We don't really know what causes endometriosis, and possible causes or factors may be different from person to person.
Family history: women who have a close relative with the condition are up to 7-10 times more likely to get endometriosis. Also, it is common with twins that both may get endometriosis, particularly if they are identical twins.
Retrograde (backwards) menstruation: When a woman has a period, the blood flows out of the vagina, but also backwards along the fallopian tubes into the pelvis. In 90% of women, the blood, which will contain endometrial cells is absorbed or broken down and causes no symptoms; however, in women with endometriosis this endometrial tissue starts to grow.
Metaplasia: the conversion of the normal pelvic tissue into endometriosis.
Other possible factors that may have a role in causing endometriosis are:
having first pregnancy at an older age
heavy bleeding during periods and periods lasting longer than five days
first period before 11 years of age
regularly having less than 27 days between periods, or having shorter regular cycles
changes in the immune cells
low body weight
alcohol use
