Endometriosis is a chronic condition in which tissue similar to the lining of the uterus grows in places outside the uterus, triggering pain, inflammation, and scar tissue. It affects roughly 10% of reproductive-age women worldwide, an estimated 190 million people. Despite being that common, it remains widely misunderstood, often dismissed as “bad periods,” and frequently takes years to diagnose.
What’s Actually Happening in Your Body
The inside of the uterus is lined with a tissue called the endometrium. Each menstrual cycle, this lining thickens, breaks down, and sheds. In endometriosis, tissue that resembles this lining appears on organs and surfaces where it doesn’t belong. These growths, called lesions or implants, respond to hormonal shifts in your cycle. They can swell, bleed, and inflame the surrounding tissue, but unlike a normal period, there’s no way for that material to leave the body.
One important distinction: endometriosis lesions are not identical to normal uterine lining. They’re cellularly more complex, containing significant populations of immune cells and blood vessel cells. Molecular studies have revealed that lesions carry distinct genetic signatures related to fibrosis (the buildup of scar-like tissue) or immune dysfunction. This helps explain why the condition behaves so differently from person to person, and why it doesn’t simply resolve when hormones are suppressed.
Over time, the chronic inflammation caused by these lesions irritates surrounding tissue. The body responds by forming scar tissue and fibrous bands called adhesions, which can bind organs together. Ovaries can stick to the pelvic wall. The bowel can adhere to the uterus. These adhesions are a major source of pain and complications.
Where Endometriosis Grows
The most common locations are within the pelvic cavity: the ovaries, the space behind the cervix, the vagina, the bladder, the large bowel, the ureters, and the ligaments that support the uterus. Ovarian endometriosis often forms fluid-filled cysts called endometriomas, sometimes referred to as “chocolate cysts” because of the dark, old blood they contain.
Less commonly, endometriosis shows up in places further from the uterus. The appendix, small bowel, umbilicus (belly button), inguinal area (groin), cesarean scars, diaphragm, and pelvic nerves can all be affected. In rare cases, lesions have been found in the lungs, skin, lymph nodes, and even the brain. This wide distribution is one reason scientists believe the condition is more complex than a simple problem of misplaced menstrual tissue.
What Causes It
No single cause has been confirmed. Several theories exist, and the reality likely involves a combination of mechanisms.
The most widely cited explanation is retrograde menstruation, first proposed a century ago. The idea is straightforward: during a period, some menstrual tissue flows backward through the fallopian tubes into the pelvic cavity, where it implants and grows. There’s solid evidence that this backward flow happens, but the theory has significant gaps. Most women experience some degree of retrograde menstruation, yet only a fraction develop endometriosis. It also can’t explain lesions found in the lungs or brain, endometriosis in newborns and adolescents, cases in women born without a uterus, or the rare instances documented in men.
Another theory proposes that endometrial cells spread through blood vessels or lymphatic channels, much like cancer cells can metastasize. This would account for lesions appearing in distant organs. A third theory focuses on embryonic development: during fetal growth, cells from the structures that form the reproductive tract may end up in the wrong location and lie dormant until puberty, when rising estrogen levels activate them. There’s also growing evidence that genetic predisposition and immune system dysfunction play central roles, potentially explaining why some people’s bodies fail to clear misplaced tissue that others eliminate naturally.
Symptoms Beyond Period Pain
Severe menstrual cramps are the hallmark symptom, but endometriosis reaches well beyond the days of your period. Many people experience chronic pelvic pain that has no clear relationship to their cycle at all. Pain during or after sex is common, particularly deep penetrative pain.
When lesions involve the bowel, you may have pain during bowel movements, along with diarrhea, constipation, or alternating between the two. Bladder involvement can cause painful urination. Some people notice blood in their stool or urine that appears cyclically, coinciding with their period. Ovulation itself can become painful. In cases affecting the diaphragm or pelvic nerves, symptoms can include chest pain or sciatica-like nerve pain radiating down the leg.
The pain can be severe enough to cause vasovagal episodes, where the body’s pain response triggers a drop in blood pressure and fainting. Heavy menstrual bleeding is also common, especially when a related condition called adenomyosis is present (where similar tissue grows into the muscular wall of the uterus itself). Fatigue, often profound and debilitating, is a symptom that rarely gets enough attention but consistently ranks among the most disruptive for people living with the condition.
How It Affects Fertility
Endometriosis is one of the leading causes of difficulty conceiving. The mechanisms are both structural and biochemical. Adhesions and scar tissue can physically distort the fallopian tubes, blocking the egg and sperm from meeting. Endometriomas on the ovaries can damage healthy egg-containing tissue.
But the impact goes beyond physical blockages. The inflammatory environment created by endometriosis appears to be toxic to eggs and sperm, reducing their quality and viability. Implantation of a fertilized egg may also be impaired. Not everyone with endometriosis will struggle with fertility, and many conceive without intervention, but the condition is found in a significant proportion of people undergoing fertility treatment.
Why Diagnosis Takes So Long
Endometriosis is notoriously slow to diagnose. Symptoms overlap with irritable bowel syndrome, urinary tract infections, pelvic inflammatory disease, and other conditions. Many people are told their pain is normal or psychological before anyone considers endometriosis. The normalization of menstrual pain in society plays a significant role in this delay.
Historically, the only way to definitively confirm endometriosis was through diagnostic laparoscopy, a surgical procedure where a camera is inserted into the abdomen to visualize and biopsy lesions. Today, dedicated ultrasound and MRI protocols performed by experienced specialists can detect many forms of the disease without surgery. Both imaging methods perform well for identifying endometriomas on the ovaries and deeper lesions on the bowel or bladder, with overall accuracy that’s comparable between the two when performed by experts. Ultrasound tends to be better at identifying how deeply lesions invade the bowel wall, while MRI is better at detecting disease along the pelvic wall and in harder-to-reach locations.
The catch is that routine, non-specialized scans perform significantly worse than dedicated protocols, which likely contributes to diagnostic delays. Superficial endometriosis, the small, flat lesions scattered across pelvic surfaces, is still difficult to see on any imaging and may require laparoscopy to confirm. A normal ultrasound or MRI does not rule out endometriosis.
How It’s Managed
There is no cure for endometriosis, but treatments aim to reduce pain, slow the growth of lesions, and preserve fertility when desired. The approach depends on symptom severity, which organs are involved, and whether you’re trying to conceive.
First-line medical treatment typically involves hormonal therapy. Combined oral contraceptives and progestin-only options work by suppressing ovulation and thinning the tissue that feeds endometriosis lesions. For many people, this significantly reduces pain and slows disease progression. These are often taken continuously, skipping the placebo week, to eliminate periods altogether.
When first-line hormones aren’t enough, medications that more aggressively suppress estrogen production are an option. These work by shutting down the hormonal signaling chain between the brain and the ovaries, creating a low-estrogen state that starves the lesions. The trade-off is that very low estrogen causes menopausal side effects: hot flashes, bone density loss, mood changes, and vaginal dryness. To counter this, a small amount of hormones can be added back, enough to protect bones and ease side effects without reactivating the endometriosis. This combination allows longer-term use.
Surgery, typically performed laparoscopically, removes or destroys visible lesions and cuts away adhesions. It can provide significant pain relief and improve fertility for some people. However, endometriosis recurs after surgery in a substantial number of cases, particularly if hormonal treatment isn’t continued afterward. For severe, widespread disease that hasn’t responded to other treatments, removal of the uterus and sometimes the ovaries may be considered, though this is a last resort and doesn’t guarantee that all symptoms will resolve.
Pain management, pelvic floor physical therapy, and psychological support also play important roles. Endometriosis is a condition that often requires a combination of strategies, adjusted over time as symptoms and life goals change.