Endometriosis is small deposits of the womb lining that are located outside of the womb cavity.
Each time that you have a normal period, so this endometriosis does also, and this leads to cyclical swelling, stretching of tissues, inflammation and scarring. Eventually all the scarring and inflammation can lead to symptoms even when you're not having a period. It is more common in women whose relatives have endometriosis, in women who have cycles shorter than 28 days and those who typically have a period lasting longer than a week. Many cases occur in women without these associations, of course, and not all women who fit into the above categories necessarily get endometriosis. There are several theories behind this, one possible cause is called retrograde menstruation .