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Endometriosis is often a “silent” condition affecting 35–50% of women with infertility. It has a significant impact on natural and assisted conception rates and probably reduces the chances of pregnancy by half.

Millions of women are affected by the condition, where cells that usually form the “functional layer” of the womb (uterus and Fallopian tubes) are growing outside the womb in the pelvic cavity. Because the womb lining has different types of cells, endometriosis varies depending on which of the three main cell types are involved:

  • Blood cells.
  • Mucous cells.
  • Connective tissue.

Together these cells create a soft, sticky and spongy surface designed to enable implantation. Changes in sex hormones still alter their function and behaviour across the month, which means that endometriosis tissue can:

  1. Grow in the follicular phase.
  2. Release mucus or form adhesions in the luteal phase.
  3. Bleed with the menstrual bleed.

Endometriosis affects roughly 6–10% of women and is typically diagnosed during childbearing years. However, it’s also been found in girls before their menarche, and 3% of cases involve post-menopausal women. Endometriosis is:

  • Found in 35–50% of infertile women with chronic pelvic pain. i
  • Present in up to 40% of women who’ve had a hysterectomy. ii
  • More likely when there’s a combination of short menstrual cycles and long periods.
  • Unaffected by race or ethnicity.
  • Possible for men who are taking high-dose estrogen therapy. iii

Types of endometriosis

There are three types, which all form “lesions”:

  1. Superficial peritoneal lesions which vary according to the tissue types:
  • Brown or pigmented lesions that look like dark spots (from trapped blood).
  • Pale pink or white lesions on the surface of organs that secrete fluids (they can form fluid-filled blisters).
  • Connective tissue creates a web and can tie organs to each other.


2. Ovarian endometriotic cysts (chocolate cysts) when endometrial tissue forms a cyst in the ovary:

chocolate cysts

3. Deeply infiltrating endometriotic lesions (DIE)
DIE is a highly unpleasant and distinct category that usually causes severe pelvic pain that penetrates over 5mm under the peritoneum and other organ tissues. iv v

Stages of endometriosis

There are endometriosis “Stages” as well the different “types”:

Stage I: Lesions are confined to the “Pouch of Douglas” between the back wall of the uterus and the rectum.
Stage II: Lesions invade the cervix, penetrate the vaginal wall, and cause fibrosis and small cysts.
Stage III: Lesions spread into the sacro-uterine ligaments and around the rectum.
Stage IV: The lesions extend until they involve the rectal wall, recto-sigmoid zone and the peritoneum, with the “Pouch of Douglas” filled with lesions.


endometriosis is more likely to be found close to the uterus:

  • On and in the ovaries (this is the most common site)
  • In or around the Fallopian tubes
  • At the front of the uterus or on the ligaments that attach the uterus
  • In the “Pouch of Douglas”
  • On the intestines; usually, the recto-sigmoid (in 10% of cases), which can cause severe pain with bowel movements
  • On the bladder and the ureters that lead from the kidneys
  • On the cervix and vagina or sites of abdominal surgery

Occasionally endometriosis is found outside the pelvic cavity:

  • On the skin where it causes cutaneous endometriosis”.
  • On the diaphragm, which can cause pain at the right shoulder just before and during the period.


Although many women get symptoms, they’re not a good indicator of the extent of their endometriosis. vi Recurrent pain in the lower abdomen is the main symptom, which tends to be caused by:

    1. Cells bleeding during the period. The blood accumulates, causing swelling and triggering inflammatory responses.
    2. Adhesions are binding internal organs and structures to each other, with pain coming from movement between attachment sites.

The symptoms are generally: vii

  • Increased pain and bloating around the period.
  • Pelvic pain outside of the period.
  • Period pain that’s throbbing, gnawing and dragging down the legs.
  • Sex that’s painful or difficult (dyspareunia).
  • Painful bowel movements that can contain blood (DIE can cause shooting rectal pain or a down pulling sensation).
  • Infertility.
  • Painful and frequent urination (or blood in urine during the period).


  • Symptoms aren’t reliable, but abnormal abdominal pain or bloating raises the possibility of endometriosis.
  • Ultrasound can identify abnormal structures with ‘3D’ or ‘4D’ techniques.
  • Laparoscopy is the only definitive way to diagnose endometriosis.
  • Uterine biopsies can indicate a 98% chance of endometriosis when abnormally high nerve fibres are in the womb lining.