The endometrium is the specialized lining of the uterus which undergoes dramatic changes in thickness and glandular activity during the menstrual cycle as it prepares itself to receive the embryo. This development is synchronized with the ovarian activity so that in the perfect situation follicular developmentand endometrial growth are synchronised ready for the arrival of the embryo after fertilization and early cleavage stages. If at the end of the cycle there is no pregnancy menstruation occur and the lining is lost - hence the bleeding.
Usually these specialized endometrial cells are confined to the uterus.
Endometriosis is a situation where the endometrial cells are present at other sites in the body. It is a common situation which can result from 'retrograde' menstruation through the fallopian tubes. In other words instead of the products of menstruation passing through the cervix and vagina they pass in the opposite direction, through the fallopian tubes, and into the peritoneal cavity covering organs in the pelvic region- e.g. ovaries, bowel, bladder. Many women are unaware that they 'suffer' from endometriosis and it is only when some investigative procedure such as laparoscopy is carried out that is noticed. However the endometrial cells can also be spread through the blood vessels and lymphatics to reach more distant location - the lungs. These 'external' sites of endometrial tissue also respond to cyclical changes - they increase in size and bleeding occurs, but because there is no escape route for the blood there is frequently pain particularly in the pelvic area.
There is much debate as to whether infertility potentiates endometriosis or whether endometriosis causes the infertility. It is known that women with endometriosis do have lower conception rates. Adhesions around the ovary and fallopian tubes do interfere with the pick-up of eggs and transport of the eggs.
IVF has been shown to be an effective form of fertility treatment in
cases of endometriosis, particularly when other forms of treatment have failed
(drugs, surgery, laser vaporization etc.).