Endometriosis: What It Is and Why It Hurts

A patient tells you their periods have always been painful, but over the past few years things have changed. They now dread the days before bleeding starts. Opening their bowels can be painful, they sometimes miss work, and the discomfort does not always disappear when the period ends. They've had an ultrasound and been told it looked normal. They want to know why they are still in so much pain.

This is the sort of history that should make us think about endometriosis. It does not confirm the diagnosis, because pelvic pain and bowel symptoms have several possible causes. But it is important not to dismiss the possibility simply because the scan did not show anything unusual.

Endometriosis occurs when tissue similar to the endometrium, the lining inside the uterus (womb), is found outside the uterus. These patches are called lesions. Some people with endometriosis have few or no symptoms; others experience pain that disrupts work, relationships, sleep and everyday life. To understand the difference, it helps to start with the anatomy rather than the scan result.

How tissue outside the uterus causes pain

The normal endometrium lines the inside of the uterus. It changes through the menstrual cycle in response to hormones, and part of it sheds during a period. Endometriosis lesions develop in other places, most commonly around the pelvis. They may appear on the peritoneum, the thin membrane lining the pelvic cavity, or involve the ovaries and tissues near the uterus. More complex disease can affect structures around the bowel, bladder or ureters.

The lesions are like endometrial tissue, but they are not identical to the lining of the womb. Endometriosis is not simply menstrual blood that has become trapped somewhere else. The lesions can respond to hormonal changes and trigger inflammation in the surrounding tissues. Over time, inflammation and healing may also cause fibrosis (scar-like tissue) and adhesions, which are bands of tissue that can make structures that normally move independently stick together.

Think about a bowel loop moving as food passes through it, or the tissues of the pelvis moving during intercourse. If nearby tissue is inflamed or tethered by adhesions, movement may be painful. Lesions close to nerves can also contribute to pain. This helps explain why endometriosis is not only associated with painful periods. The location of the disease can influence what hurts and when.

There are several patterns of endometriosis. Superficial peritoneal endometriosis is found on the surface lining of the pelvis. An ovarian endometrioma is a cyst associated with endometriosis, often containing altered blood. Deep endometriosis extends beneath the tissue surface and can involve organs or supporting structures, sometimes including the bowel, bladder or ureter. These patterns can occur together, and the symptoms do not tell us with certainty which one is present.

The photograph above is a view obtained during laparoscopy, when a camera is used to examine the pelvis. A small superficial lesion may be visible this way even though it was not identified on a routine ultrasound. Endometriosis can also be examined under the microscope; the image below shows tissue from an ovarian endometrioma.

An important part of understanding endometriosis is recognising that the amount of visible disease does not reliably predict the amount of pain. Some people have relatively limited-looking lesions and very severe symptoms. Others have extensive disease but little pain. Inflammation, the site of the lesions, adhesions and the way the nervous system responds over time all contribute.

With persistent pain, nerves in and around the pelvis may become more sensitive. Pain processing in the spinal cord and brain can also change, a process often called sensitisation. That can help explain pain between periods or discomfort that spreads beyond one specific location. It is a physical pain-processing response, not evidence that the symptoms are imagined. It also explains why some patients need help with long-term pain management as well as treatment aimed at the lesions themselves.

Listening to the symptoms, then choosing the right investigation

Return to the patient with painful bowel movements. Rather than starting by asking only how painful their periods are, you would want to hear the pattern. Does bowel pain become worse around menstruation? Is there deep pain during or after sex? Is passing urine uncomfortable at certain times in the cycle? Do symptoms continue between periods? Have they changed over time?

Severe period pain that affects ordinary activities, ongoing pelvic pain, pain associated with sex, period-related bowel or urinary symptoms, and fertility difficulties alongside these symptoms can all raise suspicion of endometriosis. Fatigue may also be part of the picture. Some people describe being told for years that they simply have bad periods. Asking what the pain has stopped them doing can reveal more than a pain score on its own.

Family history can be useful too. NICE advises asking whether a first-degree relative has endometriosis, because this increases the likelihood of the condition. A symptom diary may help the patient show changes across the month, although symptoms do not have to be strictly cyclical for endometriosis to remain a possibility. Remember to consider other causes, including adenomyosis, fibroids, pelvic inflammatory disease, bowel conditions and urinary problems, rather than attributing everything to a suspected endometriosis diagnosis.

NICE recommends offering a transvaginal ultrasound to people with suspected endometriosis even when the pelvic or abdominal examination is normal. It may identify an ovarian endometrioma or some deep lesions, help find other causes of the symptoms and guide referral. The person can decline an internal scan; if it is unsuitable or declined, a transabdominal ultrasound may be considered, with its limitations explained.

The ultrasound image shows an ovarian endometrioma. What it cannot show you is every type of endometriosis. Small superficial lesions, for example, may not be seen on routine scanning. So when someone says, “But my scan was normal,” the important response is that a normal scan does not rule out endometriosis. The same is true of a normal pelvic examination.

If symptoms persist, return or interfere with day-to-day life, further assessment and referral may still be appropriate, even after normal imaging. Suspected or confirmed endometriomas, deep disease involving organs such as the bowel, bladder or ureter, or disease outside the pelvis call for a specialist endometriosis service. Specialist ultrasound or MRI can help map deep disease. Laparoscopy, a keyhole procedure to inspect the pelvis, may be considered when endometriosis is suspected, including after normal imaging, with the benefits and risks discussed. It is not something every patient must undergo before their pain is taken seriously or treatment can begin.

A blood test is not a shortcut around this assessment. NICE specifically advises against using CA125 to diagnose endometriosis. The clinical history still matters when routine test results offer no obvious explanation.

Treatment begins with what the person needs

There is no single treatment plan that suits everyone with endometriosis. One patient may mainly want relief from severe monthly pain. Another may have pain between periods or deep disease affecting the bowel. Someone else may be thinking about pregnancy. Those differences change what treatment is trying to achieve.

Pain relief is often part of initial care. Depending on the person's circumstances, this may include an appropriate trial of analgesics, such as paracetamol or a non-steroidal anti-inflammatory drug, with attention to contraindications and response. Hormonal treatment, including combined hormonal contraception or progestogen-based options, may also reduce symptoms by changing ovarian hormonal stimulation. These treatments can be offered for suspected as well as confirmed endometriosis when appropriate, rather than waiting for an operation to establish a diagnosis.

Hormonal treatments may reduce pain, but that does not mean every lesion has disappeared. If symptoms remain difficult to manage, specialist care may include different hormonal approaches, multidisciplinary pain support or surgery. NICE also recommends selected newer GnRH antagonist treatments for eligible adults after previous medical or surgical treatment; these are not simply first-line medicines for everyone with painful periods.

Surgery may be considered to treat lesions or particular problems such as endometriomas or deep disease. The decision involves more than whether something appears on a scan. The team needs to consider the likely benefit for symptoms, surgical risks, the possibility of recurrence and whether the procedure could affect healthy tissue. Deep endometriosis close to the bowel, bladder or ureter needs specialist planning.

Fertility deserves its own conversation. Endometriosis may affect conception through inflammation, adhesions or changes involving the ovaries and tubes, but it does not mean pregnancy is impossible; many people with endometriosis conceive naturally. If someone is trying to conceive, treatments that suppress ovulation are not suitable for achieving pregnancy while they are being taken. Surgery involving an ovarian endometrioma also requires care because operating on the cyst can affect ovarian reserve. A specialist discussion can weigh symptom control, future fertility, age, previous surgery and the person's preferences.

The treatment goal may also change over time. Someone may need pain relief now, but want to discuss fertility later, or may find a previously helpful treatment no longer controls their symptoms. Reviewing what matters to the patient is part of managing a long-term condition, not an optional extra.

Bringing it back to nursing practice

As a student nurse, you may meet someone while they are still waiting for an explanation, during assessment, or after laparoscopic surgery. In each situation, there is useful information to gather. Ask where and when the pain occurs, what makes it worse, what treatment has already been tried and what the person has had to change in everyday life. Listen for pain with bowel movements or urination, deep pain during sex, and any fertility concerns they wish to discuss. Not everyone will be comfortable sharing these details immediately, so privacy and sensitive questions matter.

Good observations remain important, but someone can have disabling endometriosis symptoms while their routine observations are normal. Equally, do not assume that every episode of pelvic pain is caused by known endometriosis. A sudden change in pain, collapse, signs of acute illness, or significant pain when pregnancy is possible needs prompt reassessment for other urgent causes, following local escalation guidance.

Following a procedure, nursing care includes observations, pain management, watching for bleeding or other complications, supporting mobilisation and explaining the agreed recovery advice. Before discharge or follow-up, check that the patient understands what was found, what treatment was performed, and what symptoms should prompt them to seek help. These conversations make a difference, particularly when someone has spent a long time feeling that their symptoms were not understood.

If you take one thing from this article, make it this: the patient's symptoms and their impact matter even when a scan looks normal. Understanding where endometriosis can develop, how inflammation and nerve sensitivity contribute to pain, and what each investigation can actually show helps you recognise why the condition is sometimes difficult to diagnose.

Related Bleepbook reading: Adenomyosis explains endometrial-type tissue within the uterine muscle, while Uterine fibroids explores benign growths arising from that muscle. The menstrual cycle follows the normal hormonal and endometrial changes behind menstruation.

References

National Institute for Health and Care Excellence (NICE) (2017, updated 2024). Endometriosis: diagnosis and management (NG73). www.nice.org.uk/guidance/ng73. NICE. Endometriosis: diagnosis and referral recommendations, including imaging and normal ultrasound results. www.nice.org.uk/guidance/ng73/chapter/Recommendations.

Becker, C.M. et al. (2022). ESHRE guideline: endometriosis. Human Reproduction Open. academic.oup.com/hropen/article/2022/2/hoac009/6537540. NHS. Endometriosis: symptoms, tests and treatment. www.nhs.uk/conditions/endometriosis/.

NICE (2025). Relugolix–estradiol–norethisterone for treating endometriosis symptoms (TA1057). www.nice.org.uk/guidance/ta1057. NICE (2025). Linzagolix with hormonal add-back therapy for endometriosis symptoms (TA1067). www.nice.org.uk/guidance/ta1067. Endometriosis UK. Information and support for people living with endometriosis. www.endometriosis-uk.org/.