Normally the inner lining of the uterus (endometrium) sheds out through the cervix every month. We call this phenomenon - menstruation. Sometimes the endometrial cells, instead of coming out from the cervix go in the pelvis through the fallopian tubes or in the myometrium through the blood vessels.
Here the endometrial cells get lodged and start growing like the endometrium in response to ovarian hormones. Growth of endometrium is completely dependent on estrogen and progesterone (ovarian hormones).
This ectopic endometrium also sheds and bleeds with every menstrual cycle, causing severe pain during menstruation.
With repeated menstruation adhesions are formed. Blood filled cysts in the ovary are called chocolate cysts. Adhesions behind the uterus cause pain during intercourse.
When endometrium gets lodged in uterine muscles, uterus gets enlarged, this condition is called adenomyosis. Women with adenomyosis complaint of heavy bleeding and severe pain during menses.
Due to adhesions, women with endometriosis have a distorted tubo-ovarian relation and often fail to conceive. The disease becomes silent when ovarian hormones are suppressed. (Pregnancy or medicines) depending on the extent of endometriosis in the pelvis, the disease is staged from 1 to 4.
In the earlier stages laparoscopic adhesiolysis and burning of endometriosis deposits, allows conception in 40% patients. In advanced stages of the disease IVF is preferable (with ovarian endometrial blood filled cysts). Ovarian follicles are destroyed hence number of eggs in a women decreases with repeated surgical removal of endometrium as there is a risk of ovarian failure, hence should be avoided. IVF is the best way to conceive with advanced endometriosis.