Endometriosis is a condition where the tissue that lines inside of the uterus actually tends to grow outside the uterus!.
The endometrial tissue can technically grow anywhere. This includes the birthing tubes, ovaries and other regions in and around the female reproductive system.
With endometriosis what basically happens is that, the tissue starts to thicken, break down and bleed just as a normal endometrium would inside the uterus with each menstrual period.