Understanding Endometriosis -- the Basics

Reviewed by Traci C. Johnson, MD on March 30, 2019

What Is Endometriosis?

Endometriosis is a common condition in women. It's chronic, painful, and often gets steadily worse.

Normally, the tissue that lines a woman's uterus, known as the endometrium, is found only in the uterus. But when a woman develops endometriosis, microscopic bits of this tissue escape from the uterus and grow on other organs such as the ovaries, the outer wall of the uterus, the fallopian tubes, the ligaments that support the uterus, the space between the uterus and the rectum, and the space between the uterus and the bladder. In rare cases, they can spread outside the abdomen and grow on other organs, such as the lungs.

Just like the endometrium, the escaped tissue responds to the hormones estrogen and progesterone by thickening, and it may bleed every month. But because the escaped tissue is growing in other tissues, the blood it makes cannot escape. This causes irritation to the surrounding tissue, which causes cysts, scars, and the fusing of body tissues. This can eventually bind the reproductive organs together and lead to infertility.


Cases of endometriosis are classified as minimal, mild, moderate, or severe, depending on the size of the lesions and how deeply they reach into the other organs. They are also referred to as stage I-IV.

Endometriosis affects 3%-10% of women of reproductive age, and 25%-50% of infertile women. It affects about 40%-80% of women suffering from pelvic pain. Most women are diagnosed in their 20s, and it affects all races equally. Symptoms usually get better after menopause.

What Causes Endometriosis?

Researchers don't know why or how endometrial tissue reaches other parts of the body. But there are some trends. Endometriosis tends to run in families. Endometriosis occurs more often in women who have short menstrual cycles or a longer-than-normal flow: Women who have fewer than 25 days between periods or who menstruate for more than seven days are twice as likely to develop endometriosis. And dioxin, an industrial chemical, may be a cause.

There seems to be no direct relationship between the size of lesions and the severity of pelvic pain. Some women with small lesions have terrible pain, while others with large lesions have no symptoms. Pain probably comes from the scarring and irritation caused by bleeding, or from endometrial tissue growing on a nerve.

How the disease causes infertility also is unclear. Some researchers think that the escaped endometrial tissue upsets the process of ovulation. The tissue may also block eggs from moving through in the fallopian tubes. Others think that the escaped tissue makes chemicals that interfere with fertilization. But more study is needed to provide answers.

WebMD Medical Reference



Eunice Kennedy Shriver National Institute of Child Health & Human Development: "Endometriosis."

Lebovic, D., Gordon, J., Taylor, R. Reproductive Endocrinology & Infertility, Scrubb Hill Press, 2005.

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