Medically reviewed by Carina Fung, PharmD, BCPPS

What is endometriosis?

Endometriosis1, sometimes referred to as “endo,” is a disorder in which the tissues that normally line the uterus (called the endometrium) grow outside the uterus on other parts of the body. This disorder is often painful and can lead to painful periods, excessive bleeding, and infertility.

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Endometriosis most commonly affects the ovaries, fallopian tubes, outer surface of the uterus, and tissue lining the pelvis. It may also cause abnormal tissues to grow on the vulva or in the vagina, cervix, bowel, bladder, or rectum. In rare cases, endometriosis can spread to organs beyond the pelvic region, such as the skin, lungs, and brain.

While endometriosis growths are benign (non-cancerous), they can still cause problems. They still act like normal endometrial tissues—they thicken, break down, and bleed with each menstrual cycle. During regular menstrual periods, this bleeding exits the body from the uterus through the vagina. In endometriosis, however, abnormal tissues and bleeding blood cannot easily leave the body. This can lead to abdominal pain and inflammation (swelling).

When endometriosis affects the ovaries, this bleeding can also become trapped in the normal ovarian tissue. This causes a “blood blister” surrounded by a fibrous cyst, called an endometrioma, to form. The tissue surrounding these cysts can become inflamed and irritated. Over time, this can lead to the growth of scar tissue and adhesions (abnormal bands of fibrous tissue that can cause pelvic organs and tissues to stick together).

While endometriosis can be painful (particularly during menstruation), effective treatments, such as hormonal treatments and laparoscopic surgery, can help improve its signs and symptoms.

How common is endometriosis?

Endometriosis is a common health problem among women. Researchers believe that the condition affects at least 11% of women2 in the United States between the ages of 15 and 44. Endometriosis is especially common among women in their 30s and 40s and is thought to affect 70% of women3 with chronic pelvic pain.

Unfortunately, the diagnosis of endometriosis often comes long after a woman has already experienced signs and symptoms of the condition: the average delay in diagnosis in patients between the ages of 18 and 45 is 6.7 years3. As most patients with the condition report that their symptoms began during adolescence, many have suffered through unnecessarily prolonged pain and reduced quality of life.

Ultimately, receiving a timely diagnosis of endometriosis is crucial to help prevent pain, slow or prevent the progression of the disease, and reduce the risk of infertility.

Endometriosis causes

The menstrual cycle is the process by which the body prepares itself for pregnancy. The average menstrual cycle lasts for a total of 28 days4. During this time, the tissue found on the inside of the uterus (called the endometrium or uterine lining) builds up in order to prepare for a potential pregnancy.

If no pregnancy occurs, the levels of two hormones—estrogen and progesterone—start to fall. This eventually leads to menstruation, which occurs when the uterus sheds its lining. During menstruation, a combination of these uterine tissues and blood passes out of the uterus through a small opening called the cervix, eventually exiting the body through the vagina.

In women with endometriosis5, tissues that look and act like endometrial tissue develop outside the uterus, usually in the pelvis on their other reproductive organs or within other areas of the abdominal cavity.

During each menstrual cycle, these “misplaced” tissues respond to the body’s hormonal changes in the same way that the uterus does: they build up and break down, just like the endometrium. This “shedding” causes small amounts of bleeding within the pelvis or abdomen, leading to inflammation, swelling, and scarring of the normal tissues surrounding the endometriosis growths.

Researchers are not yet sure what causes endometriosis. Several possible causes being studied include6:

  • Retrograde menstruation: This is thought to be the most likely cause of endometriosis. In retrograde menstruation, some of the blood and tissue shed during a menstrual period flows back through the fallopian tubes into other areas of the body, such as the pelvic cavity. These endometrial cells then stick to the pelvic walls, organs, or structures, leading to endometriosis.
  • Genetics: Endometriosis has been shown to run in families, suggesting a genetic component to the development of the disease.
  • Hormonal abnormalities: The hormone estrogen seems to play a role in the development of endometriosis. Researchers are looking into the possibility that the condition stems from hormonal problems.
  • Immune system disorders: Immune system disorders—as well as certain types of cancer—are more common in women with endometriosis. Problems with the immune system may make the body fail to recognize and destroy endometrial tissue growing outside the uterus.
  • Surgical scar implantation: Abdominal surgery, such as a Cesarean section (C-section) or hysterectomy, may unintentionally cause endometrial tissue to move around and implant on the resulting scars.
  • Embryonic cell transformation: Estrogen, as well as other hormones, may cause embryonic cells (cells in the earliest stages of development) to transform into the cells that make up endometriosis, which then activate and develop into endometriosis during puberty.
  • Transformation of peritoneal cells: One theory, known as the “induction theory7,” proposes that hormonal or immune factors may cause peritoneal cells (the cells lining the inner side of the abdomen) to transform into endometrial-like cells.
  • Endometrial cell transport: It may be possible that the blood vessels or lymphatic system can transport endometrial cells from the uterus to other parts of the body.

Risk factors for endometriosis

Endometriosis can affect anyone who menstruates. However, several factors may increase your risk of developing endometriosis, including8:

  • Starting menstruation prior to age 11
  • Having a menstrual cycle shorter than 27 days
  • Having menstrual periods that are heavy and last longer than 7 days
  • Having higher levels of estrogen or having a longer lifetime exposure to the estrogen your body produces
  • Having abnormalities in your reproductive tract
  • Having a medical condition that prevents the normal flow of menstrual blood from the body
  • Having one or more blood relatives with endometriosis
  • Going through menopause at an older age
  • Never having given birth

Conversely, your risk of developing endometriosis is lower9 if:

  • Your menstrual periods began later in adolescence
  • You have been pregnant
  • You regularly exercise for more than 4 hours each week
  • You have a low percentage of body fat

Endometriosis symptoms

Endometriosis usually develops a few years after the onset of menstruation (when a woman gets her first period). The two main symptoms10 of endometriosis are:

  • Pelvic pain: This affects roughly 75% of all women with endometriosis, and usually occurs or worsens during menstruation.

While cramping during menstruation is common, those with endometriosis typically report experiencing menstrual pain that’s much more severe than usual. This pain may begin before and last several days into menstruation and may worsen over time.

  • Infertility: Up to half of all women with endometriosis become infertile (unable to become pregnant). In some cases, endometriosis is diagnosed while a woman is seeking infertility treatment.

Other signs and symptoms of endometriosis include11:

  • Painful periods (dysmenorrhea): Dysmenorrhea, which includes pelvic pain and cramping, may also feature lower back pain and abdominal pain.
  • Excessive bleeding: Endometriosis may cause occasional heavy periods or bleeding between periods (called intermenstrual bleeding).
  • Painful sexual intercourse: It’s common for women with endometriosis to experience pain during or after sex.
  • Painful urination or bowel movements: These symptoms are most likely to occur during menstruation.
  • Digestive problems: In some cases, endometriosis causes problems like constipation, diarrhea, bloating, nausea, or abdominal cramping, especially during menstruation. For this reason, endometriosis is sometimes mistaken for the chronic condition irritable bowel syndrome (IBS).

The signs and symptoms of endometriosis may temporarily improve12 during pregnancy. Unless you’re taking the hormone estrogen, they may even go away entirely.

Ultimately, the severity of your pain or other symptoms doesn’t necessarily indicate the extent of your endometriosis. Some women may have mild endometriosis and experience severe pain, while others with advanced endometriosis may have little to no pain.

It’s important that you see your healthcare provider if you experience any of the signs and symptoms of endometriosis. Receiving the right diagnosis early on can help ensure that your signs and symptoms are managed as best as possible.

Complications from endometriosis

Endometriosis may result in a number of health complications, including13:

  • Infertility: Reduced fertility is the most common complication of endometriosis. Endometriosis can cause the fallopian tube to become obstructed, blocking eggs from being released and preventing fertilization from taking place. Endometriosis also seems to affect fertility in more indirect ways, such as by damaging eggs or sperm.

Between one-third and one-half of women with the condition experience difficulties becoming pregnant. It’s often advised that women with endometriosis who would like to have children try to conceive as soon as possible, as the condition may worsen over time, decreasing the chances of pregnancy.

  • Cancer: Women with endometriosis have a higher incidence of ovarian cancer than those without the condition. Some studies, however, suggest that even though endometriosis increases this risk, it remains relatively low.

    Rarely, another form of cancer known as endometriosis-associated adenocarcinoma can develop later in life in those who have had endometriosis.

Additionally, endometriosis growths that continue to expand may cause further problems, such as14:

  • Blockages in the fallopian tubes: This occurs when endometrial-like tissue covers or grows inside the ovaries. When trapped in the ovaries, the blood that flows from these growths may cause a cyst (called an endometrioma) to form.
  • Inflammation
  • Scar tissue and adhesions: These can cause pelvic pain and may decrease the chances of becoming pregnant.
  • Problems in the intestines and bladder

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