The endometrium is the tissue that lines the womb. During the menstrual cycle this gets thicker, ready to receive a fertilised egg. If pregnancy doesn't occur, the lining is shed as a 'period' and a new layer starts to grow as part of the next monthly cycle.

In endometriosis, endometrial cells are found outside their normal location: on the ovaries, for example, or on the outside of the womb. Very occasionally, they're found as far away as the lungs.

Monthly hormonal cycles make these cells grow and break down as they would in the womb. However, this internal bleeding can't leave the body, which leads to pain and internal scarring.

When endometrial tissue is found in the ovary, it can form painful 'chocolate' cysts.

The pain of endometriosis tends to worsen over time

Endometriosis can cause pain from a young age, and severe period pain should always be investigated. The pain of endometriosis tends to worsen over time, as more and more damage is done with each menstrual cycle.

If you've had severe period pains since around the first time your periods started, endometriosis may be the reason.

The pattern of pain in endometriosis tends to differ slightly from normal period pain, building up before the bleeding really gets going and carrying on throughout the period as a more diffuse pain in the lower abdomen.

Another symptom that can occur is pain during sex at any time in the monthly cycle.

There's also an increase risk of infertility, as the scarring can interfere with normal reproductive function.

Endometriosis can occur at any time, from the onset of menstrual periods to the menopause.

In earlier times, many women spent most of their 20s and 30s either pregnant or breastfeeding, so they had few periods. In modern times, a woman experiences about 450 periods, so endometriosis has the chance to wreak much more damage than before.

For the majority of women, the condition ceases at the menopause.

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