What is endometriosis?
Endometriosis is a condition in which tissue similar to the lining of the womb — the endometrium (the layer of tissue that normally lines the inside of the womb and sheds during a period) — grows in places outside the womb. It most commonly develops on the ovaries, the fallopian tubes (the tubes that carry eggs from the ovaries to the womb), the peritoneum (the thin lining that covers the inside of the abdomen and pelvis), and the tissue between the uterus and the rectum. In rarer cases, deposits can form on the bowel, bladder, the uterosacral ligaments (bands of fibrous tissue that anchor the womb), and — very occasionally — in sites beyond the pelvis entirely.[1]
Each month, under the influence of the hormone oestrogen, this misplaced tissue behaves in much the same way as the normal womb lining: it thickens, then breaks down and bleeds. Unlike the lining of the womb, however, this blood has nowhere to go. Over time, it causes local inflammation, the gradual formation of adhesions (bands of scar tissue that can cause organs and structures to stick together when they normally would not), and sometimes the development of endometriomas (ovarian cysts — sometimes called "chocolate cysts" — that fill with old blood and endometrial tissue).[1]
The condition is considered oestrogen-dependent (driven by the hormone oestrogen), which is why it predominantly affects women during their reproductive years — from puberty through to the menopause — and why reducing oestrogen levels, whether through hormonal treatment or after the natural menopause, tends to reduce activity in the deposits.[5]
The exact cause is not fully understood. The most widely discussed theory is retrograde menstruation (a theory that menstrual blood flows backwards through the fallopian tubes into the pelvis, depositing endometrial cells outside the womb), but this does not account for all cases. Genetic factors, immune system abnormalities, and environmental influences are all thought to play a role. What is known is that endometriosis is not caused by lifestyle choices, and it is not contagious.[5]
Endometriosis is classified into four stages — from Stage I (minimal) through to Stage IV (severe) — based on the location, extent, and depth of deposits. A particularly severe form is deep infiltrating endometriosis (a more advanced pattern in which endometrial-like tissue grows more than 5mm deep into surrounding structures, such as the bowel, bladder, or ligaments). Staging is a clinical tool; it does not reliably predict symptom severity.[1]
Endometriosis affects around 1 in 10 women of reproductive age — an estimated 1.5 million women in the UK alone.[1] Despite this, it remains one of the most frequently under-recognised conditions in primary care.
Key Terms
- Endometrium — the tissue that normally lines the inside of the womb. Each month it thickens in preparation for a potential pregnancy, then sheds during a period.
- Endometriosis — a condition in which tissue similar to the endometrium grows outside the womb — most commonly in the pelvis. It responds to hormonal changes in the same way as normal womb lining, causing monthly bleeding with nowhere to go.
- Peritoneum — the thin sheet of tissue lining the inside of the abdominal and pelvic cavity and covering most of the organs within it.
- Adhesions — bands of scar tissue that form as a result of chronic inflammation, causing organs and structures to stick together in ways they normally would not.
- Endometrioma — a type of ovarian cyst that forms when endometrial tissue grows inside the ovary and fills with old blood. Often called a "chocolate cyst" because of its dark appearance.
- Dysmenorrhoea — painful periods. In endometriosis, this pain is often severe — well beyond typical menstrual cramping — and may persist throughout the cycle.
- Dyspareunia — pain during or after sexual intercourse. A recognised symptom of endometriosis, particularly in deep infiltrating disease.
- Laparoscopy — a keyhole surgical procedure performed under general anaesthetic, in which a thin camera is inserted through a small incision to view the pelvis and, where needed, treat endometriosis deposits directly.
- Transvaginal ultrasound — an ultrasound scan using a small probe gently placed inside the vagina. Provides a clearer image of the womb and ovaries than a standard external abdominal scan. Now recommended as a first investigation step by NICE (2024).
- Retrograde menstruation — the most widely discussed theory of how endometriosis develops, proposing that menstrual blood flows backwards through the fallopian tubes into the pelvis, depositing endometrial cells in places they do not belong.
- Deep infiltrating endometriosis — a more advanced form in which endometrial-like tissue grows more than 5mm into surrounding structures such as the bowel, bladder, or the ligaments that anchor the womb.
- GnRH analogues — a class of medication (for example, goserelin or leuprorelin) that temporarily suppresses oestrogen production by the ovaries, placing the body in a short-term menopause-like state to reduce endometriosis activity.
- Progestins — a group of hormonal medications — including the progestogen-only pill, injections, implant, and the Mirena intrauterine system (a small T-shaped device placed in the womb that releases progestogen locally) — used to suppress endometrial tissue growth and manage pain.
- Oestrogen-dependent — describes a condition driven or worsened by the hormone oestrogen. Endometriosis tends to improve when oestrogen levels fall — whether through hormonal treatment, pregnancy, or after the natural menopause.
Why does it matter?
Endometriosis matters because it is common, chronic, and — for far too many people — unrecognised for years. The pain it causes can be relentless: severe period pain (dysmenorrhoea), pelvic pain throughout the month, pain during intercourse (dyspareunia), pain on opening the bowel, pain on passing urine. In some cases, every day is shaped around managing pain that has never been properly explained or treated.
One of the defining features of endometriosis is the diagnostic delay. The average wait from first experiencing symptoms to receiving a diagnosis is between seven and nine years globally.[2] That is not a rare outlier — it is the pattern. During those years, symptoms are often normalised ("it's just bad periods"), investigation is delayed, and the condition may progress. Understanding that severe period pain and related symptoms warrant investigation is one of the most clinically important things this article can offer.
Endometriosis also affects fertility — not inevitably, and not for everyone, but in a meaningful proportion of cases. It is present in a significantly higher proportion of those who experience difficulty conceiving: estimates across peer-reviewed studies place the figure at 25 to 40% of women attending fertility clinics.[6] The mechanisms are not fully understood, but may involve disrupted pelvic anatomy, altered hormonal environment, reduced ovarian reserve (the number of healthy eggs remaining), or impaired implantation. Importantly, many people with endometriosis do conceive naturally, and treatment options exist for those who do not.
Beyond the physical, the mental health impact is considerable. Living with chronic, frequently unrecognised pain — and navigating a prolonged, often invalidating diagnostic journey — is associated with higher rates of anxiety and depression. The condition affects education, employment, relationships, and quality of life in ways that go well beyond the monthly cycle.
👆 Tap any card to reveal what the evidence shows
Continue reading — members only
The rest of this article — what your doctor might do, what the research shows, and the putting it all together summary — is available to helf.school members.
Explorer membership is free — Section 1 always open.
Quick Visual Summary
A slide-by-slide visual overview of this article. Use the ← → arrow keys on your keyboard, or tap the dots at the bottom of the panel to move between slides.
What your doctor might do
Recognising the symptoms
NICE NG73 (updated November 2024) identifies the key symptoms that should prompt clinical assessment. These include:[1]
- Chronic pelvic pain — pain lasting more than six months, in and around the pelvis, not limited to the period itself
- Dysmenorrhoea (painful periods) — pain severe enough to interfere with normal daily activities; not simply "bad cramping"
- Dyspareunia (pain during or after sex) — particularly deep pain, which may be associated with deposits in the area between the uterus and rectum
- Cyclical bowel or bladder symptoms — including pain on opening the bowel (dyschezia — pain specifically related to passing stools), or pain on passing urine, that arise or worsen around the time of menstruation
- Difficulty getting pregnant — endometriosis is a recognised cause of subfertility and should be considered in the assessment of fertility difficulties
- A palpable pelvic mass — a mass felt on examination, which may represent an endometrioma (ovarian cyst) or other endometriosis-related structure
An important change introduced in the 2024 update to NICE NG73 is the firm statement that a normal ultrasound scan does not rule out endometriosis. Endometriosis deposits — particularly superficial peritoneal (affecting only the surface of the pelvic lining) disease — may not be visible on standard imaging. A normal scan should not be used to dismiss symptoms.[1]
Investigation
The 2024 update to NICE NG73 made a significant practical change to where investigation begins. Previously, transvaginal ultrasound (a scan using a small probe placed inside the vagina — giving clearer images of the womb and ovaries than an external scan) was primarily arranged in secondary care, after referral. The updated guideline now recommends that GPs offer transvaginal ultrasound to all people with suspected endometriosis — bringing earlier investigation into primary care.[1]
For suspected deep infiltrating endometriosis — where deposits are thought to involve the bowel, bladder, or deep pelvic structures — pelvic MRI (magnetic resonance imaging — a detailed scan using magnetic fields and radio waves, with no radiation) may be used alongside or instead of ultrasound. Definitive diagnosis has historically required laparoscopy (keyhole surgery), but the 2024 NICE update explicitly states that hormonal treatment can be offered on the basis of symptoms alone — the person does not always need to wait for a confirmed surgical diagnosis before treatment begins.[1]
Referral
The language of referral was also strengthened in the 2024 update. Where the previous guideline said "consider referring," the updated guideline says to refer — a firmer clinical instruction. Referral to a gynaecologist or specialist endometriosis centre is appropriate when:[1]
Medical treatment for pain
NICE NG73 recommends a stepwise approach to pain management.[1] NSAIDs (non-steroidal anti-inflammatory drugs — such as ibuprofen or naproxen) and paracetamol are tried first for pain relief. If these do not provide sufficient relief, hormonal treatment is the next step.
The three main hormonal options are:[1]
- The combined contraceptive pill — taken continuously (without the usual break) to suppress the monthly hormonal cycle and reduce endometrial activity
- Progestins — including the progestogen-only pill, injectable progestogens (given as an injection every few months), the hormonal implant, and the Mirena intrauterine system (a small T-shaped device placed inside the womb that releases progestogen locally)
- GnRH analogues — medication that temporarily suppresses oestrogen production by the ovaries, effectively putting the body into a temporary menopause-like state; usually given with "add-back" therapy (a small amount of oestrogen and progestogen to protect against bone thinning and manage menopausal symptoms)
Evidence from randomised controlled trials (studies in which participants are randomly assigned to different treatment groups — the most reliable way to test a treatment) confirms that these three approaches are broadly comparable in their effectiveness for pain control. The choice between them is guided by individual factors — including whether the person is trying to conceive, their tolerance of potential side effects, and any medical contraindications (reasons a particular treatment should not be used).[1]
Surgical treatment
Laparoscopic (keyhole) surgery may be offered when hormonal treatment has not provided adequate pain control, when diagnostic uncertainty remains, or when the assessment of fertility is a priority. Two techniques exist: excision (cutting out deposits) and ablation (burning or destroying them). Both are used; the evidence on which is better for pain is not yet conclusive, and NICE NG73 currently does not firmly favour one over the other for pain outcomes.[1]
For those where fertility is a priority, the April 2024 update to NICE NG73 introduced new guidance on surgical management specifically in this context — reflecting the growing evidence that surgery can improve conception chances in appropriately selected cases.[1]
What the research shows
Three areas attract the most research attention and the most questions from people affected by endometriosis: how to manage pain, what surgery can and cannot offer, and what the condition means for fertility. Here is what the evidence currently shows.
NICE NG73 (updated November 2024) recommends hormonal treatment as the primary medical approach for endometriosis-associated pain, following initial use of standard pain relief such as ibuprofen or paracetamol. The combined contraceptive pill, progestins (including the progestogen-only pill, the Mirena intrauterine system, and injectable progestogens), and GnRH analogues are all endorsed as appropriate first-line options.
Evidence from randomised controlled trials confirms that these three approaches are broadly comparable in their effectiveness for pain relief — there is no strong evidence that one consistently outperforms the others. The choice is therefore guided by individual circumstances: whether the person is trying to conceive, their tolerance of side effects, any contraindications, and personal preference. Hormonal treatment does not cure endometriosis, but is well-established as an effective long-term management strategy for pain — and can be started on the basis of symptoms alone, without waiting for a surgical diagnosis.[1]
A Cochrane Database systematic review (covering 14 randomised controlled trials and 1,563 women, with evidence to April 2020) assessed laparoscopic surgery for both pain and fertility in endometriosis. For women where fertility was a priority, operative laparoscopy — surgery that actively identifies and treats visible deposits, rather than simply looking — was associated with approximately 89% higher odds of achieving a viable pregnancy compared with diagnostic laparoscopy alone (which only examines without treating).[4]
For pain management, the evidence from the same Cochrane review is less definitive: it was uncertain whether laparoscopic surgery meaningfully reduces overall pain compared with diagnostic laparoscopy alone, particularly for minimal to moderate disease. NICE NG73 reflects this by recommending surgery for pain only when medical treatment has not provided adequate control. The April 2024 update to NICE NG73 introduced specific guidance on surgical management when fertility is a priority — recognising the growing body of evidence in this area.[1][4]
Endometriosis is present in a significantly higher proportion of those attending infertility clinics — with estimates from peer-reviewed research typically ranging from 25 to 40%.[6] This association reflects the multiple ways in which endometriosis can affect the reproductive system: disrupted pelvic anatomy caused by adhesions, altered hormonal environment, reduced ovarian reserve in those with endometriomas, or impaired implantation. The exact mechanisms are not fully understood.
Crucially, this does not mean endometriosis inevitably causes infertility. Many people with the condition conceive naturally, and the NICE NG73 April 2024 fertility update reflects a clearer evidence base for guiding those who do experience difficulties — including recommendations on the surgical management of endometriosis when fertility is a priority, and when assisted reproductive technologies such as IVF (in vitro fertilisation — fertilisation of an egg outside the body) may be appropriate to consider.[1]
📋 Putting it all together
Endometriosis is a condition that affects around 1 in 10 women of reproductive age — yet the average wait from first symptoms to diagnosis is between seven and nine years. That delay is not inevitable. It shortens when severe period pain, persistent pelvic pain, pain during sex, or cyclical bowel and bladder symptoms are recognised as signals that warrant investigation rather than normalisation.
The treatment picture has improved, and the 2024 update to NICE NG73 reflects that progress: earlier investigation in primary care, firmer referral language, and the explicit message that hormonal treatment can begin on the basis of symptoms alone — without waiting for surgical confirmation. Effective medical options exist. Surgical options exist for those where medical treatment is insufficient or where fertility is a priority. And for those who do experience fertility difficulties, the evidence base continues to grow.
There is no cure for endometriosis — but the tools to manage it, reduce its impact, and protect fertility are well-established. Understanding what they are, and knowing when to push for investigation, is where that process begins.
References
- NICE (2024). Endometriosis: diagnosis and management. NICE guideline NG73. Published 6 September 2017; last updated 11 November 2024. nice.org.uk/guidance/ng73
- The Lancet (2024). Endometriosis: addressing the roots of slow progress. The Lancet. October 2024. DOI: 10.1016/S0140-6736(24)02179-2
- Office for National Statistics (2024). Characteristics of women with an endometriosis diagnosis in England: 27 March 2011 to 31 December 2021. Released 10 December 2024. ons.gov.uk
- Cochrane Database of Systematic Reviews (2020). Laparoscopic surgery for pain and infertility associated with endometriosis. CD011031. Evidence to April 2020. cochrane.org
- NHS (2024). Endometriosis. nhs.uk/conditions/endometriosis
- Endometriosis and infertility: epidemiology and evidence-based treatments. PubMed. PMID: 18443335