Health A-Z

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Harvard Medical School

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What Is It?

Endometrial tissue lines the inside of the uterus. In endometriosis, the same type of tissue also grows in places outside of the uterus.

Implants or patches of endometriosis may develop in the:

  • Ovaries

  • Outside surface of the uterus

  • Pelvis and lower abdomen

  • Fallopian tubes

  • Spaces between the bladder, uterus and rectum

  • Wall of the rectum, bladder, intestines or appendix (less commonly)

  • Lung, arm, thigh and skin. (This is rare.)

Misplaced endometrial tissue behaves like endometrial tissue in the uterus. It responds to the monthly rise and fall of female hormones. It also can ooze blood during menstruation. This can cause pelvic or abdominal pain.

If the misplaced endometrial tissue enlarges to cover or grow into the ovaries, or if it blocks the fallopian tubes, it can it can interfere with a woman's fertility. .

Endometrial tissue on the ovaries may form into large fluid-filled cysts. These are called endometriomas.

A woman may have an increased risk of endometriosis if:

  • She has a heavy menstrual flow.

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

  • She has a close female relative with endometriosis.

A woman's risk is probably lower than average if:

  • She is slightly underweight.

  • She exercises regularly.

  • She has had multiple pregnancies.

  • She has used oral contraceptives.

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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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