The endometrium is the inner lining of the uterus, which thickens throughout the menstrual cycle in preparation for a possible pregnancy and is shed as menstrual bleeding if pregnancy does not occur. Endometriosis is a condition in which endometrial-like tissue grows outside the uterus — most often on the ovaries, fallopian tubes, and the tissue lining the pelvis, though occasionally on the bowel, bladder, or, rarely, further afield.
This misplaced tissue behaves much like the normal endometrium: it thickens, breaks down, and bleeds in response to the menstrual cycle. Unlike normal menstrual tissue, however, it has no way of leaving the body. Over time, this trapped tissue can trigger chronic inflammation, lead to the formation of cysts on the ovaries (endometriomas), and cause scar tissue and adhesions that bind pelvic organs together — changes that underlie the pain and, in some cases, the fertility difficulties associated with the condition.
How Endometriosis Develops
The exact cause of endometriosis is not fully understood, but several mechanisms are thought to contribute:
- Retrograde menstruation: The leading theory behind endometriosis, in which menstrual blood flows backward through the fallopian tubes into the pelvic cavity instead of leaving the body. This blood carries endometrial cells, which may implant and grow on pelvic surfaces.
- Cellular transformation: Cells outside the uterus may transform into endometrial-like cells under certain hormonal or immune influences.
- Surgical scar implantation: Endometrial cells can occasionally attach to a surgical scar, such as one from a caesarean section, and grow there.
- Immune system factors: In a healthy pelvis, the immune system may clear stray endometrial cells before they can implant; an altered immune response may allow this tissue to establish and persist instead.
Endometriosis is commonly staged from I to IV based on the location, extent, and depth of the tissue found, usually at the time of surgery. Importantly, the stage does not reliably predict how severe a person's symptoms will be — some patients with minimal disease have significant pain, while others with extensive disease have few symptoms.
- Stage 1 (Minimal): A few small, superficial implants, typically without significant scar tissue.
- Stage II (Mild): More implants, still relatively superficial, with mild adhesions possible.
- Stage III (Moderate): Multiple implants, some deeper, often with small ovarian cysts (endometriomas) and more noticeable adhesions.
- Stage IV (Severe): Extensive, deep implants, larger ovarian cysts, and dense adhesions that may bind pelvic organs together, sometimes involving the bowel or bladder.
Symptoms of Endometriosis
The most common symptom is pelvic pain, which may be mild or severe and often worsens just before or during menstruation, though the pattern and severity vary widely between individuals and, notably, do not always correspond to how extensive the disease is.
Other symptoms of endometriosis include:
- Painful menstrual cramps, often more severe than typical period pain
- Abdominal or back pain during menstruation
- Heavy menstrual bleeding
- Spotting or bleeding between periods
- Pain during or after sex
- Pain when urinating or passing a bowel motion, particularly during menstruation
- Digestive symptoms such as bloating, diarrhoea, or constipation
- Difficulty getting pregnant
- Fatigue
Sometimes patients may have no symptoms and may not know they have it until they are unable to get pregnant.
Endometriosis and Fertility
Endometriosis is one of the most common findings in women being investigated for infertility, and understanding this connection is important both for family planning and for making sense of a diagnosis. Endometriosis can affect fertility through several mechanisms: distorted pelvic anatomy from scar tissue and adhesions can interfere with the normal movement of an egg from the ovary to the uterus; ovarian endometriomas can reduce the number and quality of eggs in the affected ovary; and chronic pelvic inflammation may affect the receptiveness of the uterine lining and the function of the fallopian tubes.
It's important to know that not everyone with endometriosis experiences fertility difficulties, and the relationship between disease stage and fertility impact is not always straightforward — some women with minimal disease have difficulty conceiving, while others with more extensive disease conceive without difficulty. For those trying to conceive, treatment options range from surgical removal of endometriosis tissue to improve the chances of natural conception, through to assisted reproductive technologies such as IVF, and the right approach depends on age, disease severity, and how long a couple has been trying to conceive — a discussion best had directly with a specialist familiar with both endometriosis and fertility.
Risk Factors for Endometriosis
Some factors that raise the risk of endometriosis include:
- Never having given birth
- Short menstrual cycles (fewer than 27 days between periods)
- Long or heavy menstrual periods (longer than eight days)
- A family history of endometriosis, particularly in a mother, sister, or aunt
- Starting menstruation at an early age
- Low body mass index
- Conditions that obstruct the normal outflow of menstrual blood
How Endometriosis is Diagnosed?
- Symptom History and Pelvic Examination: A detailed discussion of symptoms and menstrual history, together with a physical examination, performed with one or two gloved fingers, to check for tenderness, cysts, nodules, or irregular growths on the reproductive organs.
- Transvaginal Ultrasound: Often the first imaging test used, providing a detailed view of the uterus and ovaries and able to detect ovarian endometriomas, though it cannot reliably identify smaller or more superficial implants.
- MRI: Provides more detailed imaging of the pelvis, helpful for assessing the size and location of endometriosis, particularly deeper disease, and for surgical planning.
- Laparoscopy: A minimally invasive surgical procedure that remains the definitive way to confirm endometriosis, allowing direct visualisation and, where appropriate, biopsy or removal of endometriosis tissue in the same procedure. Increasingly, treatment is started based on symptoms and imaging findings alone, without waiting for surgical confirmation, particularly when the clinical picture is fairly clear.
Living with Endometriosis
- Track your symptoms and their relationship to your menstrual cycle — this information is genuinely useful for your care team in tailoring treatment
- Don't dismiss period pain that is significantly affecting your daily life as something to simply endure — persistent, severe menstrual pain is not something you have to accept as normal
- Discuss family planning timelines openly with your specialist, since this shapes which treatment approach makes the most sense for you
- Heat therapy, gentle exercise, and anti-inflammatory dietary choices may help some people manage day-to-day symptoms alongside medical treatment
- Seek support from family and friends. Living with chronic pelvic pain can affect mental wellbeing and relationships, and this is a valid part of the condition to address, not a separate issue
