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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Endometriosis

What Is It?

The tissue that lines the inside of the uterus is called endometrial tissue. Endometriosis is a condition in which this tissue grows in places outside of the uterus. Endometriosis usually develops in the pelvis and abdomen in these locations: the ovaries, the outside surface of the uterus, the membranes and ligaments of the pelvis and lower abdomen, the fallopian tubes, and the spaces between the bladder, uterus and rectum. Less commonly, misplaced endometrial tissue grows in the wall of the rectum, bladder, intestines or appendix. Rarely, endometriosis develops in areas very far from the reproductive tract, such as the lung, arm, thigh and skin.

Misplaced endometrial tissue tends to behave like the normal lining of the uterus. It can respond to the normal rise and fall of female hormones during the month. It also can ooze blood at the time of menstruation, which can cause episodes of pelvic or abdominal pain. As misplaced endometrial tissue grows, it also can interfere with a woman's fertility by covering or growing into the ovaries or by distorting or blocking the fallopian tubes. Endometrial tissue on the ovaries may form into large fluid-filled cysts called endometriomas. Endometriomas sometimes are called chocolate cysts because they typically contain a thick, brownish mixture of blood and dead tissue that looks like chocolate.

Researchers have several different theories about the cause of endometriosis. According to one explanation, the condition develops when menstrual fluid flows backward and carries bits of uterine tissue upward through the fallopian tubes into the pelvis instead of flowing downward toward the vagina. This seems to be a reasonable explanation for areas of endometrial tissue found around the uterus or ovaries.

However, this does not explain why endometriosis sometimes is found in distant sites such as the lung or skin. In these cases, endometrial cells may move through the bloodstream or lymph channels. Another theory is that certain special types of cells found throughout the body may transform themselves into endometrial cells and then join together to form large deposits of endometrial tissue.

Whatever the cause of endometriosis, it is thought that faulty immune defenses play some role in its development. The immune system apparently fails to identify and destroy endometrial tissue growing outside of the uterus where it does not belong, and may even promote the growth and proliferation of this tissue. Research is being done to understand the relationship between the immune system and endometriosis.

In the United States, endometriosis affects an estimated 10% to 20% of women of childbearing age. The average person with endometriosis is diagnosed at age 27, although she may have had symptoms for two to five years before the diagnosis is confirmed.

In general, a woman probably has a higher-than-average risk of endometriosis if any of the following is true:

• She has a heavy menstrual flow.

• She has a short menstrual cycle (27 days or less).

• She has a close female relative (mother, sister, daughter) with endometriosis.

A woman's risk is probably lower than average if any of the following is true:

• She is slightly underweight.

• She exercises regularly.

• She has had multiple pregnancies.

• She has used oral contraceptives (birth control pills).

From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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