Endometriosis is a condition in which the tissue that normally lines the uterus (endometrium) grows in other areas of the body, causing pain, irregular bleeding, and possible infertility.

The tissue growth (implant) typically occurs in the pelvic area, outside of the uterus, on the ovaries, bowel, rectum, bladder, and the delicate lining of the pelvis. However, the implants can occur in other areas of the body, too.

The cause of endometriosis is unknown. However, there are a number of theories. One suggests that the endometrial cells (loosened during menstruation) may "back up" through the fallopian tubes into the pelvis, where they implant and grow in the pelvic or abdominal cavities. This is called retrograde-menstruation.

Other theories include:

• A faulty immune system causes menstrual tissue to implant and grow in areas other than the uterine lining. 
• Cells lining the abdominal cavity may spontaneously develop endometriosis.
• Certain families may have genetic factors that make a woman more prone to endometriosis

Each month the ovaries produce hormones that stimulate the cells of the uterine lining (endometrium) to multiply and prepare for a fertilized egg. The lining swells and gets thicker. 

If endometrial cells are implanted outside the uterus, or elsewhere, it can cause problems. These cells also respond to the monthly hormone stimulation. Unlike cells normally found in the uterus that fall off during menstruation, the ones outside the uterus stay in place. They sometimes bleed a little bit, but they heal and are stimulated again during the next cycle.

This ongoing process can cause scarring and adhesions in the tubes and ovaries, and at the end of the fallopian tubes. (The adhesions can make it hard for reproductive cells to move from the ovary to the fallopian tube. They can also stop a fertilized egg from passing down the fallopian tube to the uterus.)

Endometriosis is a common problem. It occurs in an estimated 10% of women during their reproductive years. The rate may be as high as 35% among infertile women. Although endometriosis is typically diagnosed between the ages of 25 and 35, the problem probably begins about the time that regular menstruation begins.

A woman who has a mother or sister with endometriosis has a 6 times greater risk of developing endometriosis than the general population. Other possible risk factors include starting menstruation at an early age, regular menstrual cycles, and long periods (lasting 7 or more days).

• Increasingly painful periods
• Lower abdominal pain or pelvic cramps that can by felt for a week or two before menstruation
• Lower abdominal pain felt during menstruation (the pain and cramps may be steady and dull or quite severe)
• Pelvic or low back pain that may occur at any time during the menstrual cycle
• Pain during or following sexual intercourse
• Pain with bowel movements
• Premenstrual spotting
• Infertility

Note: Frequently, symptoms may not be present. In fact, some women with severe cases of endometriosis have no pain at all, while some women with only a few small implants have severe pain.

A pelvic examination may reveal the presence of tender nodules, with a lumpy consistency. These are often found in the posterior vaginal wall or adnexa (ovary regions), and they may sometimes be felt in healed wound scars (especially those from episiotomy and C-section). There may be pain with uterine motion.

The uterus may be fixed or retroverted. A pelvic ultrasound test may detect an endometrioma on an ovary. A laparoscopy is necessary for a definite diagnosis, but most patients can start treatment without this.