Endometriosis is a common condition, although it is difficult to determine exactly how commonly women are affected. Between 1% and 15% of pre-menopausal women are thought to be affected although the exact figure is unknown because surgery is needed to make a diagnosis.
Symptoms typically begin between the ages of 25 and 40 years. The severity of symptoms and the likelihood of diagnosis increase with age, peaking at around 40 years. Endometriosis is rare after the menopause as it depends on the hormone oestrogen, but it can reoccur in women who use hormone replacement therapy (HRT).
Risk factors for endometriosis
Any woman can have endometriosis, but there are a number of factors which can increase the risk for an individual woman.
Endometriosis sometimes runs in families and it is more common in close relatives of affected women. The disease almost certainly therefore has a genetic basis.
Endometriosis is dependent on the female hormone oestrogen which explains why it is rarely found before periods start or after the menopause when oestrogen levels are low. Early first period (menarche) and late menopause are associated with increased risk for endometriosis. The oral contraceptive pill reduces the risk of developing endometriosis and this protective effect may last for up to 12 months after stopping the pill. The risk is also decreased in women who have previously been pregnant.
Endometriosis is present in 40-60% of women who have painful periods. Women with short menstrual cycles and those who have prolonged periods are also more likely to have endometriosis.
Endometriosis is found in 20-30% of women who have difficulty or delay in becoming pregnant.