Endometriosis can cause rectal bleeding, though it does so less often than many people assume. The condition involves tissue similar to the uterine lining growing in places it shouldn’t, and the bowel is one of the most common sites outside the reproductive organs. When endometriotic tissue invades deep enough into the rectal or sigmoid wall, it can produce bleeding that shows up in the stool, typically in a pattern tied to the menstrual cycle. That cyclical link is often the strongest clue, but the diagnosis is routinely missed or confused with other conditions for years.
How Endometriosis Reaches the Bowel Wall
Endometriotic tissue tends to grow on the outer surface of the bowel first, settling on the serosa (the outermost lining) and then working inward through the muscle layers. In a study of 50 patients with rectovaginal endometriosis, the tissue involved the serosa and the muscular wall in every single case. It reached the submucosa in about a third of patients and penetrated all the way to the inner mucosal lining in roughly one in ten.1Human Reproduction. Histopathological extent of rectal invasion by rectovaginal endometriosis That distinction matters because the mucosa is what lines the inside of the bowel and comes into contact with stool. When endometriotic deposits reach it, bleeding becomes more likely and more visible.
But here is what makes rectal bleeding from endometriosis confusing: the tissue does not always have to reach the mucosa for bleeding to occur. One theory is that swelling of an endometriotic deposit deeper in the wall can create enough pressure to tear the normal overlying mucosa, producing bleeding even when the deposit itself sits in the muscle layer.2PubMed. Cyclical rectal bleeding in colorectal endometriosis That means the absence of visible mucosal disease on an endoscopy does not rule out endometriosis as the source of rectal bleeding.
Why the Bleeding Tends to Follow Your Period
Bowel endometriosis produces two broad categories of symptoms. One set is functional and irritative: diarrhea, intestinal cramping, rectal bleeding, and passage of mucus. These arise from the cyclic release of inflammatory mediators as the ectopic tissue responds to hormonal shifts. The other set is mechanical and obstructive: constipation, bloating, and a feeling of incomplete evacuation, caused by enlarging nodules, intestinal narrowing, and fibrotic scarring. Symptoms like painful bowel movements and a persistent urge to defecate are particularly associated with rectal involvement.3Best Practice & Research Clinical Obstetrics & Gynaecology. Advances in the medical management of bowel endometriosis
The classic presentation is a woman who notices worsening bowel symptoms, including rectal bleeding, around the time of her period. One case report describes a 35-year-old with four years of cyclical lower abdominal pain radiating to the perianal area, alternating diarrhea and constipation, and symptoms that intensified during menstruation.4Europe PMC / International Journal of Surgery Case Reports. Rectosigmoid endometriosis, a rare cause of lower GI bleeding in a premenopausal woman: Case report That cyclical worsening is a red flag, but not everyone tracks their symptoms closely enough to notice the pattern, and some people with bowel endometriosis have symptoms throughout the month rather than strictly around menstruation.
Researchers have also questioned the old assumption that endometriotic deposits on the bowel literally “menstruate” and shed blood into the gut lumen. The frequent absence of visible bleeding within the bowel wall on pathology specimens suggests the mechanism is more subtle. A transient mucosal tear caused by swelling of the underlying deposit remains a plausible explanation.2PubMed. Cyclical rectal bleeding in colorectal endometriosis
What It Gets Mistaken For
Rectal bleeding in a premenopausal woman triggers a differential diagnosis that rarely starts with endometriosis. Clinicians tend to think of hemorrhoids, inflammatory bowel disease, polyps, or colorectal cancer first. Endometriosis can convincingly mimic several of these.
Rectal endometriosis can look so much like rectal cancer on imaging that it has led to unnecessary staging workups. The imaging characteristics of a rectal endometriotic nodule overlap substantially with those of a malignant mass.5International Journal of Surgery Case Reports. A case report: Rectal endometriosis mimicking rectal cancer In at least one documented case, endometriotic tissue in the rectal wall was initially misinterpreted as adenocarcinoma under the microscope because the glandular pattern resembled dysplastic cancer cells in a fibrotic setting. It took immunohistochemistry staining to correctly identify the tissue as ectopic endometrium.6PubMed Central. Rectal mucosal endometriosis primarily misinterpreted as adenocarcinoma: a case report and review of literature
Endometriosis at the terminal ileum, a less common location than the rectosigmoid, has been mistaken for Crohn’s disease. The clinical overlap includes abdominal pain, bowel irregularity, and even bowel obstruction or perforation, making the two conditions difficult to tell apart without tissue diagnosis.7PubMed Central. Terminal Ileal Endometriosis Masquerading as Crohn’s Disease: A Rare Cause of Small Bowel Obstruction and Perforation in a Middle-Aged Woman These diagnostic mixups are not academic curiosities. They delay the correct treatment by years and sometimes lead to inappropriate surgery.
How Long It Takes to Get Diagnosed
Diagnostic delay is one of the most frustrating aspects of endometriosis in general, and bowel involvement makes it worse. In a recent observational study, the median diagnostic delay for all endometriosis patients was five years. But patients whose endometriosis involved the vagina, bowel, or bladder waited a median of nine years from symptom onset to diagnosis, compared with just two years for those without involvement at those sites. Having deep disease in one of those locations increased the odds of a diagnostic delay beyond five years by roughly fivefold.8Human Reproduction. Patients with endometriosis in the vagina, bowel, or bladder experience a prolonged diagnostic delay: an observational study
Part of the problem is that bowel symptoms send patients to gastroenterologists, who may not think of endometriosis, while gynecologists may not routinely ask about bowel symptoms. A person with cyclical rectal bleeding can bounce between specialists for years, getting treated for irritable bowel syndrome or investigated for inflammatory bowel disease, before someone connects the dots. If you are experiencing rectal bleeding that seems to correlate with your menstrual cycle, mentioning that timing to your doctor is one of the most useful things you can do to speed things along.
Imaging That Actually Helps
Transvaginal ultrasound has become the go-to first-line imaging tool for suspected bowel endometriosis. Two separate systematic reviews and meta-analyses found pooled sensitivity and specificity of about 91% and 97-98% for detecting deep infiltrating endometriosis in the rectosigmoid.9PubMed. Diagnostic accuracy of transvaginal ultrasound for non-invasive diagnosis of bowel endometriosis: systematic review and meta-analysis10PubMed. Accuracy of transvaginal ultrasound for diagnosis of deep endometriosis in the rectosigmoid: systematic review and meta-analysis That is strong performance for a test that is relatively inexpensive and widely available.
Bowel preparation before the scan improves accuracy further. A prospective study comparing ultrasound with and without bowel prep found that sensitivity jumped from 73% to 100% and agreement with surgical findings increased substantially when patients had prepped beforehand.11Journal of Minimally Invasive Gynecology. Bowel Preparation Improves the Accuracy of Transvaginal Ultrasound in the Diagnosis of Rectosigmoid Deep Infiltrating Endometriosis: A Prospective Study Ultrasound is excellent at identifying whether tissue has invaded the outer muscle layer of the bowel, though it is less reliable for detecting invasion of the innermost mucosal layer.12Human Reproduction. Can transvaginal sonography predict infiltration depth in patients with deep infiltrating endometriosis of the rectum?
MRI offers a complementary view, especially when surgical planning requires a detailed map of where and how deeply the disease has spread. One study found MRI had a sensitivity of 89% and a positive predictive value of 97% for predicting muscular involvement of the bowel wall.13Clinical Radiology. Deep endometriosis muscular infiltration of the bowel wall: correlation between MRI and histopathology Another found 100% sensitivity for muscular invasion, though specificity was lower, meaning MRI sometimes overestimates the extent of disease.14PubMed Central. Deep infiltrating endometriosis of the bowel: MR imaging as a method to predict muscular invasion Neither imaging modality replaces the other. Ultrasound often comes first; MRI fills in the details when surgery is being planned or when ultrasound findings are ambiguous.
Why Colonoscopy Often Misses Bowel Endometriosis
A colonoscopy looks at the inside surface of the bowel, and endometriosis usually grows from the outside in. That mismatch explains the frustratingly poor sensitivity of colonoscopy for this condition. In a study of 174 colonoscopies performed in women with suspected bowel endometriosis, the procedure detected findings suggestive of the disease in only 4% of cases. Among the 76 women who turned out to have confirmed bowel endometriosis, colonoscopy missed the diagnosis in over 92% of them. The overall sensitivity was just 7%.15PubMed Central. Role of colonoscopy in the diagnostic work-up of bowel endometriosis
Colonoscopy did pick up all cases where endometriosis had penetrated to the mucosa, and it caught about a third of submucosal cases. But when disease was limited to the muscular layer or the serosa, which accounts for the majority of bowel endometriosis, colonoscopy was essentially blind to it. Even when biopsies are taken from visible mucosal irregularities, histologic confirmation rates hover around 47% to 67% depending on whether surface nodularity is present.16Journal of Clinical Gastroenterology. Colonoscopic Findings and Histologic Diagnostic Yield of Colorectal Endometriosis The practical takeaway: a normal colonoscopy does not rule out bowel endometriosis. If suspicion remains after a clean scope, imaging with transvaginal ultrasound or MRI is the appropriate next step.
Medical Treatment for Bowel Symptoms
Hormonal therapy is the first-line approach when bowel endometriosis is diagnosed and the symptoms are manageable without surgery. Progestins suppress the hormonal fluctuations that drive the cyclical inflammation, and they work reasonably well for many patients. When the rectosigmoid junction is involved, roughly 70% of patients experience relief of intestinal symptoms on hormonal treatment, with about 10% eventually needing surgery after treatment failure. For disease limited to the mid-rectum, the numbers are even better: around 80% symptom relief and only about 3% eventually requiring surgery.3Best Practice & Research Clinical Obstetrics & Gynaecology. Advances in the medical management of bowel endometriosis
Bowel obstruction during hormonal treatment is rare, occurring in roughly 1-2% of cases with rectosigmoid involvement. That low rate is reassuring, because one of the fears with medical management is that the disease will progress and cause a blockage. For most patients, hormonal therapy buys significant time and symptom control, and for some it may be the only treatment ever needed.
Surgical Approaches and Their Trade-Offs
When medical therapy fails, or when the disease is causing significant obstruction or the patient has fertility goals that hormonal suppression would interfere with, surgery becomes the conversation. Three main techniques exist for removing bowel endometriosis, and the choice between them matters.
- Shaving: The surgeon peels the endometriotic nodule off the bowel surface without opening the intestinal wall. This is the least invasive option and carries the lowest complication rate.
- Disc excision: A full-thickness disc of bowel wall is removed where the nodule sits, and the resulting hole is closed. This gives a more complete removal than shaving but involves opening the bowel lumen.
- Segmental resection: A section of bowel is removed entirely and the two ends are reconnected. This is reserved for extensive disease, multiple nodules, or significant narrowing of the bowel.
A comparative study of 364 consecutive cases found that severe postoperative complications occurred in about 12% of patients overall, but two-thirds of those complications were in the segmental resection group. Rectovaginal fistula, one of the most feared complications, occurred in about 2% of shaving patients, 4% of disc excision patients, and 6% of those who had segmental resection.17PubMed. Postoperative complications after bowel endometriosis surgery by shaving, disc excision, or segmental resection: a three-arm comparative analysis of 364 consecutive cases A review of the literature came to the same conclusion: shaving should be considered the first-line surgical technique regardless of nodule size, with disc excision as a backup and segmental resection reserved for the most advanced cases.18Fertility and Sterility. Choosing the right surgical technique for deep endometriosis: shaving, disc excision, or bowel resection?
In a large series of over 1,100 cases, the risk of bowel fistula increased significantly any time the rectal lumen was opened during surgery. Compared with shaving, disc excision raised the odds of fistula roughly sevenfold, and segmental resection raised it roughly fivefold.19Human Reproduction. Risk of bowel fistula following surgical management of deep endometriosis of the rectosigmoid: a series of 1102 cases The concern that shaving might leave residual disease and lead to higher recurrence has not been borne out in the published data so far.
Bowel Endometriosis During Pregnancy
Pregnancy is generally expected to improve endometriosis symptoms, since the sustained high progesterone levels suppress the hormonal cycling that drives the disease. For superficial endometriosis, this usually holds true. Deep infiltrating bowel endometriosis, however, can behave differently. Decidualization and hormonal changes can cause nodules to swell, and the combination of a weakened bowel wall, fibrosis, and adhesions can, in rare cases, lead to intestinal perforation.20Frontiers in Medicine. Case report: A rare cause of intestinal perforation in a third-trimester pregnant woman
This complication is genuinely rare. A review of the literature found only 15 documented cases of bowel complications from endometriosis during pregnancy or the immediate postpartum period.21PubMed Central. Deep endometriosis induced spontaneous colon rectal perforation in pregnancy: laparoscopy is advanced tool to confirm diagnosis But the cases that do occur tend to be serious and typically happen in the third trimester. At least one case report describes an acute sigmoid perforation at 34 weeks of gestation in a woman who had no prior diagnosis of endometriosis, meaning the deep bowel disease was discovered only when the emergency unfolded.22PubMed Central. Acute endometriosis-related sigmoid perforation in pregnancy- case report For women with known bowel endometriosis who become pregnant, awareness of this possibility among their obstetric team is the main practical consideration.
Blood-Based Testing on the Horizon
One of the reasons bowel endometriosis goes undiagnosed for so long is that there has never been a simple blood test for the condition. That may be changing. A recent study evaluated a multi-marker blood test combining three microRNAs, three protein biomarkers, and a steroid hormone, with the patient’s age and body mass index fed into a machine learning model. In an independent validation group, the test achieved an area under the curve of 0.94, with 80% sensitivity and nearly 98% specificity for detecting endometriosis.23Journal of Minimally Invasive Gynecology. Noninvasive Blood-Based Detection of Endometriosis Can Improve Standard-of-Care by Facilitating Early Diagnosis and Clinical Management Among Symptomatic Women
A test like this would not replace imaging or surgery for pinpointing exactly where the disease is, but it could serve as a triage tool. A woman presenting with cyclical rectal bleeding and a positive blood test would have a much faster path to the right imaging and the right specialist. The test is not yet in routine clinical use, and independent replication in larger and more diverse populations will determine whether those accuracy numbers hold up. But after decades of relying on surgical biopsy as the only definitive confirmation, even a reasonably accurate screening blood test would represent a meaningful shift in how quickly patients get answers.
The Gut Microbiome Connection
An emerging line of research is exploring whether the gut microbiome plays a role in endometriosis itself, not just as a bystander but as an active contributor. In animal studies, depleting the gut microbiota with antibiotics led to smaller endometriotic lesions, along with changes in the immune cell populations in the peritoneum. Mice with depleted microbiomes had fewer macrophages, B cells, and T cells around their lesions, suggesting the gut microbiome may help sustain the inflammatory environment that endometriosis thrives in.24Cell Death Discovery. Gut microbiota and microbiota-derived metabolites promotes endometriosis This research is still in the animal-model phase and does not yet translate to treatment recommendations for patients. But it raises the intriguing possibility that the relationship between the bowel and endometriosis runs in both directions: endometriosis invades the bowel, and the bowel’s own microbial ecosystem may influence how aggressively the disease behaves.