Endometrium is the tissue the lines the inside of the uterus. Endometriosis is the growth of endometrium outside of the uterus. The terms "implants," "nodules," or "lesions" may be used to describe areas or patches of endometriosis.

Most endometriosis patches are found in the pelvic area, in the following locations:

  • On or under the ovaries
  • On the fallopian tubes, which carry egg cells from the ovaries to the uterus
  • Behind the uterus
  • On the tissues that hold the uterus in place
  • On the bowels or bladder

In rare cases, areas of endometriosis can grow on the lungs or in other parts of the body.

Endometriosis may cause pain. Contrary to earlier thinking, the size and location of the patches does not appear to be related to the severity of pain or to the location of the pain.


The exact cause of endometriosis is not known, but researchers have some theories.

It may result from something called "retrograde menstrual flow," in which some of the tissue that a woman sheds during her period flows through her fallopian tubes into her pelvis. While most women have some retrograde menstrual flow during their periods, not all of these women have endometriosis. Researchers are trying to uncover what other factors might cause the tissue to attach and grow in some women, but not in others.

Researchers believe that endometriosis likely results from a combination of factors, including (but not limited to) some of the following:

  • Because endometriosis runs in families, genes are probably involved with endometriosis to some degree.
  • Estrogen (a hormone involved in the female reproductive cycle) also likely contributes to endometriosis, because endometriosis is an estrogen-dependent, inflammatory disease.
  • In endometriosis, the endometrium may not respond as it should to progesterone, another hormone involved in the female reproductive cycle. This means that the endometrium has "progesterone resistance."
  • In some cases of endometriosis, the immune system fails to destroy endometrial tissue, which enables it to grow outside the uterus. This means immune system dysfunction plays a role in these cases.
  • Environmental exposures in the womb, such as to chemicals like dioxin, have also been linked to developing endometriosis.

Risk Factors

How many people are affected by or at risk for endometriosis?

Endometriosis is most common in women in their 30s and 40s, but it can affect any female who menstruates.

Because some women might have endometriosis, but do not have symptoms, it is difficult to know exactly how many women have the condition. Current estimates suggest that 6% to 10% of women of reproductive age have endometriosis.

In a 2011 study, 11% of a group of women with no symptoms of endometriosis actually had the disorder.

Factors that May Increase the Risk of Endometriosis

Studies show that women are at higher risk for endometriosis if their:

  • Mother, sister, or daughter had endometriosis (raises the risk about sixfold)
  • Periods started at an early age (before age 11)
  • Monthly cycles are short (less than 27 days)
  • Menstrual cycles are heavy and last more than 7 days

Factors that May Lower the Risk of Endometriosis

Studies also show that some factors may lower the risk for endometriosis, including:

  • Pregnancy
  • Starting menstruation late in adolescence
  • Regular exercise of more than 4 hours a week
  • Low amount of body fat

FAQs about Endometriosis

If I have endometriosis, will I be able to get pregnant?

About 30% to 40% of women with endometriosis cannot get pregnant on their own, making endometriosis one of the top three causes of female infertility.

But exactly how endometriosis causes infertility is not clear. Some evidence suggests that infertility is related to the extent of the endometriosis patches, because the patches can distort the pelvic anatomy. This would make it difficult for sperm to travel to the ovary or a fertilized egg to travel to the uterus. Other evidence suggests that the inflammation in the abdomen may disrupt ovulation or fertilization, or that the endometrium may not develop properly, hampering the attachment of the embryo to the uterus.

There are treatments for endometriosis-related infertility that may help women get pregnant even with endometriosis.

Is endometriosis the same as endometrial cancer?

Endometriosis and endometrial cancer are not the same. The word "endometrium" describes the tissue that lines the inside of the uterus. Endometrial cancer is a type of cancer that affects the lining of the inside of the uterus. Endometriosis itself is not a form of cancer.

Can endometriosis lead to cancer?

There is a slight increase in the risk of ovarian cancer among women with endometriosis, particularly among women who were diagnosed with the condition at an early age. However, it is unclear whether endometriosis causes ovarian cancer or if the two conditions share risk factors or disease mechanisms that make them more likely to occur together.

In some cases, women with endometriosis also have breast cancer or non-Hodgkin's lymphoma. However, these situations are rare.

Does endometriosis ever go away?

For about one-quarter of women diagnosed with endometriosis, endometriosis patches go away on their own.

Also, after menopause, symptoms of endometriosis typically lessen because there is a drop in the woman's natural hormones and the growths gradually shrink. However, this is not true for all women. If a woman takes hormones for menopausal symptoms, both her pain symptoms and the growths may return.

Women with endometriosis who are experiencing symptoms, especially after menopause, should talk with their health care providers about treatment options.

Reference: National Institutes of Child Health and Human Development

Last updated April 20, 2017


This information is for general educational uses only. It may not apply to you and your personal medical needs. This information should not be used in place of a visit, call, consultation with or the advice of your physician or health care professional.

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