Ultrasound

Pelvic ultrasound is a less invasive first step. In acute postpartum endometritis, two ultrasound signs have shown promise: a hypoechoic (darker) rim beneath the outer uterine surface, and blurring of the boundary between the uterine muscle and the endometrium. When both signs were present together, average sensitivity reached about 80% and specificity about 92%.¹¹ Adding Doppler imaging to look at blood flow patterns can increase diagnostic accuracy further, as endometritis tends to produce abnormal venous drainage and increased blood vessel density in the uterine wall and lining.¹² For chronic endometritis, ultrasound alone is generally not sensitive enough to make the diagnosis, though it can flag thickened or irregular endometrium that warrants biopsy.

Hysteroscopy

Hysteroscopy, where a thin camera is passed through the cervix to directly view the uterine cavity, gives clinicians a real-time look at the endometrial surface. Features suggestive of chronic endometritis include stromal edema (the tissue looking swollen and pale), focal or diffuse redness, tiny polyps less than one millimeter in size (micropolyposis), and a characteristic pattern nicknamed the “strawberry aspect” for its speckled red appearance. Studies have found that the combination of these features offers good diagnostic accuracy when correlated with biopsy confirmation.¹³ Hysteroscopy can also identify other abnormalities at the same time, such as polyps, fibroids, or adhesions, making it a useful two-for-one diagnostic tool in the fertility setting.¹⁴

The Fertility Connection

Chronic endometritis has drawn intense interest from the reproductive medicine community because of its impact on IVF outcomes. A systematic review and meta-analysis found that women with untreated chronic endometritis had roughly half the odds of a live birth compared with women without the condition. Importantly, when the inflammation was successfully treated with antibiotics, IVF outcomes rebounded to levels comparable with women who never had the condition.¹⁵ The same analysis noted that the negative effect was concentrated in severe cases: women classified as having mild chronic endometritis did not show a meaningful difference in IVF success compared with unaffected women. This is a useful distinction for patients who receive the diagnosis, because “chronic endometritis” can sound alarming, and knowing that mild cases may not carry the same consequences can reduce some anxiety.

Beyond IVF, chronic uterine inflammation has been linked to the formation of intrauterine adhesions (Asherman’s syndrome). Research has found that about 40% of women with adhesions also showed signs of chronic endometritis, and those with both problems had a higher recurrence of adhesions after surgical removal.¹⁶ The theory is that prolonged inflammation disrupts normal tissue repair, favoring scar formation over healthy regeneration.

The Uterine Microbiome and Dysbiosis

One of the more active areas of research is the idea that the uterus has its own microbiome, and that shifts in that microbial community can drive or sustain inflammation. The dominant bacterium in a healthy endometrium appears to be Lactobacillus, the same genus that dominates a healthy vagina. When Lactobacillus levels drop and other organisms like Gardnerella and Escherichia move in, the resulting dysbiosis has been associated with higher levels of inflammatory molecules (such as IL-6 and IL-1β) and lower levels of anti-inflammatory signals in the uterine tissue.¹⁷

Research in women with confirmed chronic endometritis has found that the microbial diversity of their uterine lining is significantly different from that of healthy controls: there tends to be an increase in Proteobacteria and a decrease in Firmicutes and Actinobacteria.¹⁸ Antibiotic treatment aimed at restoring a Lactobacillus-dominant environment has been reported to improve implantation rates in subsequent IVF cycles.¹⁹ The microbiome angle is still evolving, and much of the evidence comes from small studies, but it is changing the way fertility specialists think about why some uterine inflammation resists standard antibiotic courses: the goal may not just be killing specific pathogens but restoring a healthy microbial ecosystem.

Rising Antibiotic Resistance

A practical concern for anyone being treated for chronic endometritis is that antibiotic resistance among the bacteria involved has been climbing. A cross-sectional study spanning 2020 to 2024 found that resistance to ampicillin reached nearly 99% of tested isolates, while resistance to tetracyclines, quinolones, and nitroimidazoles was also substantial. Extended-spectrum beta-lactamase (ESBL) positivity, a marker of broad resistance to common antibiotics, was detected in about 35% of isolates.²⁰

Multi-drug-resistant chronic endometritis is not yet the norm, but it has been increasing. In a decade-long analysis of women with repeated implantation failure, multi-drug resistance among those with chronic endometritis rose roughly eightfold over ten years. For these difficult-to-treat cases, third-line agents like moxifloxacin and azithromycin achieved cure rates around 75 to 79%.⁸ These numbers are reassuring in the short term, but the trend underscores the importance of culture-guided treatment when standard courses fail, rather than simply cycling through broader and broader antibiotics.

The Overlap With Adenomyosis

Adenomyosis, a condition in which endometrial tissue grows into the muscular wall of the uterus, shares enough symptoms with chronic endometritis (pain, abnormal bleeding, fertility problems) that the two can be hard to tell apart. They can also coexist. A systematic review and meta-analysis found that the prevalence of chronic endometritis in women with adenomyosis was about 32%, and that women with adenomyosis were nearly four times as likely to have chronic endometritis compared with controls.²¹ Whether one drives the other or they simply share underlying risk factors remains an open question. For clinicians, the co-occurrence means that a woman diagnosed with adenomyosis who is having fertility trouble may benefit from being screened for chronic endometritis as well, since the latter is treatable with antibiotics even if the former is not.

Immune Dysfunction and Autoimmunity

Endometriosis, a related but distinct condition where endometrial-like tissue grows outside the uterus, is increasingly understood as an inflammatory and possibly immune-mediated disease. Chronic inflammation is a hallmark of its progression, and it commonly co-occurs with autoimmune conditions.²² The relevance for uterine inflammation is that the immune landscape in and around the uterus may predispose some women to persistent endometritis. One study examining women with recurrent pregnancy loss and repeated implantation failure found that endometritis and elevated inflammatory and autoimmune markers were common in that population, though the presence of endometritis could not be predicted from blood-based immune markers alone.²³ In plain terms, the immune system’s role in uterine inflammation extends beyond just fighting infection. In some women, an overactive or dysregulated immune response may perpetuate inflammation even after the initial trigger has been cleared.

When Abnormal Bleeding Points to Inflammation

Abnormal uterine bleeding is one of the most common reasons a woman sees a gynecologist, and it has a long list of possible causes: fibroids, polyps, hormonal imbalance, and cancer among them. Chronic endometritis is an underappreciated entry on that list. When researchers examined endometrial biopsies from women with abnormal bleeding patterns, plasma cells turned up at high rates in samples showing disordered proliferative endometrium and proliferative endometrium with breakdown, while they were rare in samples with a normal proliferative pattern.²⁴ Stromal breakdown, a specific histological feature, was the finding most strongly associated with plasma cell infiltration. The takeaway for someone dealing with unexplained bleeding is that if standard workups for polyps, fibroids, and hormonal issues come back unremarkable, chronic endometritis is worth investigating. An endometrial biopsy with CD138 staining can confirm or rule it out in a single sample.