Endometriosis: Understanding Your Pathology Report

Section Editor: Kianoosh Keyhanian MD FRCPC
September 1, 2026

Endometriosis is a noncancerous (benign) condition in which tissue resembling the lining of the uterus grows outside the uterus. The lining of the uterus is called the endometrium. In endometriosis, tissue of the same kind is found on other surfaces in the pelvis and, less often, elsewhere in the body.

This tissue responds to the same hormones as the uterine lining. It thickens and bleeds with each cycle, but the blood has nowhere to drain. The result is repeated irritation, inflammation, and, over time, scarring of the surrounding tissue. That process produces the pain and adhesions that characterize this condition.

Endometriosis is common. It affects roughly 1 in 10 women of reproductive age, and it is found more often in those investigated for pelvic pain or difficulty becoming pregnant. It is not cancer, and the great majority of women who have it never develop cancer. This article will help you understand what this diagnosis means on your pathology report, what each term means, and why it matters for your care.

What causes endometriosis?

No single explanation accounts for every case, and research continues. Several mechanisms are thought to contribute.

Retrograde menstruation — During a period, some menstrual blood flows backward through the fallopian tubes into the pelvis instead of leaving through the vagina. Cells carried with it may settle and grow. This happens in most women, so it cannot be the whole explanation.
Cells changing type — Cells lining the pelvic cavity may transform into endometrial-type cells. This may explain endometriosis in unusual locations and in people who have never menstruated.
Spread through blood and lymph vessels — This may account for the rare cases found in distant sites such as the lung.
Immune factors — Differences in how the immune system clears displaced cells may determine who develops the condition.
Genetics — Endometriosis runs in families. Having a mother or sister with the condition raises the likelihood of developing it, though no single gene is responsible.

It is not caused by using tampons, by delaying pregnancy, or by any lifestyle choice.

Where in the body does endometriosis occur?

Endometriosis most often occurs on surfaces inside the pelvis. Common sites include the ovaries, the outer surface of the uterus, the fallopian tubes, and the ligaments that support the uterus. Two other frequent sites are the space between the uterus and the rectum, and the space between the uterus and the bladder. In some cases, the bowel, bladder, and appendix are also involved.

Less commonly, endometriosis is found in a cesarean section scar or other surgical scar, in the umbilicus, or in the wall of the abdomen. Rarely it occurs at distant sites such as the chest. Doctors describe three main patterns. These are superficial deposits on pelvic surfaces, cysts within the ovary, and deep disease growing more than 5 mm beneath a surface.

What are the symptoms?

Symptoms vary widely, and some women with extensive endometriosis have none at all. The amount of disease seen at surgery does not predict how much pain a person has, which is one of the more difficult aspects of this condition.

Painful periods — Pain that begins before or with a period and is more severe than usual. This is the most common symptom.
Ongoing pelvic pain — Pain between periods, sometimes constant.
Pain during or after intercourse — Often deep rather than at the entrance.
Bowel or bladder pain — Pain with bowel movements or urination, often worse around a period. These occur when endometriosis involves those organs.
Difficulty becoming pregnant — Sometimes the first sign, and the reason many women are investigated.
Fatigue and heavy bleeding — Both are commonly reported.

Endometriosis is often diagnosed years after symptoms begin. If your symptoms have been dismissed in the past, that experience is common, and it is not a reason to stop seeking assessment.

How is the diagnosis made?

The way endometriosis is diagnosed has changed. For many years the diagnosis required laparoscopy, a keyhole operation to look inside the pelvis, with tissue sent to a pathologist. Current international guidelines no longer treat surgery as the required standard. Endometriosis can now be diagnosed from symptoms, examination, and imaging, and treatment can begin without an operation.

Ultrasound and MRI can show ovarian cysts caused by endometriosis and deeper disease, and both are used routinely. Neither reliably shows superficial deposits, so a normal scan does not rule out endometriosis. Blood tests, including CA-125, are not recommended to diagnose or exclude the condition.

Laparoscopy is now reserved for situations where imaging is negative, but symptoms persist, treatment has not worked, or surgery is being done for another reason. When tissue is removed, a pathologist examines it and confirms the diagnosis under the microscope.

One point to know if you have had a biopsy. A negative result does not exclude endometriosis. Lesions can be missed, and long-standing deposits sometimes lose their original appearance, leaving only scarring behind. A report that does not confirm endometriosis is not proof that you do not have it.

What does endometriosis look like under the microscope?

Endometriosis is tissue normally found in the lining of the uterus that grows outside it. To the naked eye, deposits may appear as small blue-black or brown spots, white scarred areas, or a blood-filled cyst. Under the microscope, the pathologist looks for the following features.

Endometrial glands — Tube-shaped structures of the kind normally found in the lining of the uterus. Glands are one of the two structures that define this diagnosis.
Endometrial stroma — The specialized supporting tissue that normally surrounds endometrial glands. Finding this type of stroma alongside the glands distinguishes endometriosis from other glandular tissue.
Hemosiderin — A brown pigment left behind when blood breaks down. Its presence reflects repeated bleeding into the tissue.
Scarring — Fibrosis develops around the deposits over time and is what causes surfaces to stick together.
Inflammation — Immune cells gather around the deposits. This inflammation contributes to pain.

Pathologists usually require two of the three main features, most often glands together with stroma. When only scarring and pigment remain, the report may say “consistent with endometriosis” or “features suggestive of endometriosis” rather than giving a definite diagnosis. That hedged wording reflects an old, burnt-out deposit, not uncertainty about your condition.

What is an endometrioma?

An endometrioma is a cyst in the ovary formed by endometriosis. Repeated bleeding into the ovary produces a cyst filled with thick, dark brown fluid, which is why these are often called chocolate cysts. They are among the most common findings in endometriosis.

Endometriomas can be seen on ultrasound and have a characteristic appearance. When one is removed, the pathologist examines the lining to confirm endometriosis and to check that no other process is present. The lining of a long-standing endometrioma is often scarred and may have lost its glands so that the report may use cautious wording.

Removing an endometrioma involves removing some ovarian tissue with it, which can reduce the ovarian reserve. If fertility matters to you, discuss this before surgery rather than afterward.

What is the difference between endometriosis and adenomyosis?

People often confuse the two, and they can have both. The difference is location.

Endometriosis — Endometrial-type tissue outside the uterus, on pelvic surfaces and organs.
Adenomyosis — Endometrial-type tissue within the muscular wall of the uterus itself.

Adenomyosis typically causes heavy, painful periods and an enlarged, tender uterus. Doctors diagnose it on imaging or after a hysterectomy. Because the two conditions are separate, a report may describe one, the other, or both.

Does endometriosis increase the risk of cancer?

Endometriosis is not cancer and does not usually become cancer. There is a small increase in the risk of two uncommon types of ovarian cancer, and it is worth seeing that risk in absolute terms rather than as a multiple.

About 1 in 75 women in the general population develop ovarian cancer at some point in life. Among women with endometriosis, the figure is roughly 1 in 40. The increase is real, but the great majority of women with endometriosis never develop ovarian cancer. International guidelines advise that no cancer screening is recommended based on an endometriosis diagnosis.

The two cancers associated with endometriosis are clear cell carcinoma and endometrioid carcinoma of the ovary. Both are thought to arise occasionally from endometriosis within the ovary. Pathologists sometimes describe atypical endometriosis, meaning deposits whose cells look abnormal. This is uncommon, and if your report uses that term, ask your doctor what follow-up is appropriate.

How is endometriosis treated?

Treatment targets symptoms rather than eliminating the condition, and the right approach depends on whether pain, fertility, or both are the priority. Options your doctor may discuss include:

Pain medication — Anti-inflammatory medications are often the first step for period pain.
Hormonal treatment — Combined hormonal contraceptives, progestin-only medications, the hormonal IUD, and medications that suppress ovarian hormone production all work by reducing the cyclical stimulation of the deposits.
Surgery — Deposits and endometriomas can be removed or destroyed at laparoscopy. This can relieve pain and, in some situations, improve fertility.
Fertility treatment — IVF is often used when endometriosis is contributing to difficulty conceiving.
Treatment after surgery — Endometriosis recurs frequently after surgery alone. Hormonal treatment afterward is usually recommended for those not trying to conceive.

Hysterectomy with removal of both ovaries is considered only for severe symptoms that have not responded to other treatment, and it is not a guaranteed cure.

What other findings may be described in the report?

Endometriosis is commonly reported alongside other findings, particularly when tissue has been removed from several sites. Findings often described include:

The sites involved — Each piece of tissue is reported separately so that a single report may list several locations.
Adhesions — Bands of scar tissue binding surfaces together. Periovarian adhesions are common in endometriosis and are reported separately.
Changes in the fallopian tube — A blocked, fluid-filled tube is called hydrosalpinx, and endometriosis is one of its causes.
Ovarian cysts — A corpus luteum cyst or follicle cyst may be present alongside an endometrioma. Both are noncancerous.
Decidualized endometriosis — During pregnancy, endometriosis deposits change appearance and can look alarming on imaging or at surgery. This is a normal response to pregnancy hormones.

Questions to ask your doctor

Was endometriosis confirmed under the microscope, or diagnosed from symptoms and imaging?
Which sites were involved, and how extensive was the disease?
Was an endometrioma found, and was any ovarian tissue removed with it?
Does my report mention atypical endometriosis?
Was all visible endometriosis removed, or only some of it?
Do I also have adenomyosis?
What does this mean for my fertility, and should I see a fertility specialist?
Which treatment do you recommend for my symptoms, and why?
Should I take hormonal treatment after surgery to reduce the chance of recurrence?
How likely is the endometriosis to come back?
Does this diagnosis change my risk of cancer, and do I need any screening?
My biopsy did not confirm endometriosis. Could I still have it?
What symptoms should prompt me to contact you?