Endometriosis occurs when tissue of the uterine lining (endometrium) grows in areas outside the uterus. Endometrial tissue consists of glands, blood cells and connecting tissue called stromal cells.

During a woman’s monthly cycle, the endometrium responds to changing levels of the hormones estrogen and progesterone. The endometrium thickens to prepare for possible pregnancy. If a woman becomes pregnant, the endometrium provides a place for the fertilized egg to implant in the uterus and develop. If a woman does not become pregnant, the endometrium breaks down and is shed during the menstrual period.

In cases of endometriosis, endometrial tissue grows in other areas of the body. The tissue responds to the woman’s monthly hormonal changes, whether it is in the uterus or located elsewhere. The displaced tissue of endometriosis (called endometrial implants or lesions) grows and expands as it would in the uterus. But it cannot leave the body the way the uterine tissue does during menstruation. As a result, the implants can bleed into other areas or inflame other organs and form scar tissue or adhesions.

Endometrial implants usually occur in the pelvic region, including the:

• Ovaries
• Fallopian tubes
• Peritoneum (the lining of the pelvic cavity)
• Outside of the uterus and on its ligaments
• Lymph nodes
  

Many women with endometriosis have lesions in more than one location. Endometriosis sometimes occurs in other parts of the abdomen, such as the rectum and bladder. In rare cases, endometrial tissue is discovered in other body parts such as the arms, legs or lungs.

Endometrial growths begin microscopically small, often as clear lesions. With time, they become larger and darker and can take many shapes. The colors vary, ranging from red, brown or black to clear, white, yellow or pink. The prevailing color may depend on the blood supply, age of the lesion and other nearby tissue. Some endometrial lesions look like other scars or inflammations and may need an experienced physician to recognize them as endometriosis.

The endometrial implants adhere to organs and can affect their function. For example, lesions growing on the ovary or fallopian tube may stick the ovary to the pelvic wall and block the movement of an egg.

More than half of endometriosis cases involve implants that grow within the ovaries, called endometriomas. These endometriomas can fill with blood and other fluids and become darker as they age. The endometriomas are sometimes called endometrial cysts, blood cysts or chocolate cysts for their dark color. They are unrelated to cancers of the ovary.

Endometriosis usually occurs during a woman’s years of menstruation. The condition is very rarely seen before a girl’s first menstrual period (menarche). It is most commonly diagnosed among women in their late 20s, but women may have the condition for years prior to diagnosis.

Many women discover that they have endometriosis only when they seek treatment for infertility. For other women, severe pain causes them to seek treatment. At menopause, the growths can shrink and symptoms lessen in many cases. However, if a menopausal woman uses hormone replacement therapy, which mimics the menstrual hormone cycle, she may continue to experience endometriosis symptoms.

For some women, the pain associated with endometriosis is debilitating and it may worsen over time. The damage to reproductive organs caused by endometriosis is a major cause of infertility among women. It is also largely responsible for hysterectomies  and oophorectomies (ovariectomies) performed on premenopausal women. Many women experience recurring symptoms even after extensive medical or surgical treatments for endometriosis.

More than 5.5 million women in the United States have endometriosis, which is about 10 to 15 percent of all women in their reproductive years, according to the National Institutes of Health (NIH). However, the exact incidence of the condition is unknown because women without symptoms or fertility problems may have endometriosis but never seek diagnosis or treatment.