This is the first of several planned blog posts, which will cover several aspects of endometriosis. This first blog post is a primer of sorts, outlining what ‘endo’ is, how it is staged, its likely cause, and risk factors. This will set the scene for the rest of the series, which will cover endometriosis’ symptoms and signs, and both medical and surgical management.

What is endometriosis?

Endometriosis is a common condition, in which deposits of tissue that normally line the inside of the uterus (called the ‘endometrium’) are found elsewhere in the body, where they don’t belong. These endometriotic deposits most commonly occur in and around a woman’s pelvic organs, such as the Fallopian tubes, the cervix and uterus, and on the pelvic ligaments.

These deposits (or ‘nodules’) of endometrial tissue respond to the usual hormonal fluctuations that occur during every menstrual cycle, just like the uterine lining normally does. As a result, endometriotic nodules enlarge, break down, and cause local inflammation during every period. As discussed below, this inflammation can cause severe period pain, and even bowel and / or urinary symptoms (if the nodule is growing near the bowel or bladder).

What causes endometriosis?

Scientists and doctors are still working out exactly what causes endometriosis. The most widely-held theory is that of so-called ‘retrograde menstruation’, which I’ll outline below.

Periods are caused by the lining of the uterus (the ‘endometrium’) getting thicker and thicker over the course of a woman’s menstrual cycle. Every month, this uterine lining sloughs off (rather like a snake’s skin) and causes vaginal bleeding – a period. ‘Retrograde menstruation’ posits that in some women, for whatever reason, some of the uterine lining goes ‘upstream’ in the wrong direction: up through the Fallopian tubes, and into the abdominal / pelvic cavity.

These endometrial cells may then implant onto the outside of pelvic structures, such as the: peritoneum (the clingwrap-like layer of cells that lines the inside of the abdominal and pelvic cavities, and covers the organs therein); ligaments supporting the pelvic organs; bowel; bladder; and ovaries. Such endometrial implants (or ‘endometriotic nodules’) can then get bigger over time, and cause symptoms.

As mentioned above, retrograde menstruation is the most widely-held theory regarding the cause of endometriosis. However, there must be more to it (at least in some women), as retrograde menstruation can’t explain the very rare occurrence of endometriotic deposits being found in and around a patient’s lungs – not a body cavity that has easy direct access to the uterus or Fallopian tubes! Hence, it’s thought that there are several different processes that lead to endometriosis, that may or may not all occur in one patient.

What are the risk factors for endometriosis?

While retrograde menstruation (see above) is thought to occur in up to 90% of women, most of these women don’t develop endometriotic nodules. So there must be additional factors involved, which may include:

  • Family history
  • Early puberty (eg. prior to 11yo)
  • Not having had children
  • Shorter menstrual cycles (≤ 27 days from the first day of one period to the first day of the next)
  • Heavy periods
  • Late menopause (eg. after 60yo)
  • Taller height
  • Lower body mass index

How common is endometriosis?

It’s hard to say exactly how common endometriosis is. Strictly speaking, endometriosis can only be definitively diagnosed at keyhole (or ‘laparoscopic’) surgery. So many girls and women may be said to have ‘possible endometriosis’ or ‘suspected endometriosis’; if their symptoms can be managed with medicines (such as pain relief and the contraceptive pill), there may not be any need to perform keyhole surgery to confirm the diagnosis.

Most sources say that up to 1 in 8 women of reproductive age (eg. between 12yo and 50yo) have symptomatic endometriosis. Up to 50% of women with subfertility are found to have endometriosis, as are up to 70% of women with persistent pelvic pain.

How is endometriosis staged?

A patient’s endometriosis is staged based on how much endometriosis is found at keyhole surgery. There are four stages of endometriosis, from minimal to severe:

  • Stage I = minimal endometriosis = a few shallow deposits, with minimal internal scarring
  • Stage II = mild endometriosis = endometriotic deposits that are more numerous and deeper than stage I; there may be some associated internal scarring
  • Stage III = moderate endometriosis = many deep nodules, significant internal scarring with / without endometriotic ovarian cysts
  • Stage IV = severe endometriosis = widespread deep nodules, with thick bands of internal scarring between pelvic organs, with / without large endometriotic ovarian cysts and bowel or bladder endometriosis

Are symptoms and staging always correlated?

Generally, there is a reasonably good correlation between the severity of a patient’s symptoms (eg. period pain) and the stage of their endometriosis (ie. women with severe endometriosis are more likely to have severe period pain than women with minimal endo). However, this is not always the case: sometimes a woman who doesn’t experience any period pain at all may be having keyhole surgery for another reason (eg. to have her appendix removed), and the surgeon happens to notice that she has severe endometriosis. More commonly, a woman with debilitating period pain has keyhole surgery, and no endometriosis is found.

Hence, there must be additional mechanisms at play, that magnify / diminish some women’s symptoms, in relation to their endometriosis. Suggested mechanisms include:

  • Increase in nerve fibres
  • Imbalance of sympathetic and sensory nerve fibres
  • Central sensitisation

What does endometriosis look like?

Endometriosis can have several different appearances at keyhole surgery, including:

  • Shallow clear bubbles or blisters lining the inside of a patient’s pelvic cavity (or ‘peritoneum’)
  • Small patches of light brown in amongst a patient’s pelvic organs
  • Dark brown / black raised nodules
  • Collections of chocolate-covered fluid on a patient’s ovaries (such endometriotic cysts are called ‘endometriomas’)
  • Internal scarring, which can lead to internal organs having a ‘puckered’ appearance, or being stuck to one another
  • Distorted pelvic anatomy (eg. the rectum being stuck up in an unusual position, to the back of the cervix)

It takes a lot of experience to be able to recognise these changes, some of which can be quite subtle. Such experience has been gained by Maven Centre’s Dr Sneha Parghi and Alison Bryant-Smith while completing advanced laparoscopic Fellowships.

Topic area frequently asked questions (FAQs)

Is there anything I can do to prevent endometriosis?

Given it’s not exactly clear what causes endometriosis, and why some women are more prone to develop than others, there is no known way to definitively prevent endometriosis. We do know that high estrogen levels are consistently associated with endometriosis, and stimulate its growth.

Hence, there are several things that you could consider doing, in order to decrease the risk of developing severe endometriosis. These may include:

  • Having fewer periods. You could manage this by taking back-to-back hormone pills of the contraceptive pill, or having a progesterone-containing intra-uterine device inserted.
  • Having lighter periods. This could be achieved by taking the contraceptive pill, or taking a medicine called tranexamic acid to make your periods lighter.
  • Some research suggests that limiting your alcohol and caffeine intake may limit endometriosis
  • Maintaining a healthy weight

Is endometriosis genetic?

There is no one distinct gene that causes endometriosis. However, we know that endometriosis does tend to run in some families: daughter of mothers with endometriosis are more likely to have endometriosis themselves (compared to other women). So, if you know that your mother (or other female relatives) had endometriosis, and you have suggestive symptoms (such as bad period pain), have a chat with your GP about your best next steps.

How do I know whether or not I have endometriosis?

A good quality pelvic (transvaginal) ultrasound will be able to see signs of moderate or severe endometriosis (such as endometriotic cysts on your ovaries, or your pelvic organs being stuck together by internal scarring). However, mild (shallow) endometriosis can only be diagnosed by undergoing keyhole surgery.

Some women are happy to be labelled as having ‘possible endometriosis’, if their symptoms can be managed with medications. Others prefer to undergo keyhole surgery so that they know for sure whether or not they have endometriosis. This is a very personal decision; please discuss the pros and cons with your gynaecologist.


In this endometriosis ‘primer’, we’ve outlined some basic background information regarding endometriosis. Understanding what endo is, what causes it, and its risk factors provides the basis for the rest of the posts in this endometriosis-themed blog series. Watch out for our next blog post, which will cover endometriosis’ symptoms and signs!

We look forward to collaborating with you to help you to be your best.